Final Year Instrumentation and Control Projects

This post is regarding the Project topics for Instrumentation and Control Engineering Discipline final year Students.

I will first give out the instrumentation and control engineering project titles first, if you need any help in that particular topic, you may contact me in the comment form..


Industrial machine Data (sensors) Transmitting, Receiving And Control Through Walkie Talkie.
Temperature Reding And Controlling Through Fibre Cable.
Wireless Controlling For Boiler / Gas Stove temperature / Timer / Flame Controlling System.
Finger print recognition.
Using SCADA for Process Control
Data Acquisition for Process Control using Lab View Software.
PLC based Dc Servo Motor Control System
HEART BEAT MONITOR WITH WAVE ON LCD(PIC BASED)
Instrumentation Project on POWER GRID CONTROL THROUGH PC.
Temperature Reading Data - Transmitted Through Fibre Cable.

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Introduction to Correction calibration

All measuring instruments are to prove themselves their ability to measure reliably and accurately. For this, the results of measurement are to be compared with higher standards which are traceable to national or international standards. The procedure of this is termed as calibration.

Calibration is thus a set of operation that establishes the relationship between the values that are indicated by measuring instrument and the corresponding known value of measurand.

Thus calibration of measuring instrument means introducing an accurately known sample of the variable that is to be measured and then observing the system’s response. Then the measuring instrument is checked and adjusted until its scale reads the introduced accurately known sample of the variable.

It should be further noted that an instrument is calibrated at one place and is put to use at some other place. Care should be taken to see that the instrument is used at a place where the environment has the same conditions as that of the place where the instrument was calibrated to ensure that the instrument gives correct readings.

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Dew Point Meter

Basic Principle of Dew Point Meter


By Cooling at constant pressure if the temperature of air is reduced, the water-vapour in the air will start to condense at a particular temperature. This temperature is called dew point temperature.

Description of Dew Point Meter


The main Parts of arrangement are


  • A shiny surface (mirror) is fixed with a thermocouple.
  • A nozzle is providing a jet of air on the mirror.
  • A light source focused constantly on the mirror.
  • A photo cell to detect the amount of light reflected from the mirror.


Operation of Dew Point Meter


  • The mirror is constantly cooled by a cooling medium. The cooling medium is maintained at a constant temperature.
  • To this mirror is attached a thermocouple whose leads are connected to a millivoltmeter.
  • Constantly a light is made to fall in an angle on the mirror and the amount of reflected light is sensed by a photo cell.
  • Now an air jet is made to fall in an angle on the mirror and the water-vapor (moisture) contained in the air starts condensing on the mirror and they appear as small drops (dew) on the mirror.
  • This moisture (dews) formed on the mirror reduces the amount of light reflected from the mirror and it is detected by photocell. When for the first time, there is a change in amount of transmitted light; it becomes an indication of dew formation.
  • At this instance (that is, when the due formation is detected first), the temperature indicated by the thermocouple attached to the mirror becomes the dew point temperature.
  • Thus this arrangement is used to determine the time at which the dew appears for the first time and dew point temperature.

Application of dew point meter


  • This instrument is used on ships to protect cargoes from condensation damage by maintaining the dew point of air in holds lower than the cargo temperature.
  • Used in industries for determining dew point.

Limitations

There are limitations in cooling fluids and light measurement.

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Electrical Humidity Sensing Absorption Hydrometer

Basic Principle:


A change in resistance with change in humidity is taken as a measure of humidity.

This is type of Electrical Humidity sensing Absorption Hydrometer, know more about mechanical humidity sensing methods using Hair Haydrometer

Description:


The main Parts of this arrangements are, two metal electrodes which are coated and separated by a humidity sensing hygroscopic salt (lithium chloride). The leads of the electrodes are connected to a null – balance wheat stone bridge.

Operation:


When the humidity of the atmosphere is to be measured, the electrodes coated with lithium chloride are exposed to atmosphere. Humidity variation causes the resistance of the chemical (lithium chloride) to change. That is, the chemical absorbs moisture or loss moisture and causes a change in resistance.

Higher the humidity (RH) in the atmosphere more will be the humidity absorbed by lithium chloride and lower will be the resistance.

Lesser the humidity (RH) in the atmosphere less will be the humidity absorbed by lithium chloride and higher will be the resistance.

The change in resistance using a Wheatstone bridge and this change in resistance becomes a measure of humidity (RH) present in the atmosphere.

Applications


These hydrometers are used under constant temperature conditions to measure humidity of atmosphere.
The accuracy of this instrument is in +/- 25%.
The response is of the order of few seconds.

Limitations


These instruments should not be exposed to 100% humidity as the resulting absorption by the chemical (lithium chloride) might damage the instrument.
If these devices are not used under constant temperature conditions, temperature correction must be made.

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Hair Hydrometer

Basic Principle

Due to humidity, several materials experience a change in physical, chemical and electrical properties. This property is used in transducer that are designed and calibrated to read relative humidity directly.

Hair hydrometer is a type of absorption hydrometer and uses the mechanical humidity sensing technique.

Certain hygroscopic materials such as human hair, animal membranes, wood, paper, etc., undergo changes in linear dimensions when they absorb moisture from their surrounding air. This change in linear dimension is used as the measurement of humidity present in air.

Description of Hair Hydrometer

The main Parts of hair hydrometer are,

Hair HydrometerHuman hair is used as the humidity sensor. The hair is arranged in parallel beam and they are separated from one another to expose them to the surrounding air/atmosphere. Number of hairs are placed in parallel to increase mechanical strength.

This hair arrangement is placed under small tension by the use of a tension spring to ensure proper functioning.

The hair arrangement is connected to an arm and a link arrangement and the link is attached to a pointer pivoted at one end. The pointer sweeps over a humidity calibrated scale.

Operation of Hair Hydrometer

When the humidity of air is to be measured, this air is made to surround the hair arrangement and the hair arrangement absorbs the humidity from the surrounding air and expands or contracts in the linear direction.

This expansion or contraction of the hair arrangement moves the arm & link and thus the pointer to a suitable position on the calibrated scale and thus indicating the humidity present in the air/atmosphere.

Note: These Hair hydrometers are called membrane hydrometers when the sensing element is a membrane.

Applications of Hair Hydrometer

These hydrometers are used in the temperature range of 0’C to 75’C.
These hydrometers are used in the RH (Relative Humidity) range of 30 to 95%.

Limitations of Hair Hydrometer

These Hydrometers are slow in Response
If the Hair hydrometer is used constantly, its calibration tends to change.

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Pressure Measuring Instruments


McLeod Vacuum Gauge

A known volume gas is compressed to a smaller volume whose final value provides an indication of the applied pressure. I have given a brief post on the equations used with good construction diagram of McLeod Vacuum Gauge

Strain Gauge Pressure Cell

Based on a principle

When a closed container is subjected to the appilied pressure, it is strained (that is, its dimension changes). Measurement of this strain with a secondary transducer like a strain gauge ( metallic conductor) becomes a measure of the appilied pressure.

The Two types of Strain Gauge Pressure Cell, Flattened tube pressure cell and Cylindrical type pressure cell are discussed.

Elastic diaphragm gauges

When an elastic transducer (diaphragm is this case) is subjected to a pressure, it deflects. This deflection is proportional to the appilied pressure when calibrated.

Bourdon tube Pressure Gauge

when an elastic transducer ( bourdon tube in this case ) is subjected to a pressure, it defects. This deflection is proportional to the applied pressure when calibrated.

