Load cell - A Simple Device to Measure Weight


Load Cell

A strain gauge – elastic member combination arrangement used for the measurement of load or weight is called load cell. It utilizes an elastic member as primary transducer and strain gauge as a secondary transducer. A suitable plane area is placed on the eleastic member as a weight stand. Then, strain gauge is attached to the eleastic member.

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Vibration Measurement using Accelerometer.

            Measurement of vibrations of equipment is very important because the equipment should with stand the stated level of vibration which happens during any process but in every industry. Vibration monitoring is carried out on important machines such as power stations turbines and generators, to give o early warning of defective parts. If the vibration is beyond certain level which the equipment cannot withstand, then it very lead to equipment failure.

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Microprocessor based Electronic balance – A Mini Project.

      Microprocessor based electronic weighing machine is used for measuring and indicating weight on digital display.

            The item to be weighted is placed on a platform. The weight is determined by the machine and displayed on 7-segment Led display. Figure shows the schematic block diagram of microprocessor based electronic balance. It consists of load cell, amplifier, analog to digital converter and 7-segment LED display.

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Metal resistance thermometer.

This thermometer is an instrument used to measure the temperature variation in control room.

Working principle

           In each metallic conductor, their resistance changes when its temperature is changed. By calculating the variation in resistance, the temperature variations may be calculated. The thermometer which utilizes this phenomenon is called “resistance thermometer”.

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Total radiation pyrometer

            The total radiation pyrometer receives all the radiation from a particular are of hot body. The term total radiation includes both the visible and invisible radiations. It consists of radiation receiving element and a measuring device.

            The mirror type radiation pyrometer is shown in figure below. Here, the diaphragm unit along with a mirror is used to focus the radiation on a thermocouple. The distance between the mirror and the thermocouple is adjusted for proper focus.

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Radiation pyrometer:

             When temperature to be measured is high and physical contact with the heat body is impossible, radiation pyrometer are used. Radiation pyrometers are used under conditions where corrosive vapours or liquid would destroy thermocouples, resistance thermometer and Thermistor. These pyrometers also find applications where the temperatures are above the range of thermocouple. Basically, thermal radiations are electromagnetic radiation lies in the wavelength region from about 0.1 to 100 micrometer.

A typical Radiation pyrometer:

             The radiation pyrometer measures the heat emitted by a hot object. The radiation pyrometers operate on the principle that the energy radiated from a hot body is a function of its temperature.

             The energy radiated by the hot body whose temperature is measured is focused by the lens to the detector. The detector is a thermocouple or bolometer (bolometer is a thermal device that changes electrical resistance with temperature change). The detector output is given to a PMMC instrument, digital display or recorder.

Principle used for radiation pyrometer:

          There are two principle used for the consrution of radiation temperature measuring devices,

Total radiation pyrometer

         In this, total radiant energy from a heated body is measured.

Selective radiation pyrometer

                In this, the radiated energy from the heated body is measured at a given wavelength.

Check out some Error analysis related posts

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Temperature measurement using Thermistor:

In the before blog post I written about Thermistor and its types with the construction diagrams, now in this post let see how temperature measurement is made using Thermistor.

Initially, Thermistor is placed in the environment whose temperature is to be measured. Then, Thermistor is connected in a series simple circuit consisting of battery and micro-ammeter as shown below. Any change in temperature causes a change in resistance of Thermistor. Hence, corresponding change in circuit current. By directly calibrating micro ammeter interms of temperature, we can measure temperature.

 Temperature measurement using Thermistor

Advantages:


  1. Low cost
  2. Sensitivity is high
  3. Small in size.
  4. Good stability.
  5. High output signal.

Disadvantages

  1. Non-linear output is seen.
  2. Not suitable for high temperature measurement.
  3. Requires external power supply.

Appilications

  1. Used for measurement and control of temperature.
  2. Used for providing time delay.
  3. Used as temperature compensation element in electronic equipments.
  4. Used for measuring thermal conductivity of a medium.

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Construction and types of Thermistor

              Thermistor are composed of sintered mixture of metallic oxides such as manganese, nickel cobalt, copper, iron and uranium. They are available in variety of sizes and shapes. Thermistors may be in the form of beads, probes, rods and discs. Some of the commercial form are shown in figures.

