OK well kind of an electronics question I hope one of you guys can answer for me.
Can anyone explain how a multimeter/k type thermocouple setup works exactly? Now I have been doing a lot of reading on how thermocouples work because I was concerned that I wasn't properly using mine. Now from what I understand, there is a sensing junction and a reference junction, and the temperature of the reference junction must be known in order to figure out the temperature sensed. Now on my DMM that allows me to put my thermocouple directly into it with its own port, and gives me a temperature readout, how exactly does this work?
Such as this
I only have one idea...
Maybe the DMM has a thermistor built in that senses ambient temperature and uses that to calculate what the reference voltage is and what the sensed temperature is. In this case, assuming the actual reference junction is somewhere in the thermocouple wiring itself, if the DMM read 0 mv on the thermocouple leads, it would display whatever ambient was measured at the thermistor, and when voltage is applied from the thermocouple, it would calculate what the reference junction voltage would be based off the thermistor, and then calculate temperature sensed.
Is this correct? If this is the case, I could be getting faulty readings. My thermocouple sensing junction is clamped to the suction line, and the wire is run through the window inside the cab to my DMM. If the reference junction remained outside in the hot air, and the DMM inside in the cool air (or possibly hot sun), I would get a false reading.
Now I decided to hook my thermocouple leads directly to my DMM and strictly measure voltage. With the entire thermocouple in free air, I read 0 mv. As I put the sensing junction in an ice bath, I read .85mv. Using a chart, that is equal to 71 deg F assuming a 32 deg reference. Since my reference is actually about 72 deg F, that would put me right at ~31 deg F which is right where I would expect the ice bath to be. Basically this is telling me that there is a reference junction somewhere in the actual thermocouple wiring, if I am not mistaken.
Of course this stuff is new to me so I may be completely off track, but I just want to know how the system works so I know whether or not I am getting proper readings.
EDIT: Hmm I may be onto something... I decided to chill the DMM in the refrigerator for a few minutes, lo and behold after removing it and placing the thermocouple sensing junction into the ice bath, the temperature readout was about 11 deg F...
