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.
This thermometer is an instrument used to measure the temperature variation in control room.
Working principleIn 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”.
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.
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 pyrometerIn this, total radiant energy from a heated body is measured.
Selective radiation pyrometerIn this, the radiated energy from the heated body is measured at a given wavelength.
Check out some Error analysis related posts
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.
- Low cost
- Sensitivity is high
- Small in size.
- Good stability.
- High output signal.
- Non-linear output is seen.
- Not suitable for high temperature measurement.
- Requires external power supply.
- Used for measurement and control of temperature.
- Used for providing time delay.
- Used as temperature compensation element in electronic equipments.
- Used for measuring thermal conductivity of a medium.
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.
Washer type Thermistor:
Temperature measurement using Thermistor will be the next post.
- Positive Temperature Coefficient Thermistor. (PTC)
- Negative Temperature Coefficient Thermistor.(NTC)
In this type, resistance of resistance of the Thermistor increases with the increase in temperature. These Thermistors are usually made from Barium, Titanate.
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
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.
Read about thermocouple with circuit for more information
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|
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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