How thermocouple CJC's happens ?

Hi all
We all use thermocouples in industry.And all thermocouples needs cold junction compensation,and we do not use any ice cube bath for this purpose.Now does temprature controllers have thermistor located inside them for compensation or its there in control algorithm of the controller,because all temprature controllers ask to mention type of thermocouple you are going to use in there set-up.
No idea about this even though I'am dealiong with temprature cntrollers and thermocouple for a long time !
Regards
Manmeet

You have to tell the controller unit which T/C you are using for it to use the right algorithm for calculating the temperature from the mV reading. Those controllers probably have a universal input for thermocouples.

The cold junction reference is common for all types T/Cs. It is supposed to be a measurement of the temperature of the input terminals, but sometimes it's an ambient air temp on the input board.

That final temperature measurement is the sum of the temperature calculated by the T/C measurement and the cold junction temp offset above zero degrees C.

At ambient temperature, where the thermocouple's hot end and cold end are at the same temperature, the EMF generated by the thermocouple is 0 mV, which corresponds to 0°C on the T/C tables. The cold junction temperature added to freezing then gives a reading of "ambient".

Hi there

Thanx to Randylud And Danw.
Well it seems controllers do have thermistors to record the ambient temprature and then give out actual temprature using calculations in algorithm.
In case if somebody is intrested in algorithm then here it is: http://www.jashaw.com/pid/code.htm
Regards
Manmeet

The jashaw.com link (above) is for a PID algorithm, which is not the same as an algorithm for calculating the process variable or input value.

PID compares what you got (PV) with what you want (setpoint) and determines an output value according the tuning parameters.

A T/C algorithm can be either a look-up table or a polynomial which finds or calculates the temperature value from a Thermocouple's mV signal and the cold junction reading.