Manometer with equal unequal limb types

This is the most simple and precise device used for the measurement od pressure. It consists of a transparent tube constructed in the form of an elongated 'U', and partially filled with the manometeric fliud such as mercury. The purpose of using mercury as the manometeric fluid is that their specific gravity at various temperatures are known exactly and they dont stick to the tube. The two common types of manometers are the equal limb type and unequal limb type.

Dead Weight Tester

The dead weight tester apparatus consists of a chamber which is filled with oil free impurities and a piston – cylinder combination is fitted above the chamber as shown in diagram. The top portion of the piston is attached with a platform to carry weights. A plunger with a handle has been provided to vary the pressure of oil in the chamber. The pressure gauge to be tested is fitted at an appropriate plate.

Pressure Measurement using U-tube Manometer
A well known very simple device used to measure the pressure is the U-tube manometer. The name U-tube is derived from its shape. U-tube manometer is shown below.

Terms related to pressure

All the important terms related to Pressure measurement like Atmospheric pressure, Absoulte pressure, Gauge Pressure, Vacuum pressure, Static Pressure, Total or Stagnation pressure, Dynamic – or – Impact – or – Velocity pressure and more are discussed.

And This Post will be updated regularly whenever i post a topic about pressure measurement using different devices.

Please Post your Comments below.

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Sling Psychrometer

Sling Psychrometer is used to measure both the dry bulb and wet bulb temperatures at time. These temperatures are a measure of humidity content in air.

Description of Sling Psychrometer


The main parts of the instrument are

sling psychrometer
The instrument frame which holds the thermometers.

One mercury in glass thermometer whose sensing bulb is bare to directly contact the air and to measure the temperature which is called as the dry-bulb temperature.

One mercury in glass thermometer whose sensing bulb is covered with a cotton or muslin wick made wet with pure water. This sensing bulb covered with the cotton wick moistened is made to contact the air and the temperature indicated by this thermometer is called as the wet bulb-thermometer.

The instrument frame carrying the thermometer is covered by a glass casing.

A swivel handle is attached to frame-glass casing – thermometer arrangement to ensure that the air at the wet bulb always in immediate contact with the wet wick.

When a thermometer bulb is directly exposed to an air-water vapour mixture, the temperature indicated by the thermometer is the dry-bulb temperature.

When a thermometer bulb is covered by a constantly wet wick and if the bulb covered by the wet wick is exposed to air water vapour mixture, the temperature indicated by the thermometer is the wet bulb temperature.

Operation of Sling Psychrometer.


In order to measure the dry bulb and wet bulb temperature, the Psychrometer frame – glass covering – thermometer arrangement is rotated at 5 m/s to 10 m/s to get the necessary air motion.

Note: An important condition is that correct/accurate measurement of wet bulb temperature is obtained only if air moves with velocity around the wet wick. In order to get this air velocity, the Psychrometer is being rotated.

The thermometer whose bulb is bare contacts the air indicates the dry bulb temperature.

At the same time, the thermometer whose bulb is covered with the wet wick comes in contact with the air and when this pass on the wet wick present on the bulb of the thermometer, the moisture present in the wick starts evaporating and a cooling effect is produced at bulb. Now the temperature indicated by the thermometer is the wet bulb thermometer which will naturally be lesser than the dry bulb temperature.

Note: If the Psychrometer is rotated for a short period, then the wet bulb temperature recorded will not be proper.
Note: If the Psychrometer is rotated for a longer period, the wick will get dried soon and the wet bulb temperature will not be at its minimum value.

Application of Sling Psychrometer


  1. It is used for checking humidity level in air-conditioned rooms and installations.
  2. It is used to set and check hair hygrometer.
  3. It is used in the measurement range of 0 to 100% RH.
  4. It is used for measuring wet bulb temperature between 0’C to 180’C.

Limitation of Sling Psychrometer


  1. The measured medium is disturbed due to the act of measurement. The evaporation process at the wet bulb will add moisture to the air.
  2. It cannot be used in automation requirement situations.
  3. It cannot be used for continuous recording purpose.
  4. If the wick is covered with dirt, the wick will become stiff and its water absorbing capacity will reduce, however, a stiff/dirty wick will resume normalcy when boiled in hot water.

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Humidity / Dampness Measurement

Introduction:

The amount of water vapour contained in air or gas is called as humidity. This humidity affects human comforts and many industrial process as in case of chemical industries, garments industries, paper industries, food industries, leather industries, pharmaceutical industries, precision equipment manufacturing, etc. hence, study of humidity is important. Let us define some common terms related to humidity measurement.

Humidity


The amount of water vapour contained in air or gas is called as humidity. It is usually measured as absolute humidity, relative humidity or due point temperature.

Dry air


When there is no water vapour contained in the atmosphere, it is called dry air.

Moist air


When there is water vapour present in the atmosphere, it is called moist air.

Saturated air


Saturated air is the moist air where the partial pressure of water-vapour equals the saturation pressure of steam corresponding to the temperature of air.

Absolute humidity


It is the mass of water vapour present per unit volume. In other words, it is the quantity of the water vapour present in air and its unit is grams per cubic meter of air.

Relative humidity RH


Relative humidity = (water vapour pressure actually present)/(water vapour pressure required of saturation) at a given.

Here a comparison is made between the humidity of air and humidity of saturated air at the same temperature and pressure.
It should be noted that if relative humidity is 100%, it is saturated air. That is, the air contains all the moisture it can hold.
It should also be noted that the degree of saturation (percentage of relative humidity) of air keeps on changing with temperature.


Humidity ratio or specific humidity


For a given volume of air water vapour mixture,

Humidity ratio = (mass of water – vapour)/mass of dry air

Dew point temperature


By continuous cooling at constant pressure if the temperature of air is reduced, the water-vapour in the air will start to condense at a particular temperature. The temperature at which the water vapour stats condensing is called as dew point temperature.

Dry-bulb temperature


When a thermometer bulb is directly exposed to an air-water vapour mixture, the temperature indicated by the thermometer is the dry-bulb temperature.
This dry-bulb temperature is not affected by the moisture present in the air, that is, the temperature of air is measured in a normal way by the thermometer.
The dry bulb is used to distinguish the normal temperature measured from the temperature measured by the wet-bulb.

Wet-bulb temperature


When a thermometer bulb is covered by a constantly wet wick, and if the bulb covered by the wet wick is exposed to air-water vapour mixture, the temperature indicated by the thermometer is wet-bulb temperature.

When air is passed on the wet wick present on the bulb of the thermometer, the moisture present in the wick strats evaporating and this creates a cooling effect at the bulb. The bulb now measures the thermo dynamic equilibrium temperature reached between the cooling effected by the evaporation of water and heating by convection.

Wet-bulb depression


Wet-bulb depression = (dry bulb temperature) – (wet bulb temperature)

Always dry-bulb temperature is higher than the wet bulb temperature.

Percentage humidity


Percentage humidity = weight of water vapour in a unit weight of air/ weight of water vapour in same weight of air if the air were completely saturated at the same temperature.

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Virutal Instrumentation Software - LabVIEW

Today in this post, I  like to present you a very famous and helpful software in our instrumentation and control field which is nothing but LabVIEW.

It is completely related to virtual instrumentation. And to know about virtual instrumentation http://en.wikipedia.org/wiki/Virtual_instrumentation.

Here is a video about LabView.








You can download a student edition in ni.com  or download a pirated version in torrents site.

