How Well Do You Really Know Thermocouples

Many of those working with PLCs and control systems are familiar with thermocouples. Most know the basic concept of how temperatures are measured using thermocouples. But how well do you really know the principles that make them work?

Here is a thought provoking question to make you think about how they really work..... Start with a temperature indicator or controller that accepts and is setup for a J Type thermocouple. Using a very short (less than 1") piece of copper wire, jumper the 2 terminals where the thermocouple would normally be connected. With this jumper in place what would you expect the temperature reading on the indicator to be and why?

The temperature wouldn't read. Because you need two different materials with current flow to achieve a measurable temperature. Correct?

Based only on having had a propane wall heater which used a thermocouple, I'm going with 'the minimum reading'. Without the heat of the pilot flame no voltage is generated by the thermocouple and therefore no power to maintain the pilot valve nor to the mercury switch thermostat to operate the valve to cycle the heater on/off.

My guess would be the temperature of the room where the indicator is.

Changed my mind. Probably the negative temperature of the room. The (tc copper wire) input would be 0 volts but the temperature compensation would be added to that.

I'm guessing it should be about room temperature - due to the cold-junction compensator built into the thermocouple module ...

if there's no compensator circuit - then probably zero degrees ...

DISCLAIMER: I just tried it both with a regular (simple) thermocouple temperature meter ... the meter didn't budge ... then I tried it with a 1771-IXE/C thermocouple module - and it read 77 degrees F ...

what are you up to, Archie? ...

Ron Beaufort said:

I'm guessing it should be about room temperature - due to the cold-junction compensator built into the thermocouple module ...

if there's no compensator circuit - then probably zero degrees ...

Click to expand...

^^^ This
but, for no compensation circuit - I would think it would measure the lowest range value (of the module's input)

Would it matter if the controller had a CJC?

I want to say that since the CJC is used to offset the voltage reading from the hot side of the TC to compensate for the cold junction temperature (and differential nature of the TC), that if you had a "short" or essentially 0V reading on the TC input, you'd go to 32 degrees F (Type J TC is 0V at 32 degrees) minus the CJC or Ambient temperature at the connection point?

But I think I'm putting more weight on the CJC than should be.

It's going to provide a temperature measurement, but since it's a circuit of homogeneous material, it won't be dependable (maybe in the ballpark of room temperature + cold-junction compensation, +- margin of error?). The different types of thermocouples aren't classified their conductor mixture, but rather the produced voltage to temperature relationship of that mixture. Based on that, I suppose you *could* mix and match types of materials to reproduce the output of a given type of thermocouple, but that would be an exhaustive, time-consuming process since none of their outputs are linear.

JeremyM said:

It's going to provide a temperature measurement, but since it's a circuit of homogeneous material, it won't be dependable. The different types of thermocouples aren't classified their conductor mixture, but rather the produced voltage to temperature relationship of that mixture. Based on that, I suppose you *could* mix and match types of materials to reproduce the output of a given type of thermocouple, but that would be an exhaustive, time-consuming process.

Click to expand...

I think the specified short piece of copper wouldn't have enough length or dissimilar metal properties to generate any voltage differential. Which is why I thought it would be 0mV.

It will read the CJC temperature.

Thermocouples produce a voltage in proportion (but not linear!) to the temperature difference between the hot junction and the cold junction. With a jumper, the voltage is 0V, so the instrument interprets the reading as 'no difference between the cold junction and the hot junction'. It has some way of determining the cold junction temperature to do the compensation (usually an RTD) and will display that reading.

Further thought experiment Archie: extend your jumper to a few feet and place the halfway point (pseudo hot junction) in an oven operating at 350 degrees F. What does your instrument read now?

Tharon said:

I think the specified short piece of copper wouldn't have enough length or dissimilar metal properties to generate any voltage differential. Which is why I thought it would be 0mV.

Click to expand...

There is potentially still a dissimilar metal connection, depending on what the terminals of the measuring instrument are made out of. Neither of the materials for a type J thermocouple (or any thermocouple, as far as I'm aware) are copper, so I'd expect a more or less completely unpredictable reading based on the construction of the instrument.

I agree with Ron. What are you up to, Archie?

ASF said:

There is potentially still a dissimilar metal connection, depending on what the terminals of the measuring instrument are made out of. Neither of the materials for a type J thermocouple (or any thermocouple, as far as I'm aware) are copper, so I'd expect a more or less completely unpredictable reading based on the construction of the instrument.

I agree with Ron. What are you up to, Archie?

Click to expand...

FYI, the positive leg of a type T and type U thermocouple is pure copper.

If the materials of the instrument made a difference, how do you account for instruments that can be software-configured to read various different types of thermocouples with no impact to accuracy?

Well since we're talking about T/C's, I'd like some education on their interaction with a 1756-IT6I2 module....On a startup and I observed:

Temperature to be measured apx 65 deg F, Type J T/Cs
CJC Jumpers installed on onboard terminal block, correct terminals in correct orientation
T/Cs wired straight to card

With Polarity reversed, CJC Remote checked, read 92 deg F
With Polarity reversed, CJC Remote Unchecked, read 37 deg F
With Polarity corrected, CJC Remote checked, read 65 deg F
With Polarity corrected, CJC Remote Unchecked, read 28 deg F

So on a mv range, why does polarity only change the reading by a small percentage, and not have the temp go negative or way positive?

And is it correct to check CJC Remote box? The Help says it's to be checked when using a remote terminal block through a cable, but our CJC's are directly on the module terminal block. But checked (not default) seems to be the only reading that makes sense.

A J thermocouple is generally Iron/Constantan.

If the terminals of the input are matching then when you put the copper wire in you'll have one Iron/Copper junction and one Copper/Constantan junction.

So the Copper/Constantan junction is essentially like a T thermocouple.
I think the Iron/Copper junction is not a good thermocouple combination so would not produce much voltage.

If the temperature is 60F. A T thermocouple is at 0.611mV (http://www.omega.com/temperature/Z/pdf/z223.pdf) Since the indicator is set up for a J thermocouple, at 0.611mV a J-thermocouple would give about 55 degrees.

There are a lot of assumptions above and I'm not sure the significance of the "very short" part of the problem statement but at least I have a method...

It should read room temperature, or actually CJC temperature of the input module.

The thermocouple measurement is a relative measurement between the thermocouple junction (probe at end of wire) and the input module. if there is no potential difference, the the probe must be at same temp as input module. The cjc will be reading the input module temperature.

This is one reason that good thermocouple readings will rely on the cjc being as stable and as close to an isotherm as possible. If you have cold air blowing across the analog input module/cjc at diffrent times, the reading will change even if the absolute temperature at the probe does not change.