Thermocouple Question

[silberrucken]

Background. I purchased a Dwyer (Love) 16110 controller off of fleabay (cheap) and found the manual and spec sheet on the net.
Powered up just fine and noted that it was reading about 40C with a K thermocouple attached.

I muddled through the menus and changed the thermocouple to a K type and display in Farenheit.
My office runs about 75F - the unit was displaying 104. I reset the unit to factory defaults, and had the same results.
Placing the TC in a pot of boiling water showed 235F. Likely it is a calibraton problem...

A call to Dwyer Tech Support and the response was pretty much "that's a really old controller" (internal date codes show 1993).


Anyway, the question at hand us that if I understand the NIST tables a K thermocouple at 0 degrees C (32F) has an output voltage of 0.0000 volts
Am I understanding the tables correctly? (I'm curious as a J type TC has the same voltage at 0.) Or is this an offset from the measured temperature of a given TC at 0?

[crec]

Silberrucken you are reading the tables correctly. Type "K" thermocouples, per the NIST tables output 0 mv at 0 C. The type "J" thermocouple also outputs 0 mv at 0 C. The thermocouples differ in the materials that they are made from, so the output voltages will be different for each. Per the tables at 100 C

Type K = 4.096mV
Type J = 5.269mV

A couple of things to look for. Is the thermocouple actually a type K? Sheathing on Type k (here) is yellow with the yellow lead being positive and the red being negative. Are you using type K extension wire? Do you have multiple connections? The more junctions you have the more thermocouple points they can create, each one introducing it own possible error. Not sure about the Love controllers, but on many you need to keep the upper and lower bounds the thermocouple set correctly. Check the POL and POH settings. For the Love controller type K in F I think it is POL=-200 and POH =2500. Offsetting these values in some controllers will scale the output.
Also note that a poor quality thermocouple can give very strange readings. Have one back at the shop like that. Small, thin, and cheap - that one always reads higher then the good ones that I have.

Carlos