Cold Junction Compensation

If your temperature transmitter/plc input/dcs input has a thermocouple input, it would most certainly have the cold junction compensation built in. No need to worry about it. Just connect and forget.

If you're connecting the thermocouple to a mV (millivolt) input, that's a different story. mV inputs are straight voltage inputs whereas thermocouple inputs have the compensation. Better check your equipment's manual to confirm.

One method of checking if the signal input has compensation is to countercheck the thermocouple reading with a thermometer (mercury/alcohol/digital whatever). If there's a difference (assuming the input is calibrated and properly configured), it is probably not compensated.

I emphasize the "probably" because I've had one technician condemn a thermocouple transmitter once because it wasn't reading correctly. It wasn't reading correctly because his indicator was configured for 0 to 150°C while his transmitter was configured for 0 to 100°C.

I cannot crosscheck with thermometer, bcoz it is steam line of temperature range 485 deg......

we r getting temp through temp transmitter( yokokawa make)

Actually there is a temp difference of 30 deg(less) with temp gauge....

i had replaced with anothre temp gauges( 3 nos) and another make of temp transmitter, in that case,all these are showing correctly,i mean matching with gauge & transmitter value....

But, as per thermocouple chart value that yokokawa temprature( 30 deg less) value is correct........

plz suggest that, where may be thet fault....

I'm having difficulty understanding your setup.

Am I correct in that you have a temperature gauge which is independent of the thermocouple/temperature transmitter?

And am I correct in that your thermocouple, when connected to the Yokokawa temperature transmitter, is reading 30° less than the gauge?

And am I also correct in that your thermocouple, when connected to another make of temperature transmitter, is reading the same value as your temperature gauge?

I'm afraid your last statement: "But, as per thermocouple chart value that yokokawa temperature( 30 deg less) value is correct........" has completely confused me.

Please clarify.

May it be caused by a grounded TC and a non isolated transmitter VS an isolated transmitter?

I'm afraid your last statement: "But, as per thermocouple chart value that yokokawa temperature( 30 deg less) value is correct........" has completely confused me.

Me too.

To shanmugam:

What are you basing your conclusion from? In other words, how can you be sure that the reading of 30°C less is correct?

One way to get a bit of confirmation, as I said, is to use a thermometer, say in an oil bath that's heated up to around 100°C or so. You don't need to raise it to 400+. All you're interested in is that the thermometer and thermocouple are reading the same. If the 30°C difference is still there, you may have:

1. A defective transmitter - defective cjc or non-existent (check the manual)
2. A wrongly configured transmitter.
3. A thermocouple that has "micro-cracks" - microscopic defects on the thermocouple wires that are affecting the readings.

Now, it's possible that the 30°C difference may not exist or is not apparent at 100°C. The problem may now lie in the span setting (still in the configuration setting).

I'm afraid your last statement: "But, as per thermocouple chart value that yokokawa temperature( 30 deg less) value is correct........" has completely confused

mee too cofused...

Some DCS i/o only has CJC as an option. A thermocouple must be installed near the i/o and terminated on a designated CJC terminal. To test whether CJC is working, simply cool or heat the dcs i/o or transmitter. If it moves, you have no compensation. In one case, everytime a process i/o cabinet was opened, our process temperature changed and it affected out product when the controls reacted.

I'm afraid your last statement: "But, as per thermocouple chart value that yokokawa temperature( 30 deg less) value is correct........" has completely confused

mee too cofused...

Actually that was saturated temp of steam drum..the pressure was 47 kg/ cm2..

according to this condition the temp should be around 260 deg.. but that transmitter was showing 230 deg only...

i had conformed that 30 deg less by above mentioned case, also i had conformed with another gauge's and transmitter with a same range........