Temperature Transducer

Temperature Transducer:

Temperature Transducer is one of the most widely measured and controlled variable in industry, as a lot of products during manufacturing requires controlled temperature at various stages of processing.

A wide variety of Temperature Transducer and temperature measurement systems have been developed for different applications requirements.

Most of the Temperature Transducer are of Resistance Temperature Detectors (RTD), Thermistors and Thermocouples. Of these RTD’s and Thermistor are passive devices whose resistance changes with temperature hence need an electrical supply to give a voltage output. On the other hand thermocouples are active transducers and are based on the principle of generation of thermoelectricity, when two dissimilar metals are connected together to form a junction called the sensing junction, an emf is generated proportional to the temperature of the junction. Thermocouple operate on the principle of seeback effect. Thermocouple introduces errors and can be overcomed by using a reference junction compensation called as a cold junction compensation.

Thermocouples are available that span cryogenic to 2000°C temperature range. They have the highest speed of response. Thermocouples can be connected in series/parallel to obtain greater sensitivity called a Thermopile.

RTD commonly use platinum, Nickel or any resistance wire whose resistance varies with temperature and has a high intrinsic accuracy. Platinum is the most widely used RTD because of its high stability and large operating range. RTD’ s are usually connected in a Wheatstones bridge circuit. The lead wire used for connecting the RTD’ s introduces error, hence compensation is required. This is obtained by using three-wire or four wire compensation, but 3-wire compensation is mostly used in the industry.

Another form of temperature measurement is by the use of thermistor. A thermistor is a thermally sensitive resistor that exhibits change in electrical resistance with change in temperature. Thermistors made up of oxides exhibit a negative temperature coefficient (NTC), that is, their resistance decreases with increase in temperature. Thermistor are also available with positive temperature coefficient (PTC), but PTC thermistor are seldom used for measurement since they have poor sensitivity.

Thermistors are available in various sizes and shapes such as beads, rods, discs, washers and in the form of probes.

Radiation pyrometer are used where non-contact temperature is required to be measured. It measures the radiant (energy) heat emitted or reflected by a hot object. Radiation pyrometers are of two types total radiation pyrometer and infrared pyrometer.

Total radiation pyrometer virtually receives all the radiation from a heated body and measures temperature in the range around 1200°C-3500°C. Infrared pyrometers are partial or selective radiation pyrometers and are used in the range of 1000°C-1200°C.

Optical pyrometers are used in the visible wavelength. The most common type of optical pyrometer is the disappearing filament type and is used in temperature range of 1400°C and can be extended up to 3000°C. Optical pyrometers are widely used for accurate measurement of temperatures of furnaces, molten metals etc.

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