Some useful link related to Labview

http://www.cord.edu/faculty/jensen/LabVIEW/Tutorial/Default.htm
http://labviewwiki.org/LabVIEW_tutorial
http://www.eelab.usyd.edu.au/labview/main.html

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McLeod Vacuum Gauge

Basic Principle of McLeod Vacuum Gauge:


A known volume gas is compressed to a smaller volume whose final value provides an indication of the applied pressure. The gas used must obey Boyle’s law given by;

P1V1=P2V2

Where, P1 = Pressure of gas at initial condition (applied pressure).
P2 = Pressure of gas at final condition.
V1 = Volume of gas at initial Condition.
V2 = Volume of gas at final Condition.

Initial Condition == Before Compression.
Final Condition == After Compression.

A known volume gas (with low pressure) is compressed to a smaller volume (with high pressure), and using the resulting volume and pressure, the initial pressure can be calculated. This is the principle behind the McLeod gauge operation.

Description of McLeod Vacuum Gauge:


The main parts of McLeod gauge are as follows:
McLeod vacuum gauge

A reference column with reference capillary tube. The reference capillary tube has a point called zero reference point. This reference column is connected to a bulb and measuring capillary and the place of connection of the bulb with reference column is called as cut off point. (It is called the cut off point, since if the mercury level is raised above this point, it will cut off the entry of the applied pressure to the bulb and measuring capillary. Below the reference column and the bulb, there is a mercury reservoir operated by a piston.

Operation of McLeod Vacuum gauge:


The McLeod gauge is operated as follows:

The pressure to be measured (P1) is applied to the top of the reference column of the McLeod Gauge as shown in diagram. The mercury level in the gauge is raised by operating the piston to fill the volume as shown by the dark shade in the diagram. When this is the case (condition – 1), the applied pressure fills the bulb and the capillary.
Now again the piston is operated so that the mercury level in the gauge increases.

When the mercury level reaches the cutoff point, a known volume of gas (V1) is trapped in the bulb and measuring capillary tube. The mercury level is further raised by operating the piston so the trapped gas in the bulb and measuring capillary tube are compressed. This is done until the mercury level reaches the “Zero reference Point” marked on the reference capillary (condition – 2). In this condition, the volume of the gas in the measuring capillary tube is read directly by a scale besides it. That is, the difference in height ‘H’ of the measuring capillary and the reference capillary becomes a measure of the volume (V2) and pressure (P2) of the trapped gas.

Now as V1,V2 and P2 are known, the applied pressure P1 can be calculated using Boyle’s Law given by;

P1V1 = P2V2

Let the volume of the bulb from the cutoff point upto the beginning of the measuring capillary tube = V

Let area of cross – section of the measuring capillary tube = a
Let height of measuring capillary tube = hc.

Therefore,

Initial Volume of gas entrapped in the bulb plus measuring capillary tube = V1 = V+ahc.

When the mercury has been forced upwards to reach the zero reference point in the reference capillary, the final volume of the gas = V2 +ah.

Where, h = height of the compressed gas in the measuring capillary tube
P1 = Applied pressure of the gas unknown.
P2 = Pressure of gas at final condition, that is, after compression
= P1+h

We have, P1V1 = P2V2 (Boyle’s Law)
Therefore, P1V1= (P1+h)ah

P1V1 = P1ah + ah^2

P1V1-P1ah = ah^2

P1 = ah^2/(V1-ah)

Since ah is very small when compared to V1, it can be neglected.

Therefore, P1 = ah^2/V1

Thus the applied pressure is calculated using the McLeod Gauge.

Applications

The McLeod Gauge is used to measure vacuum pressure.

Advantages of the McLeod Gauge:


  • It is independent of the gas composition.
  • It serves as a reference standard to calibrate other low pressure gauges.
  • A linear relationship exists between the applied pressure and h
  • There is no need to apply corrections to the McLeod Gauge readings.

Limitations of McLeod Gauge:


  • The gas whose pressure is to be measured should obey the Boyle’s law
  • Moisture traps must be provided to avoid any considerable vapor into the gauge.
  • It measure only on a sampling basis.
  • It cannot give a continuous output.

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Surface Preparation and bonding techniques

Surface preparation and bonding techniques have been discussed under the following three topics namely:

  1. Backing, base or carrier material.
  2. Bonding material or cement.
  3. Surface preparation and mounting of strain gauges.

Backing, Base or Carrier Material.


The purpose of providing the carrier/backing material ina strain gauge arrangement has been listed as follows;

  1. The backing material provides support to the resistance wire (grid) of the strain gauge arrangement.
  2. The backing material provides protection to the sensing resistance wire of the strain gauge arrangement. It also provides dimensional stability for the resistance wire of the strain gauge arrangement.

Characteristics Required for Backing Materials

  1. The backing material should be an insulator of electricity.
  2. The backing material should not absorb humidity, that is should be non-hygroscopic.
  3. The backing material should be very thin.
  4. It should go along with the adhesive material used to fix (bond) it on the structure under study.
  5. It should not be affected by temperature changes.
  6. It should be strong enough to transmit the force from the structure under study to the sensing resistance wire.

Bonding Materials or Cements (Adhesive)


The strain gauge has to be fixed (bonded) on the structure under study using an adhesive or paste. These adhesive are called as bonding material or cements.
The different adhesive, their composition and the temperature for which they can be used are shown in following table.

Adhesive, that is, Bonding Material       Composition                 For Temperature
Thermo-plastic cement                               Celluloid dissolved in acetone    Upto 75’C
Thermo-setting Cement                              Phenol resin             From 150’C to 210’C
Special Ceramic  – cement                                 -                                  Above 175’C


Characteristics Required


  1. The characteristics required for a bonding material are listed.
  2. The bonding material should be an insulator of electricity.
  3. The bonding material should not absorb humidity, that is, it should be non-hygroscopic.
  4. It should go along with the backing material so that the backing material is fixed (bonded) rigidly on the structure under study.
  5. It should not be affected by temperature changes.
  6. It should have good shear strength to transmit the force from the structure under study to the sensing resistive wire.
  7. It should be easy to apply and should spread easily and should provide good bonding adhesion.
  8. The bonding material should have a high creep resistance.

Surface preparation and mounting of strain gauges


The steps involved in preparing a surface to mount a strain gauge are listed:

  1. The structure under study is made even and free from dust and dirt by rubbing with an emery sheet or by sand blasting.
  2. The even surface is then cleaned by a volatile solution (acetone) using a cloth to remove oil/grease.
  3. The bottom side of the backing (gauge carrier) is also cleaned by a solvent using a cloth.

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Bonded Strain Gauges

These gauges are directly bonded (that is pasted) on the surface of the structure under study. Hence they are termed as bonded strain gauges. The three types of bonded strain gauges are

  1. Fine wire strain gauge
  2. Metal foil strain gauge
  3. Semi-conductor gauge

Fine wire strain gauge

This is the first type of Bonded Strain Gauges.

Description


The arrangement consists of following parts,

A fine resistance wire diameter 0.025 mm which is bent again and again as shown in diagram. This is done to increase the length of the wire so that it permits a uniform distribution of stress. This resistance wire is placed between the two carrier bases (paper, Bakelite or Teflon) which are cemented to each other. The carrier base protects the gauge from damages. Leads are provided for electrically connecting the strain gauge to a measuring instrument (Wheatstone bridge).
fine wire strain gauge


Operation


With the help of an adhesive material, the strain gauge is pasted/bonded on the structure under study. Now the structure is subjected to a force (tensile or compressive). Due to the force, the structure will change the dimension. As the strain gauge is bonded to the structure, the stain gauge will also undergo change in both in length and cross-section (that is, it strained). This strain (change in dimension) changes the resistance of the strain gauge which can be measured using a wheat stone bridge. This change in resistance of the strain gauge becomes a measure of the extent to which the structure is strained and a measure of the applied force when calibrated.