Bead thermistors

bead thermistor
                Smallest Thermistors are in the form of heads with a diameter of 0.15mm to 1.25mm. This is the most familiar type of Thermistor usually glass coated.







Probe Thermistor

probe thermistor
Beads may be sealed in the tips of soild glass rods to form probes. Glass probe have a diameter of about 2.5mm. the probes are used for measuring temperature of liquids.




Disc Thermistor

disc thermistor
Discs are made by pressing material under high pressure into cylindrical flat shapes with a diameters ranging from 2.5mm to 25mm. they are mainly used for temperature control.



Washer type Thermistor:

washer type thermistor
Washer type is usually long cylindrical units. Leads are attached to the ends of the rods. The advantage of this type is, it produce high resistance under moderate power.

Temperature measurement using Thermistor will be the next post.

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Thermistor and Types of Thermistor

         Thermistor or thermal resistor are thermal sensitive resistor. It means its resistance changes with temperature. This change in resistance character divides the Thermistor into two types, depending in the way it responds to the temperature.

  1. Positive Temperature Coefficient Thermistor. (PTC)
  2. Negative Temperature Coefficient Thermistor.(NTC)
PTC Thermistor

           In this type, resistance of resistance of the Thermistor increases with the increase in temperature. These Thermistors are usually made from Barium, Titanate.

NTC Thermistor

Thermistor characteristics 

                In this type, resistance of Thermistor decreases with increase in temperature. Thermistor are semiconductor component that behave as a resistor with usually, negative temperature coefficient of resistance. In some cases, resistance of a Thermistor at room temperature may decrease to 5 percent for each one degree Celsius rise in temperature.

Construction of Thermistor coming soon at next post.


Read some related posts:


Thermocouple with Circuit
The Thermocouple is explained with
circuit Diagram in this post

Temperature Measurement using thermocouple:
How Temperature is measured using the thermocouple
and what are the basic things

Construction of thermistor and types of Thermistor
construction and types of thermistor with neat
examples and circuit diagram

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Temperature Measurement using thermocouple:

Temperature Measurement using thermocouple is very simple procedure and easy to understand.

Circuit of temperature measurement using thermocouple:


Simple temperature measurement using thermocouple is shown in the figure above. Hot junction is placed near the heat source, whose temperature is to be measured. A sensitive mill voltmeter is directly connected across the reference junction. Now the deflection of the meter is directly proportional to the difference in temperature between the hot junction and reference junction.

Temperature measurement using thermocouple

Advantages of thermocouple:


Cheaper than RTD.
Rugged in construction.
Used for wide temperature range.
No external power is required.

Disadvantages of thermocouple:


Non-linear output is produced.
Low output signal.
Instability.

Read about thermocouple with circuit for more information

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Explanation of Thermocouple with Circuit

             A thermocouple consists of two dissimilar metal wires. They are joined together at one end. This junction is called sensing or hot junction. The two wires are terminated at the other end. This junction is called reference or cold junction. The cold junction is maintained at a known constant temperature called as reference temperature.

                The scientist Thomson see back discovered that, when a temperature difference exists between the sensing and reference function, an emf is produced. This emf causes a current in the circuit. When a meter or recorder is connected to the reference junction, the meter indication will be proportional to the temperature difference between the hot junction and reference junction. This thermo-electric effect caused by contact potential at the junction is known as the “See back effect”.
basic thermocouple circuit
Basic Thermocouple circuit

               The magnitude of the emf depends on the wire material used and also on the temperature difference between the junctions. The figure shows the thermal emf of some common thermocouple material.

                 For long life, thermocouple is placed in a protecting tube. To prevent chemical reactions the protective tube is both chemical inert and vacuum tight. Connection from thermocouple is made by special extension wires called compensating wires. The measurement is very accurate when compensating wires are of the same material as the thermocouple materials.

In the next blog I explain about Temperature measurement using thermocouple.
Before that read related posts:
Pressure Measurement using U-tube Manometer
Transducer: Part 2 - Classification of transducers...

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PAControl.com - Instrumentation and control Site review.

 If you are interested in instrumentation and control engineering and if you want online training, you should need to know about this site PAControl.com. You can get all sort of information about instrumentation and control engineering and process control in this site. Even though it not a blog, it is updated regularly with really good looking images and well designed layout.
The topics covered are :

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Download tons of PDFs related to the Instrumentation and process control for free in this site PAControl.com

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