Type K Thermocouple w/ MC050

[Jerry B]

I'm new to using thermocouples and it's proving a little trickier than I was expecting. I'm using a type K thermocouple and was going to connect it to a "AIN/Temp/rheo" pin (c1-p28). I connected the red line of the thermocouple to the neutral line and the yellow line to c1-p28 and have tried inputMode 0 and 1 monitoring the ".Voltage" (mode 0 is what I would expect is needed to be used). I've looked for information about using the "temp" input, but haven't been able to find much about it. Any help would be greatly appreciated.

[Jerry B]

Looking into a good temperature range RTD to use instead of the type K thermocouple. Then use the resistance mode with an equation to get the temperature reading since it should be linear with the resistance.

[FluidPowerTom]

Sounds like you're on the right track.  It looks like a type K thermocouple has a very low millivolt output such that you'd have to be able to parse fractions of a millivolt accurately.  If you had to do this then you'd want to look at .AnIn to get the raw 0-32767 ADC signal, and that will maybe be higher resolution than the 0 - 5,250 mV.  It shouldn't really be higher resolution since the 12-bit ADC has 4,096 steps.

Going with a RTD or thermistor is a better bet for sure.  You've got 0 - 10 kOhm on the input.  Just keep in mind that only a few of the controllers have resistance inputs, and those only have a few of that type.

[acmall]

Also note if you want accurate measurements with the thermocouple the resistance vs temperature difference between hot and cold junction while almost linear is not perfectly linear. Depending on the application this can be significant. You would also need to take care of cold junction compensation.

Using a RTD or thermistor will be easier if it works for your application. If you have to use a thermocouple the easiest approach would be to use a suitable signal conditioner that will take care of the above and give either a voltage or 4 to 20mA output.

Alastair

[oiltronic]

Thermocouples cannot be connected directly to any Plus+1 input unless maybe you only care about very high temperatures, with low accuracy, and with calibration on each unit. 

A Type-K thermocouple produces only 12.2mV at 300 deg C, and 25mV at 600 C.  The difference between 1000 and 1010C is only 0.39mV.  Meanwhile, the Plus+1 analog inputs throw away a bunch of counts at the bottom due to input offset limitations of the electronics, which means even at the lowest measuring range you won't read anything below 12.9 mV, and at the 5V range it starts measuring at 20mV.   Sure, there's the low-end precision spec of 0.09mV, but there's also the "Worst case error" of 27mV.

Read all about the analog input limitations here: Plus 1 MC050-1xx Controller Family Technical Information

If you cannot use a RTD or another type of sensor then you will need to use a signal conditioner to convert the thermocouple signal to something like 0-5V, which is what I do for all thermocouples. Other than expanding the measurement voltage to a useful range it electrically isolates the signal from other electrical noise, has a better input impedance, and many conditioner models linearize the output so every step change in temperature has the same step change in output voltage.