Fine Wire strain gauge Materials


Material Composition
Nichrome Ni - 80% ; Cr – 20%
Constantan Ni – 45%; Cu – 55%
Nickel ----
Platinum ----
Isoelastic Ni – 36%; Cr – 8%; Mo – 0.5%

Advantages of Fine Wire Strain Gauge


The range of this gauge is +/- 0.3% of strain.
This gauge has a high accuracy.
Has a linearity of +/- 1%.

Limitation of Fine Wire strain gauge


  • These gauges cannot be detached and used again (because the gauges are bonded to the structure).
  • These gauges are costly.

Metal Foil Strain Gauge


Description of Metal Foil Strain Gauge


The arrangement consists of the following;

The metal foil of 0.02mm thick is produced using the printed circuit technique. This metal foil is produced on one side of the plastic backing. Leads are soldered to the metal foil for electrically connecting the strain gauge to a measuring instrument (wheat stone bridge).
metal foil strain gauge

Operations of Metal foil Strain gauge


With the help of an adhesive material, the strain gauge is pasted/bonded on the structure under study. Now the structure is subjected to a force (tensile or compressive). Due to the force, the structure will change the dimension. As the strain gauge is bonded to the structure, the stain gauge will also undergo change in both in length and cross-section (that is, it strained). This strain (change in dimension) changes the resistance of the strain gauge which can be measured using a wheat stone bridge. This change in resistance of the strain gauge becomes a measure of the extent to which the structure is strained and a measure of the applied force when calibrated. Same as Fine Wire strain gauge operation.

Advantages of Metal foil Strain gauge


  • These strain gauges can be manufactured in any shape.
  • Perfect bonding of the strain gauge is possible with structure under study.
  • The backing can be peeled off and the metal foil with leads can be used directly on the structure under study. In such cases, a ceramic adhesive is to be used.
  • These gauges have a better fatigue life.
  • Has good sensitivity and have stability even at high temperatures.

Semi – conductor or Piezo Resistive Strain Gauge


Description of Piezo Resistive Strain Gauge.


The arrangement of a semi-conductor strain gauge is as follows:

The sensing element is rectangular filament made as a wafer from silicon or geranium crystals. To these crystals, boron is added to get some desired properties and this process is called doping and the crystals are called doped crystals. This sensing element is attched to a plastics or stainless steel backing. Leads made of gold are drawn out from the sensing element for electrically connecting the strain gauge to a measuring instrument (wheat stone bridge).

There are two types of sensing element namely:

  • Negative or n-type (resistance decrease with respect to tensile strain).
  • Positive or P-type ( resistance increase with respect to tensile strain).
piezo resistive strain gauge


Operation


With the help of an adhesive material, the strain gauge is pasted/bonded on the structure under study. Now the structure is subjected to a force (tensile or compressive). Due to the force, the structure will change the dimension. As the strain gauge is bonded to the structure, the stain gauge will also undergo change in both in length and cross-section (that is, it strained). When the sensing element (crystal) of the semiconductor strain gauge is strained, its resistivity changes contributing to a change in the resistance of the strain gauge. The change in the resistance of the strain gauge is measured using a wheat stone bridge. . This change in resistance of the strain gauge becomes a measure of the extent to which the structure is strained and a measure of the applied force when calibrated.

Advantages of semi-conductor Strain gauges


  • These gauges have high gauge factor and hence they can measure very small strains.
  • They can be manufactured to very small sizes.
  • They have an accuracy of 2.3%
  • They have excellent hysteresis characteristics.
  • They have a good frequency of response.
  • They have good fatigue life.

Limitation of semi-conductor Strain gauges


  • These gauges are brittle and hence they cannot be used for measuring large strain.
  • The gauge factor is not constant.
  • These gauges have poor linearity.
  • These gauges are very costly and are difficult to be bonded onto the structure under study.
  • These gauges are sensitive to change in temperature.

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Installation of Strain Gauge Video Post

Hello Reader time for real watch about strain gauge and how to install them, this video is Presented by a YouTube user Binsfledengineering


Post you comments below.


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Unbonded Strain Gauges

These strain gauges are not directly bonded (that is, pasted) onto the surface of the structure under study. Hence they are termed as unbounded strain gauges.


Description of the Unbonded Strain gauges:
     The arrangement of an unbonded strain gauges consists of the following. Two frames P and Q carrying rigidly fixed insulated pins as shown in diagram. these two frames can move relative with respect to each other and they are held together by a spring loaded mechanism. A fine wire resistance strain gauge is stretched around the insulated pins. The strain gauge is connected to a wheat stone bridge.

unbonded strain gauge

Operation of Unbonded strain gauges:

    When a force is applied on the structure under study (frames P & Q), frames P moves relative to frame Q, and due to this strain gauge will change in length and cross section. That is, the strain gauge is strained. This strain changes the resistance of the strain gauge and this change in resistance of the strain gauge is measured using a wheat stone bridge. This change in resistance when calibrated becomes a measure of the applied force and change in dimensions of the structure under study.

Application of Unbonded strain gauge:

     Unbonded strain gauge is usedin places where the gauge is to be detached and used again and again.
unbonded strain gauges are used in force, pressure and acceleration measurement.

Advantages of Unbonded strain gauge:


  • The range of this gauge is +/- 0.15% strain.
  • This gauge has a very high accuracy.


Limitation of unbonded strain gauges

It occupies more space.

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Load cell and Load cell Types

If need to read by post on Load Cell, if you are new to my site and read about strain gauge load cell to have a better idea about load cells

There are two types of Load Cells, they are
  1. Hydraulic load cells
  2. Pneumatic load cells
Hydraulic Load Cell

Basic Priniple of Hydraulic Load cell

When a force is applied on a liquid medium contained in a confined space, the pressure of the liquid increases. This increase in pressure of the liquid is proportional to the appilied force. Hence a measure of the increase in pressure of the liquid becomes a measure of the appilied force when calibrated.

Description of Hydraulic Load Cell


construction of hydraulic load cellThe main parts of a hydraulic load cell are as follows

A dirphragm
A piston with a loading platform (as shown in figure) placed on top of the diaphragm.
A liquid medium which is under a pre-loaded pressure is on the other side of the diaphragm.
A pressure gauge (bourdon tube type) connected to the liquid medium.

Operation of Hydraulic Load Cell


The force to be measured is applied to the piston.
The appilied force moves the piston downwards and deflects the diaphragm and this deflection of the diaphragm increases the pressure in the liquid medium (oil).
This increase in pressure of the liquid medium is proportional to the applied force. The increase in pressure is measured by the pressure gauge which is connected to the liquid meduim.
The pressure is calibrated in force units and hence the indication in the pressure gauge becomes a measure of the force applied on the piston.

Note about Hydraulic Load cell:

As the hydraulic load cell is sensitive to pressure changes, the load cell should be adjusted to zero setting before using it to measure force.
This hydraulic load cell have an accuracy of the order of 0.1 percent of its scale and can measure loads upto upto 2.5*10^5 Kgf
The resolution is about 0.02 prcent.


