RTD and Thermister.


Back to Basic: RTD and ThermistorsRTD: Resistance temperature Detectors
resistance thermometers
the resistance of conductors change when its temperature is changed. the property is utilized for measurement of temperatures.the variation of resistance R with temperature T can be represented by the following relationship as R= R0(1 + a1T + a2T2 + ...)

the resistivity of metals showed a marked dependence on temperature which was dicovered by Sir Humphry Davy. A few years later Sir Williams Siemens preferred the use of platinum as an element in the resistance thermomoeter. his choice proved most propitious, as platinum is the primary element in all high accuracy thermometer. platinum is suitable for this purpose as it can withstand high temperatures while maintaining high stability. as a noble metal, it shows limited suscepatbility to contamination. all metals produce a positive change in resistence with temperature whihc is the main function of RTD.

the requirements for a conductor to be used as RTD are:
1. the change of resistance of metal per unit change in temperature should be as large as possible.
2. the material should have a high value of resistivity so that the minimum volume of material should be used in the construction.
3. the resistance of materials should have a continous and stable relationship with temperature.

Thermistors:
thermistor is a term " thermal resistors". thermistors are generally composed of semi-conductor materials. although positive temperature co-efficients of units are available, most thermistors have a negative coefficient of temperature resistance. the negative temperature coefficient of resistance can be as large as several percent per degree celcius. this allows the thermistor circuits to detect very small changes in temperatures which could not be observed in RTDs.

thermistors are widely used in applications which involve measuremnts in the range of -60 deg c  to 15 deg c. the resistance of thermistors range from 0.5 ohms to 0.75M ohms. it is a highly sensitive device.the rpice to be paid off for the high sensitivity is in terms of linearity. it exhibits a non linear curve of resistance versus temperature.

Applications:
1.measurement of temperature.
2.control of temperature.
3.temperature compensation
4.measurement of power at high frequencies
5.measurement of thermal conductivity
6.measurement of compositon of gases
7.vacuum measurements
8.providing time delays.


Thermocouples


A Thermocouple is a sensor used to measure temperature. Thermocouples consist of two wire legs made from different metals. The wires legs are welded together at one end, creating a junction. This junction is where the temperature is measured. When the junction experiences a change in temperature, a voltage is created. The voltage can then be interpreted using thermocouple reference tables to calculate the temperature.

There are many types of thermocouples, each with its own unique characteristics in terms of temperature range, durability, vibration resistance, chemical resistance, and application compatibility. Type J, K, T, & E are “Base Metal” thermocouples, the most common types of thermocouples.Type R, S, and B thermocouples are “Noble Metal” thermocouples, which are used in high temperature applications (see thermocouple temperature ranges for details).

Thermocouples are used in many industrial, scientific, and OEM applications. They can be found in nearly all industrial markets: Power Generation, Oil/Gas, Pharmaceutical, BioTech, Cement, Paper & Pulp, etc. Thermocouples are also used in everyday appliances like stoves, furnaces, and toasters.

Thermocouples are typically selected because of their low cost, high temperature limits, wide temperature ranges, and durable nature.

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