Electrical – Using a type K thermocouple ADC to measure a type E probe

arduinothermocouple

I'm currently trying to measure the temperature of an off-the-shelf soldering iron.
After realizing that the temperature-sensor in the soldering iron is a thermocouple, i ordered a type-K thermocouple ADC board, more specifically a MAX6675-based breakout board.
While I am able to get temperature readings out of the chip, they are way to high in comparison to the real temperature.
For example: I'm getting a reading of about 550°C, when in reality the iron has a temperature of slightly under 300°C.

Looking at this chart in the Wikipedia I think that my iron might be a type E thermocouple.
https://de.wikipedia.org/wiki/Thermoelement#/media/File:Thermocouple_voltages.PNG

Now for the final question:
Is it possible/feasible to convert/calculate the correct temperature (of a type E probe) from the type-K-thermocouple ADC reading?
Has anyone done this in the past?
I don't really need high absolute accuracy, +-10°C will be present due to the PID regulation anyway.

My last solution would be to order/sample a MAX31855E-chip from maxim and then swap the chip out on the board. I don't really want to do that, because the MAX31855E isn't that easy/cheap to source.

Additional information: I'm using an ATmega328 microcontroller, firmware is written in C, so any example code would be highly appreciated as well.

Best Answer

This is how I see it: -

  • You have a K type TC connected to a proper measurement interface
  • The above is telling you 550 degC
  • Your TC in your soldering iron is telling you about 300degC when connected to your "proper measurement interface"
  • You have concluded that the TC in your iron is an E type.

enter image description here

Looking at the graph above you would expect an E TC to produce a bigger voltage than a K TC for the same temperature so, you have to ask yourself if your beliefs are founded.

EDIT section

It now appears that the OP is using the internal TC connected to a K type interface circuit and this does tend to justify that the likely internal device is an E type TC. The graphs are fairly linear and both fall through 0 uV and 0 degC so a first order approximation is to treat the conversion as linear. At 1000 degC a K type produces 41 mV and an E type produces about 76 mV.

If you want a more precise polynomial try THIS data sheet: -

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