Pneumatic Load Cell


Basic Principle of Pneumatic Load Cell

If a force is applied to one side of a diaphragm and an air pressure is applied to the other side, some particular value of pressure will be necessary to exactly balance the force. This pressure is proportional to the applied force.

Description of pneumatic Load cell


The main parts of a pneumatic load cell are as follows:

A corrugated diaphragm with its top surface attached with arrangements to apply force.
An air supply regulator, nozzle and a pressure gauge arranged as shown in figure.
A flapper arranged above the nozzle as shown in figure.

Operation of Pneumatic Load cell


pneumatic load cells
The force to be measured is applied to the top side of the diaphragm. Due to this force, the diaphragm deflects and causes the flapper to shut-off the nozzle opening.Now an air supply is provided at the bottom of the diaphragm. As the flapper closes the nozzle opening, a back pressure results underneath the diagram. This back presssure acts on the diaphragm producing an upward force. Air pressure is regulated until the diaphragm returns to the pre-loaded position which is indicated by air which comes out of the nozzle. At this stage, the corresponding pressure indicated by the pressure gauge becomes a measure of the appilied force when calibrated.

Note:


The pneumatic load cell can measure loads upto 2.5*10^3 Kgf.
The accuracy of this system is 0.5 percent of the full scale.

Read more...

Effects of Feedback

The effects of feedback in systems on other working parameters of the system has been analysed. The parameter considered for analysis are gain, sensitivity, distortion, impedance and bandwidth. A feedback system has been shown in the figure.

Effect of feedback on overall Gain:


From figure it is seen that the transfer function is given by the equation:

M= A/(1- bA)

Hence the feedback reduces the overall gain of the system by a fator of (1-bA).
The quantities A and B are function of frequency and can be adjusted to make the denominator greater than unity.
Hence the gain increases for a particular frequency range and decreases for another frequency range.

Effect of Feedback on Sensitivity:


Sensitivity is the extent to which the system responds to changes in parameters like gain,impedance,etc. Sensitivity is also said to be the ratio of the extent of change of one of the above mentioned parameter to a small change of the determining parameters.

For example, if

M= transfer function
K= Determining Parameter

Then the sensitivity (S) is given by:

S= Percentage change in M/ Percentage change in K

Following are the effect of feedback on Sensitivity.



Feedback may reduce sensitivity with respect to certain parameters.
Feedback doesnot affect variations of elements in the feedback path.
Feedback reduces the sensitivity of the system based on variation of parameter in the forward path of the loop. Larger the loop gain Ab, more effective is the feedback in reducing sensitivity.

Effect of Feedback on Distortion

effect of feedback on distortion

Feedback is used in communication systems to reduce noise and other distortion signals which it might pickup from extraneous sources.
The place of insertion of the extraneous noise to the signal flow is the main factor that determines the extent to which the feedback reduces the effects due to distortion.
Consider the figure which shows a signal flow graph of a system.

A noise signal is inserted at the point shown.

In the absence of the feedback, the output is given by

e0 = A1A2es + A2en
= e0s + e0n

where, e0s = single component of the output.
e0n=The component of the output due to noise.

Output signal to noise ration = output due to signal/output due to noise


Signal to noise ratio = A1A2es/A2en

= A1es/en

Hence to increase the signal to noise ratio, either A1 and /or es is to be increased or en is to be decreased.

If the system is aided by a feedback circuit, the output is given by:


e0 = (A1A2es/1-A1A2b) + (A2en/1-A1A2b)

From the above equation it is clear that the noise component of the output has its gain reduced by a factor 1-A1A2b. Thus the noise is reduced and the overall distortion of the output is reduced.

Effect of Feedback on Impedance

effect of feedback on impedance

In practice , the system is bound be connected to an external circuit. The working of the system depends on the input and output impedance. Consider the figure shown.


Za = (rpRl)/(rp+Rl+µKRl)

Here the amplifier gain is given by:


A = - µRl/(rp + Rl)

Thus the shunt impedance is reduced by a factor (1-AK). The series impedance is increased by a factor (1- AK).

Effect of Feedback on Bandwidth


In electronic frequency depent circuits, bandwidth is an important characteristic.
Bandwidth is the parameter that measure the ability of the system to reproduce its input signal with high quality and least noise.
It is to be noted that the bandwidth increases with feedback.

Read more...

Feedback Principle

As I said in the previous post Open loop control system dont give the required level of control as they depend on human judgement.

Example of feedback prnciple:


Home furnace control system must control the temperature in the room and kept it constant. As in open loop system a timer is used to switch on the furnace for some time and then switch it off, accuracy is not obtained. This is because the system doesot act according to the room temperature but according to a preset value of time.

example of closed loop control system
A closed loop control system takes care of this problem. The feedback unit (the main component of the closed loop control system) senses the room temperature and accordingly turns on or off the furnace. The feedback unit feeds the output back to a comparator which is provided with a reference value with which the output is compared to generate an error signal. This error signal generates the required control action. In the home furnace control system, a temperture sensor is used to sense the temperature in the room and this is feedback to an error detecting device. The error detecting device comparres the room temperature with the reference value. If it detects that the room temperature is higher than the reference value, if generates a signal to switch off the furnace. On the other hand, if it detects that the room temperature is lower than the reference value, it generates a signal to switch on the furnace. This has been shown in the figure.

Hence it is clear that a system with feedback (closed loop control system) is efficient than that of a system without feedback (open loop control system).

example of closed loop control system
Another examples of a closed loop control system is the working of a human brain. If a person wants to pick up an article say bag or book, the brain instructs the hand to reach the article and the eyes constantly keep giving the feedback to the brai regrading the posistion of the hand relative to the article. But if the person is asked to reach the article closing his eyes, he can reach it only approximately. Thus the human system is a very accurate feedback control system. The control system taking human as an example is shown in the figure.

While designing a closed loop system, a compromise between stability and accuracy is to be astablished. This is because the gain of the system may exceed over a limit which may cause the system to be over correct, leading the system to become unstable, that is, oscillation of the output without bound.

Note

Gain is the ratio between the amplitude of the output to the amplitude of input. The variation of the gain over a range of the frequencies is called frequency response. Moreover, due to mechanical problems like friction, the system might tend to have a steady state error.

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Types of control systems


There are two types of control systems namely:

  1. Open loop control systems (non-feedback control systems)
  2. Closed loop control systems (feedback control systems)

Open loop control system


If in a physical system there is no automatic correction of the variation in its output, it is called an open loop control system. That is, in this type of system, sensing of the actual output and comparing of this output (through feedback) with the desired input doesnot take place. The system on its own is not in a position to give the desired output and it cannot take into account the disturbances. In these systems, the changes in output can be corrected only by changing the input manually.
open loop control system




These systems are simple in construction, stable and cost cheap. But these systems are inaccurate and unreliable. Moreover these systems donot take account of external disurbances that affect the output and they donot initiate corrective actions automatically.

Examples of open loop control systems:

  1. Automatic washing machine
  2. traffic signal system
  3. home heating system( without sensing, feedback and control)

Any non-feedback control system can be considered as a feedback control system if it is under the supervisio of someone. Although open loop control systems have economical components and are simpe in design, they largley depend on human judgement. As an example, let us consider a home furnace control system. This system must control the temperature in a room, keeping it constant. An open loop system usually has a timer which instructs the system to switch on the furnace for some time and then switch it off. Accuracy cannot be achieved as the system doesnot swith on/off based on the room temperature but it does as per the preset value of time.

