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Adafruit Universal Thermocouple Amplifier MAX31856
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Adafruit Universal Thermocouple Amplifier MAX31856

by ericson95 on Wed Jul 21, 2021 7:32 pm

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

ericson95
 
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Re: Adafruit Universal Thermocouple Amplifier MAX31856

by mikeysklar on Thu Jul 22, 2021 6:31 pm

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 | TOGGLE FULL SIZE
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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Re: Adafruit Universal Thermocouple Amplifier MAX31856

by ericson95 on Fri Jul 23, 2021 1:16 pm

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?

ericson95
 
Posts: 10
Joined: Tue Jun 01, 2021 3:59 pm

Re: Adafruit Universal Thermocouple Amplifier MAX31856

by mikeysklar on Fri Jul 23, 2021 1:54 pm

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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Re: Adafruit Universal Thermocouple Amplifier MAX31856

by ericson95 on Mon Jul 26, 2021 10:10 pm

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?

ericson95
 
Posts: 10
Joined: Tue Jun 01, 2021 3:59 pm

Please be positive and constructive with your questions and comments.