Max6675 problem with very low temperatures

[Cadius]

I set up an Arduino Max + Ethernet Shield with two Max6675 breakouts. On my work bench, both Max boards give readings close enough to actual room temp -- and close enough to each other -- to trust them as valid.

But when I walked the setup down the hall and inserted one of the K-class thermocouple leads into an ultra-low freezer, I get bad data back. The freezer tells me it's currently at -43c and is heading down to it's set-point of -80c (we turned it off to defrost it so we could put the thermocouple in). The max6675 breakout board is in a small plastic box taped to the outside of the freezer.

I was getting reading of 0c until I searched the forums here and saw several posts about the red and yellow labels possibly being reversed. So I reversed the leads and now get readings around 92c to 100c. The freezer isn't that hot, obviously.

If I attach a free-air thermocouple to the red/yellow as labeled, I get room-temp readings.
If I attach to the freezer's thermocouple, I get either 0c or +92 to +100c, depending on whether I use the labeled yellow/red or reversed.

What am I doing wrong?

[Cadius]

My code:

Code: Select all

  MAX6675 thermocoupleA(28, 26, 24); //set up Max circuit, clk, cs, do
  delay(500); 
  client.print(thermocoupleA.readCelsius());

[adafruit_support_rick]

According to the data sheet, the measurement range of the Max6675 is 0°C to +1024°C.

You can try the Max31855 breakout board instead. That has a measurement range of -200°C to +1350°C.

[Cadius]

Ah BANNED.

I was caught up in the thermocouple ranges for our equipment and never once considered whether the chip could handle the data or not. time for me to RTFM!

Thanks for pointing out my error.