Closed loop control system


A closed loop control system is a system where the output has an effect upon the input quantity in such a manner as to maintain the desired output value.

closed loop control system
An open loop control system becomes a closed loop control system by including a feedback. This feedback will automatically correct the change in output due to disturbances. This is why a closed loop control system is called as an automatic control system. The block diagram of a closed loop control system is shown in figure.

In a closed loop control system, the controlled variable (output) of the system is sensed at every instant of time, feedback and compared with the desired input resulting in an error signal. This error signal directs the control elements in the system to do the necessary corrective action such that the output of the system is obtained as desired.

The feedback control system takes into account the disturbances also and makes the corrective action. These control systems are accurate, stable and less affected by noise. But these control systems are sophisticated and hence costly. They are also complicated to design for stability, give oscillatory response and feedback brings down the overall gain of the control system.

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Measurement of Strain

Strain gauges are devices used to measure the dimensional changes of components under test. Strain gauges are used in a number of applications, some of them have been listed below:


  1. Strain gauges are used in force measuring devices such as strain gauge load cell.
  2. Strain gauge are used in measurement of vibration / acceleration such as strain gauge accelerometer.
  3. Strain gauges along with diaphragm are used in the measurement of pressure.
Some important terms have been explained below:

Strain


Strain is the relative change in dimensions, that is, change in length of given original length.

Strain = change in length/original length = mm/mm (dimensionless).

Strain Gauges 

When a metallic conductor is stretched or compressed, its resistance changes due to a change in the length and diameter (cross section) of the conductor.Hence a strain gauge is a measurement transducer used to measure strain (that is, relative changes in dimension). It is a transducer because it converts information about relative change in dimension to a change in resistance.

Positive Strain


When a strain gauge (metallic conductor) is subjected to tension, it is said to be positively strained. That is, when the strain gauge is subjected to positive strain (tension), its length increases and its area of cross section decreases. As the resistance of a conductor is proportional to its length and inversely proportional to its area of cross section, the resistance of the strain gauge increases with the positive strain.

Negative Strain.


When a strain gauge (metallic Conductor) is subjected to compression , it is said to negatively strained. that is, when the strain gauge is subjected to negative strain (compression), its length decreases and its area of cross section increases. As the resistance of the conductor is proportional to its length and inversely proportional to its area of cross section, the resistance of the strain gauge decreases with negative strain.

Piezoresistivity


There will be a change in resistivity of a conductor when it is strained and this property is called as piezoresistivity.

Poisson's Ratio


Poisson's Ratio = lateral strain, that is, the relative change in dimension in the cross section / Longitudinal strain, that is the relative change in dimension in the length.

                        = (dD/D)/(dL/L)
where, D=Diameter:    L=Length

Gauge Factor (Strain Sensitivity Factor)

The fractional change in resistance due to unit change in length (unit strain) is called as gauge factor.

Gauge Factor, F = (dR/R)/(dL/L)

Where, R = Resistance, L=Length

The magnitude of the strain gauge factor indicates the sensitivity of the strain gauge. the high gauge factor implies that there will be a large change in resistance for a given strain input.

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Requirements of a Control System

Stability, accuracy and speed of response are the three requirement s of a control system.

Stability:
A system is to be stable if the output of the system after fluctions, variation or oscillation, if any, settles at a reasonable value for any change in input or change in disturbance.

Accuracy:
A system is said to be 100 percent accurate if the error ( different between input and output ) is zero. An accurate system is costly. There is no point in going for 100 percent accurate system when that much of accuracy is not really required.

Example of accuracy:
When a variation of say 0.2 degree centigrade cannot be sensed by a human being, there is no need to have a home heating system of temperature variation equal to zero.

Speed of Response:
This refers to time taken by the system to respond to the given input and give that as the output. Theoritically the speed of response should be infinity, that is, the system should have an instantaneous response. This requirement is prime concern with follow-up systems.

Any ideal system is perfectly stable, 100 percent accurate and has instantaneous speed of response. Unfortunately, the requirements are incompatiable. Hence there should be a compromise between these requirements.

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Basic control system definitions

controlled variable:

the quality or condition characterizing a process whose value is held constant by a controller or is changed according to a certain algorithm designed with the interests of the nature of the function the system is performing.

Controlled medium:

the process material in the control system in which the variable to be controlled exists.

Command
An input that is established or varied by some means external to and independent of the feedback system.

Set point or Reference input:

A signal established as a standard of comparison for a feedback control system by virtue of it's relation to command. The setpoint either remains the same or is varied with respect to time depending on a preset algorithm.

Actuating signal:

An algebraic sum of reference input and primary feedback. It is also called error or control signal.

Manipulated variable:

The quality or condition that is varied as a function of the actuating signal so as to change the value of the controlled variable.

Primary feedback signal:

The function of thecontrolled output which is compared to reference input to obtain the actuating signal.

Error detector:

An element that detects feedback; essentially a summing point which gives the algebraic sum of it's inputs.

Disturbance:

An unwanted variable in the system which tends to affect the system adversely by changing the controlled variable. Disturbance may due to change in set point, supply, demand, environment and the other associated variables.

Feedback element:

An element of the feedback control system which establishes a functional relationship between controlled variable and feedback signal.

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Automatic Control Systems

With the development of technology, man has learnt to use very reliable and accurate systems that require least manpower. Automatic control systems are one such development. Control is defined as the science of regulation of a parameter by comparing it with a standard value. This is the aim of an automatic control system. They constantly monitor the output of system and if this output is found to deviate from the desired value, it produces a control signal that server to bring down the change by providing a driving energy to the component that is responsible for the deviation.
typical control system
A system is said to be made of a number of components such that the behaviour of the overall combination can be predicted if

1. the behaviour of the each component can be predicted, and
2. the interaction between each component is known.

Hence, a system is obtained when a number of components are connected in a sequence to perform a specific function. Suppose in a system, the output quantity is controlled by altering the input quantity, then such a system is called control system.

The output quantity is called the controlled variable and the input quantity is called the input signal. Automatic control system have become an integral part of modern manufacturing and industrial processes.

Examples:

manufacturing industry : numerical control of machine tool.
Industrial process : control of pressure, temperature and flow.

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Laws of thermoelectricity

Actual applications of the thermocouple to measure require the consideration of following laws of thermo electricity.

First law of homogeneous circuit


An electric current cannot be sustained in a circuit of single homogenous by application of heat done. This law is generally accepted to an experimental. In thermocouple, an emf is formed by joining two dissimilar wires/metals.
law of homogeneous circuit

Second law of intermediate metals


It states that intersection of third metal into a thermocouple circuit will have no effects, as long as a junction by the third metal with thermocouple at the same temperature.

Applications


This law makes it possible to use extension wires at metals different from the thermocouple because platinum extension wires are at the same temperature high cost, copper can be used without any change in performance.

The law enables an instrument to be introduced into the circuit to be means the emf produced.

This law allows the use of joining material such or hard solder(silver) in fabricating the thermocouple and junction.

Law of intermediate thermocouple


The emf generated in a thermocouple with junction at temperature T1 and T3 is equal to sum of emf generated by similar thermocouple one acting between T2 + T3 and where T2 between T1+T3

This law is used when making or reference junction temperature is different from temperature at which it is calibrated. Thus a thermocouple is calibrated with reference junction at 0’C is used and with the junction at 20’C.

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Download Instrumentation and control Ebooks

instrumentation ebooks

Hello guys, i have small set of books related to Instrumentation and control engineering that you can download now and if you have any problem use the comments link to post comments to me..

All the ebooks that i am going to give is in PDF format to have a adobe reader installed in your computer.



