Adafruit Universal Thermocouple Amplifier MAX31856

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ericson95
 
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Adafruit Universal Thermocouple Amplifier MAX31856

Post by ericson95 »

Hello,

for my project I use a Adafruit Universal Thermocouple Amplifier MAX31856 (Breakout) with a Type T cable to measure temperatures in the area of liquid Nitrogen (-196 °C). The room temperature is shown correct (23 °C), but if I try to measure the temperature on a metal plate on the surface of the liquid Nitrogen I got a temperature round about -240 °C. This is not possible, because the normal temperature measure devise (which I used to compare the temperatures) shows -190 °C and the highest possible temperature could be not lower then -196°C (cause of the liquid Nitrogen). Also the description of this thermocouple says (on the Adafruit website) that this one has a measure area from "-210°C to +1800°C" with a precision of "output in 0.0078125° resolution". So what can I do?

Thank you in advance and best regards

Erik

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mikeysklar
 
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Re: Adafruit Universal Thermocouple Amplifier MAX31856

Post by mikeysklar »

Hi Erik,

It sounds like you have things connected correctly and are getting a result outside our expected range for this amp. That is to be expected when you are so close the extreme end of the range. More here in the FAQ.

Code: Select all

At extremely high or low temperatures, the measurements are not correct
The 31855 chip handles the linear range of the K-type thermocouples very well.  It does not provide correction for the non-linearities that occur at the extremes of the measurement range.  Thermocouple linearization for temperature extremes requires some curve fitting.  See this guide for more information and example code:
https://learn.adafruit.com/calibrating- ... earization

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ericson95
 
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Re: Adafruit Universal Thermocouple Amplifier MAX31856

Post by ericson95 »

Hi Mikey,

thank you for your help.

Yesterday I tried the thermo sensor a second time and it showed me -174°C on the screen. This is also not the correct temperature. The thing is, I bought this one, because on the website they are saying it works good until -210 °C and my used temperature is -196°C... so normally it should work.

Is there a way to calibrate the thermo sensor?

Or is there a other good one from Adafruit, which I could use?

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mikeysklar
 
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Re: Adafruit Universal Thermocouple Amplifier MAX31856

Post by mikeysklar »

We don't have a pre-calibrated ready to go unit that will work so close to the extremes. You will need to use the link in my previous reply to setup the linear handling and do your own confirmation calibrations.

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ericson95
 
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Re: Adafruit Universal Thermocouple Amplifier MAX31856

Post by ericson95 »

Okay. Thank you very much...that looks really difficult for a beginner, is there a way to get help in coding? I use a Type T, is it possible to use the same script from the Type K?

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mikeysklar
 
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Re: Adafruit Universal Thermocouple Amplifier MAX31856

Post by mikeysklar »

No the tables will be different for type-K vs type-T.

Since you know you are dealing with extremely low temperatures in a very specific range you can use these tables and write your own code to respond to voltage in the specific ranges you are work with.

https://www.thermocoupleinfo.com/type-t ... couple.htm

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ericson95
 
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Re: Adafruit Universal Thermocouple Amplifier MAX31856

Post by ericson95 »

Thank you Mike!

I will try out the two way calibration first and if it doesn't work I will try out this one.

Have a nice weekend!

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