Thermocouples

[speedtronic]

Thermocouples generate voltage in response to temperature, but in control rooms we see temp only. How this measurement is done?

 

[Phil Mumford]

Not sure if I understand the question, but usually there is an electronic interface or controller that looks at the thermocouple/sensor and converts it into a display we can understand. Fairly basic electronics, I expect.

 

[Darkwood]

Thermocouples are usually made using 2 different cores made from different metals each with a specific resistance value, the end is usually terminated with a protective housing suitable for various moughting applications or enviroments but all its housing is the joint between core 1 and 2.
A reference voltage or current is monitored and as the temp' drops or rises so does the resistance of this joint and its this what is converted to a readout on a display via basic mathematics, because the resistance of the coupler cable is known and use to measure the temp its vital that any extension to the coupler is done with thermocouple connectors and thermocouple wire or dont expect a accurate readout.

 

[Notsosmart]

Various methods ;

For two wire J,K,T,R.S etc Thermocouples Utilising compensating cable. ~

A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference. They are reasonably inexpensive and interchangeable, are supplied fitted with standard connectors, and can measure a wide range of temperatures. The main limitation is accuracy: system errors of less than one Kelvin(K) can be difficult to achieve.
Any junction of dissimilar metals will produce an electric potential related to temperature. Thermocouples for practical measurement of temperature are junctions of specific alloys which have a predictable and repeatable relationship between temperature and voltage. Different alloys are used for different temperature ranges. The correct type of Compensating cable matched to the type of thermocouple is required to connect the device to the outside world.

Generally in simple terms the compensating cable is connected to a converter (either in the Thermocouple head or within a remote panel). The converter puts out 4-20mA (in most cases) which when connected to an instrument (or Analogue input etc) has a 250 Ohm resistor across it which converts the signal to 1-5 volts. (0.004 x 250 = 1, 0.020 x 250 = 5) Cold junction monitoring is required to enable the device to measure correctly.

Three wire RTD's (Resistive thermic device) Monitors the resistance across the device again these are connected across converters to output 4-20mA

RTDs are manufactured from metals whose resistance increases with temperature.
Within a limited temperature range, this resistivity increases linearly with temperature:
In theory, any metal could be used to measure temperature (the metal selected should have a high melting point and an ability to withstand the effects of corrosion), Platinum has therefore become the metal of choice for RTDs Its desirable characteristics include chemical stability, availability in a pure form, and electrical properties that are highly reproducible.

RTD's are more accurate than thermocouples.