Instrumentation and Control Fundamentals Handbook
Instrumentation and Control Volume 1 of 2
Instrumentation and Control Volume 2 of 2
Instrumentation and Control Tutorial 1
Instrumentation and Control Tutorial 2
Instrumentation and Control Tutorial 3
Instrumentation and Control Tutorial 4

Level and Pressure Instruments
Level Measurement In Liquids - Ultrasonic
Liquid Level Controllers

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Strain Gauge Pressure Cell:

Basic Principle:


When a closed container is subjected to the appilied pressure, it is strained (that is, its dimension changes). Measurement of this strain with a secondary transducer like a strain gauge ( metallic conductor) becomes a measure of the appilied pressure.

That is, if strain gauges are attached to the container subjected to the applied pressure, the strain guages also will change in dimension depending on the expansion or contraction of the container. The change in dimension of the strain guage will make its resistance to change. This change in resistance of the strain gauge becomes a measure of pressure appilied to the container (elastic container or cell).

There are two types of strain gauge pressure cells namely:

  1. flattened tube pressure cell.
  2. Cylindrical type pressure cell.

Flattened tube pressure cell.


The main parts of the arrangement are as follows:

An elastic tube which is flat and pinched at its two end as shown in diagram.
Two strain gauges are placed on this elastic tube: one is on the top and other is at the bottom of this elastic tube.
One end of the elastic tube is open to receive the appilied pressure and its other end is closed.

Operation:


The pressure to be measured is appilied to the open of the tube. Due to pressure, the tube tends to round off, that is, the dimension changes (strained). As the strain gauge are mounted on the tube, the dimension of the strain gauges also change proportional to the change in dimension of the tube, causing a resistance change of the strain gauges. The change in dimension of the tube is proportional to the applied pressure. Hence the measurement of the resistance change of the strain gauges becomes a measure of the appilied pressure when calibrated.

Cylindrical Type pressure cells:


Description


The main parts of this arrangement ar as follows:

A cylindrical tube with hexagonal step at its centre. This hexagonal step helps fixing this device on to place where the pressure is to be measured.

The bottom portion of this cylindrical tube is thearded at its external and is open to receive the pressure to be measured.

The top portion of this cylindrical tube is closed and has a cap screwed to it.

On the periphery of the top portion of the cylindrical tube are placed two sensing resistance strain gauges.

On the cap (unstrained location) are placed two temperature compensating strain gauges.

Operation.

The pressure to be measured is appiled to the open end of the cylindrical tube. Due to the pressure, the cylindrical tube is strained, that is its dimension changes. As the strain gauges are mounted on the cylindrical tube, the dimension of the sensing strain gauges also change proportional to the change in dimension of the cylindrical tube, causing a resistance changes of the strain gauges.

The change in dimension of the cylindrical tube is proportional to appilied pressure.

Hence the measurement of the resistance change of the sensing strain gauges becomes a measure of the appilied pressure when calibrated.

Appilications of the strain gauge pressure cells


the flattened tube pressure cell is used for low pressure measurement.
The cylindrical type pressure cell is used for medium and high pressure measurement.

Read more...

Microprocessor based data logging, processing and output – A mini Project

This post deals with using a microprocessor in instrumentation for collecting/acquiring data, processing it and displaying the output using the suitable display such as digits on an led or a computer screen. The basic elements of the data logger has been shown in the figure.


A typical data logger can handle 20 to 100 inputs. (some are even capable of handling around 1000 inputs). Such a unit is used to monitor the inputs from a large number of sensors or used to give outputs to number of display units or actuators. With the help of the signal conditioners, the output signals from the sensors are processed to make it suitable for measuring the input.
microprocessor based mini project


Mini Project based on Microprocessor


Lets assume the project is to data log the temperature of a liquid in a tank in a chemical industry and a thermocouple is placed inside the tank and the reading are to be seen in a display and recorded.

The output from the thermocouple is a small voltage is a small voltage in millivolts. Signal conditioning is done to convert this small voltage into suitable size current signal with noise rejection, linearisation and cold junction compensation for not being at 0'C.

The input and output devices are connected to a microprocessor system through ports. Inputs can be from sensors, switches, keyboards, etc.. and the output can be to displays, actuators, etc.,

Microprocessors require inputs that are digital. Hence, if the output from the sensor is analogue to digital converter ADC needed. Further, if the signal generated by he sensor is very small, amplification of the signal is first done before it is fed to an ADC. Even for digital signals, signal conditioning may be required to improve thier quality.

After suitable signal conditioning, the signal from the individual sensors are fed to a multiplexer. A multiplexer is circuit that can take-up inputs from a number of sources and then by selecting an input channel, give an output from just on of them.

In some appilications, there might be a need for measurements to be made at a number of different locations. In such a situation, instead of using a separate ADC and microprocessor for each measurement, a multiplexer is used to select each input one after the other, and switch it through a single ADC and microprocessor.

The output of the ADC is a digital signal which is processed using a microprocessor. The output from the microprocessor is displayed on a digital meter that indicates output and channel number or a printout record from a printer or the output of the microprocesor can be stored on a floppy disk or transferred to a computer for analysis but new need to program the microprocessor for every different output to needed.

A data logger can make around 1000 reading per second with an accuracy of 0.01% of full scale.

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Elastic diaphragm gauges

Basic principle of Elastic diaphragm gauges:


when an elastic transducer (diaphragm is this case) is subjected to a pressure, it deflects. This deflection is proportional to the applied pressure when calibrated.

Description of Elastic diaphragm gauges:


The main parts of the diaphragm which is a thin circular plate (made of springy metal) is fixed firmly around its edges. The diaphragm may either be flat or corrugated.

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Bourdon tube Pressure Gauge


Basic Principle of Bourdon tube pressure gauge:


when an elastic transducer ( bourdon tube in this case ) is subjected to a pressure, it defects. This deflection is proportional to the applied pressure when calibrated.

Description of Bourdon tube Pressure Gauge:


The main parts of this instruments are as follows:

An elastic transducer, that is bourdon tube which is fixed and open at one end to receive the pressure which is to be measured. The other end of the bourdon tube is free and closed.

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U-tube Manometer with equal unequal limb types

Description of manometer:


This is the most simple and precise device used for the measurement od pressure. It consists of a transparent tube constructed in the form of an elongated 'U', and partially filled with the manometeric fliud such as mercury. The purpose of using mercury as the manometeric fluid is that their specific gravity at various temperatures are known exactly and they dont stick to the tube. The two common types of manometers are the equal limb type and unequal limb type.

Operation of Manometer :


to measure the pressure of a fluid which is less dense and immisible with the manometeric fluid, it is appilied to the top of one of the limbs of the manometer while a reference fluid pressure (generally atmospheric ) is appilied to the other limb.

Let : P1 = unknown pressure with specific weight w1.
And : P2 = reference pressure with specific weight w2.
And : wm = specfic weight of the manometeric fluid.

The difference between the pressure on the two limbs of the manometer is a function of 'h', the difference between the levels of the manometeric fluid in the two limbs. 'h', can be read directly by placing a scale near the manometer.

For Equal Limb type:

equal limb type manometer



the pressure balance equation is

P1 + gh * w1 = P2 + gh * wm

therefore, differential pressure

P = P1 – p2 = gh(wm – w1)

 

For unequal Limb type


unequal limb type manometer
the pressure balance equation is : (section-XX)

P1+w1(h1+h2+h) = P2 + w2h2 + wmh

Therefore, differential pressure:

P = P1 – P2 = w2h2 + wm * h – w1(h1 + h2 + h)

= wm*h [ 1 + (w2/wm * h2/h) – w1/wm ( h1+h2/h + 1) ]

= wmh * Cn

where; Cn = [ 1 + ( w2/wm – h2/h) – w1/wm ( h1+h2/h + 1)]

Cn is known as hydraulic correction factor.

