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

Question

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.

Answer 1

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.

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.

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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: -

Answer 2

I have now found a solution to the problem.

double TypeEVoltageToTemperature(double voltage)
{
    return
        1.7057035 * pow(10, -2) * voltage +
        -2.3301759 * pow(10,-7) * pow(voltage, 2) +
        6.5435585 * pow(10,-12) * pow(voltage, 3) +
        -7.3562749 * pow(10,-17) * pow(voltage, 4) +
        -1.7896001 * pow(10,-21) * pow(voltage, 5) +
        8.4036165 * pow(10,-26) * pow(voltage, 6) +
        -1.3735879 * pow(10,-30) * pow(voltage, 7) +
        1.0629823 * pow(10,-35) * pow(voltage, 8);
}
double TypeKTemperatureToVoltage(double temperature)
{
    double precal = (1.185976 * pow(10,2)) * pow(2.7182818284590452 ,(-1.183432 * pow(10,-4)) * pow((temperature - 126.9686),2));

    return  (-1.7600413686 * pow(10,1))     * pow(temperature,0.0) + precal +
            ( 3.8921204975 * pow(10,1))     * pow(temperature,1.0) + precal +
            (1.8558770032 * pow(10,-2.0))   * pow(temperature,2.0) + precal +
            (-9.9457592874 * pow(10,-5.0))  * pow(temperature,3.0) + precal +
            (3.1840945719 * pow(10,-7.0))   * pow(temperature,4.0) + precal +
            (-5.6072844889 * pow(10,-10.0)) * pow(temperature,5.0) + precal +
            ( 5.6075059059 * pow(10,-13.0)) * pow(temperature,6.0) + precal +
            (-3.2020720003 * pow(10,-16.0)) * pow(temperature,7.0) + precal +
            ( 9.7151147152 * pow(10,-20.0)) * pow(temperature,8.0) + precal +
            (-1.2104721275 * pow(10,-23.0)) * pow(temperature,9.0) + precal;
}

This solution takes around 80,000 cycles on an ATmega328. (5 milliseconds per both calls).

Thanks to @Toble_Miner and @Andy aka for their help with fixing this problem!