I'm not a control engineer nor have I played one on television. Never the less this question has landed on my desk. I've done a google search as well as a search on this site without success. All I can find is voltage to temperature conversion. It seems a piece of equipment was ordered specifying T type TC's. Unknown to the engineer and technician it arrived with J type TC's but they used T type equipment to measure temps. and now want to know if there is a way to convert the temperature readings they recorded back to the correct equivalent T type reading. I've told them I don't believe that it is possible without knowing what the cold junction temperature was and maybe not even then. Is it possible to do this even if one had to approximate the CJ temperature? Thanks

 

> I'm not a control engineer nor have I played one on television.

Neither am I, but I wear the hat at work.

I think you're on the right track.

Assume a CJ temperature (take a thermometer and place it next to the terminals and see what the terminal temperature is at operating temperature). Look up the EMF value for Type T (this is "Type T CJ EMF") at the assumed CJ temperature, because Type T was the instrument in use for the readings you're using.

Convert stated temperatures to equivalent EMF values for Type T. These are Type T temp EMF values.

Subtract the Type T CJ EMF from each Type T temp EMF. This is the Type T 'difference' EMF

Look up the Type J CJ temp EMF for your assumed CJ temperature; this is Type J CJ temp EMF

Add the Type J CJ temp EMF to the difference EMF to get the Type J EMF.

Convert the Type J EMF to a temperature value.

For explanation, see page 12 of Conversion of Thermocouple Voltage to Temperature by Gerald Recktenwald, March 25, 2010
"Summary: The following steps are used to compute the temperature of a thermocouple with a reference junction at an arbitrary temperature Tr:"
http://tinyurl.com/7elzmkx
http://preview.tinyurl.com/7elzmkx

David

 

don't fool with it, just replace your element.

 

Thanks David,

I knew it had to be something like that but with little math background I wasn't sure what needed to be added/subtracted to what to get a correct reading. I'll pass this along to the engineer who thought he could just take equivalent emf TC values to get a corrected reading without knowing CJ temps.

 

> don't fool with it, just replace your element.

Exactly! No easy shortcuts here, the tests will simply have to be rerun using correct equipment. Someone was just inquiring if it was possible to do a simple conversion and avoid the time lost. Not a good idea.

 

You haven’t mentioned the installation including any thermocouple leadwire, - between the thermocouple and the equipment – which, if used, I assume was a Type T compensation wire?

So if you have a Type J thermocouple connected to Type T leadwire, I believe you would have a complex (in more than one sense) thermocouple connected to the cold junction and receiver. This “combination” thermocouple probably won’t win any prizes for temperature measurement accuracy, and due to the extra junctions in the circuit at the thermocouple/leadwire transitions, won’t help with CJ checks either.

Have you thought of replacing the thermocouple with a Type T?