Applications of Manometer :


  • they are used to sense differential pressure in venturimeter and other flow meters.
  • They are used as level devices to sense liquid heads.
  • There are used as primary standard for pressure measurements.

Advantages of Manometer :


  • they are simple is construction.
  • They give accurate results.
  • Wide range of manometeric fluids ar available such as mercury, water, aniline, tetrabromethane, bromoform and carbon tetrachloride.

Disadvantages of Manometer :


  • They might break during transport.
  • Certain manometeric fluids cause hazards when exposed to atmosphere.
  • Error is introduced if the diameter of the tube is less.
  • Leveling is required.

Read more...

Dead Weight Tester

Description


The dead weight tester apparatus consists of a chamber which is filled with oil free impurities and a piston – cylinder combination is fitted above the chamber as shown in diagram. The top portion of the piston is attached with a platform to carry weights. A plunger with a handle has been provided to vary the pressure of oil in the chamber. The pressure gauge to be tested is fitted at an appropriate plate.

Operation


the dead weight tester is basically a pressure producing and pressure measuring device. It is used to calibrate pressure gauges. The following procedure is adopted for calibrating pressure gauges. Calibration of pressure gauge means introducing an accurately known sample of pressure to the gauge under test and then observing the response of the gauge. In order to create this accurately known pressure, the following steps are followed.

The valve of the apparatus is closed.
A known weight is placed on the platform.
Now by operating the plunger, fluid pressure is applied to the other side of the piston until enough force is developed to lift the piston-weight combination. When this happens, the piston weight combination floats freely within the cylinder between limit stops.

In this condition of equilibrium, the pressure force of fluid is balanced against the gravitational force of the weights puls the friction drag.

Therefore, PA = Mg + F

Hence : P = Mg + F / A

where, P = pressure
M = Mass; Kg
g = Acceleratoion due to gravity ; m/s²
F = Friction drag; N
A = Eqivalent area of piston – cylinder combination; m²

Thus the pressure P which is caused due to the weights placed on the platform is calculated.

After calculating P , the plunger is released.

Now the pressure gauge to be calibrated is fitted at an appropriate place on the dead weight tester. The same known weight which was used to calucated P is placed on the platform. Due to the weight, the piston moves downwards and exerts a pressure P on the fluid. Now the valve in the apparatus is opened so that the fluid pressure P is transmitted to the gauge, which makes the gauge indicate a pressure value. This pressure value shown by the gauge should be equal to the known input pressure P. If the gauge indicates some other value other than p the gauge is adjusted so that it reads a value equal to p. Thus the gauge is calibrated.

Applications:


It is used to calibrated all kinds of pressure gauges such as industrial pressure gauges, engine indicators and piezoelectric transducers.

Advantages:


it is simple in construction and easy to use.
It can be used to calibrated a wide range of pressure measuring devices.
Fluid pressure can be easily varied by adding weights or by changing the piston cylinder combination.

Limitations:


the accuracy of the dead weight tester is affected due to the friction between the piston and cylinder, and due to the uncertainty of the value of gravitational constant 'g'.

Read more...

Pressure Measurement Devices

Gravitational transducers

  1. A dead weight tester
  2. Manometer

Elastic transducers ( Force summing Transducers)

  1. Bourdon tube pressure gauge
  2. elastic diaphragm gauges
  3. bellow gagues
  4. Bellow gaugues to measure gauge pressure
  5. Bellow gauge to measure differential pressure

Strain gauge pressure cells.

  1. Flattened tube pressure cell (PINCHED TUBE).
  2. Cylindrical type pressure cell.

Mcleod vacuum gague.

  1. Thermal conductivity gauges.

Pirani gauge.
  1. Thermocouple typre conductity gauge.

Ionisation gauge
  1. bulk modulus or electrical resistance pressure gauge.

The above instruments are used in following situations:


Type of pressure to be measured
Pressure Measuring instrument to be used
Low pressure
Manometer
High and medium pressure
Bourdon tube pressure gauge.
Diaphragm gauge.
Bellows Gauges.
Low vacuum and ultra high vacuum
Mcleod vacuum gauge
thermal conductivity gauges.
Ionisation gauges.
Very high presures
Bourdon tube pressure gauge.
Diphragm gauge.
Bulk modulus pressure gauge.

Read more...

Terms related to pressure

Atmospheric pressure:


The pressure due to air surrounding the earths surface is called as atmosperic pressure.

Absoulte pressure:

It is known that pressure is force per unit area when the interaction of fluid particles among themselves is zero, a zero pressure intensity will occur. This is possible only when the population of molecules is negligibly small which is nothing but perfect vacuum. Hence the pressure intensity measured from a state of prefect vacuum is called as absoulte pressure.

Gauge Pressure:


A pressure measuring instrument generally measures the difference between the uknown pressure (p) and the atmospheric pressure (pa). When the unknown pressure (P) is greater than the atmospheric pressure (Pa), the pressure measured by the instrument is called as the gauge pressure.

Vacuum pressure:


A Pressure measuring instrument generally measures the difference between the unknown pressure (P) and the atmospheric pressure (Pa). When the atmospheric pressure (Pa) is greater than the unknown pressure(P), the pressure mesured by the instrument is called as the vacuum pressure.

Static Pressure:


the pressure caused on the walls of the pipe due to a fluid at rest inside the pipe or due to the flow of a fluid parallel to the walls of the pipe is called as static pressure. This static pressure is measured by inserting a pressure measuring tube into the pipe carrying the fluid, so that the tube is at right angle to the fluid flow path.
Pressure relation


Total or Stagnation pressure:


the pressure which is obtained by bringing the flowing fluid to rest isentropically is called as total or stagnation pressure. Hence the pressure will be a sum of static pressure and impact pressure.

Dynamic – or – Impact – or – Velocity pressure.


The pressure due to fluid velocity (flow speed) is called as impact pressure.

Impact pressure = Total pressure – static pressure.

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Introduction to pressure measurement

It is time to know about pressure measurement..

    In general, pressure is represented as force per unit area. The measurement of pressure is one of the most important measurements, as it is used in almost all industries. Some important appilications of pressure measurement is listed.

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Bimetallic Thermometer

Basic Principle:


These thermometers use the following two principles:

1. All metals change in dimension, that is expand or contract when there is a change in temperature.
2. The rate at which this expansion or contraction takes place depend on the temperature co-efficient of expansion of the metal and this temperature coefficient of expansion is different for different metals.Hence the difference in thermal expansion rates is used to produce deflections which is proportional to temperature changes.

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Optical pyrometer( Disappearing filament Type)

Monochromatic brithness radiation thermometer

Basic Principle of optical pyrometer:


The principle of temperature measurement by brightness comparision is used in optical pyrometer. A colour variation with the growth in temperature is taken as an index of temperature.

This optical pyrometer compares the brightness of image produced by temperature source with that of reference temperature lamp. The current in the lamp is adjusted until the brightness of the lamp is equal to the brightness of the image produced by the temperature source. Since the intensity of light of any wave length depends on the temperature of the radiating object, the current passing through the lamp becomes a measure of the temperature of the temperature source when calibrated.

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