Bad temp readings with MAX31855 breakout board

by Scott216 on Mon Feb 11, 2013 12:40 pm

I have four MAX31855 breakout boards connected to an arduino mega. They breakout boards are close to the thermocouples. Three of the thermocouples have a 5-conductor 18 gauges wire connecting the breakout board to my arduino mega. This 5 conductor wire is about 10 feet long. The fourth breakout board is right next to the arduino. I have the little capacitor soldered to the t/c terminal block. The actual temperature for all the thermocouples is room temp, high 60s low 70s. But they are all reading in the low 30s. The temperature jumps around a lot and it's frequently exactly 32 degrees. Anyone have some suggestions to what might be causing this?
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Re: Bad temp readings with MAX31855 breakout board

by adafruit_support_bill on Mon Feb 11, 2013 12:48 pm

You can't extend thermocouples with copper wire. They must be connected directly to the amplifiers, or extended with type K thermocouple extension wire.
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Re: Bad temp readings with MAX31855 breakout board

by Scott216 on Mon Feb 11, 2013 2:27 pm

adafruit_support wrote:You can't extend thermocouples with copper wire. They must be connected directly to the amplifiers, or extended with type K thermocouple extension wire.


I'm not extending the thermocouple wires. The thermocouples are wired directly into the breakout board. On the other side of the breakout board I soldered in at 6-position terminal block. I have a 5-conductor cable connected to this terminal block (I didn't connect anything to 3v pin). 10 feet away, the other side of the 5-conductor cable goes to a terminal block that's connected to my Arduino mega.
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Re: Bad temp readings with MAX31855 breakout board

by adafruit_support_bill on Mon Feb 11, 2013 2:38 pm

10 feet is rather long. But if you are seeing the same results from the closer one that is not likely the problem.

What does the internal temperature report?
Code: Select all | TOGGLE FULL SIZE
  // basic readout test, just print the current temp
   Serial.print("Internal Temp = ");
   Serial.println(thermocouple.readInternal());
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Re: Bad temp readings with MAX31855 breakout board

by Scott216 on Tue Feb 12, 2013 8:44 am

I won't be able to test that until this weekend. Any additional things you suggest I test while I'm at it?
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Re: Bad temp readings with MAX31855 breakout board

by adafruit_support_bill on Tue Feb 12, 2013 9:43 am

Any additional things you suggest I test while I'm at it?

Make sure that the thermocouple wires are firmly clamped in the screw-terminals. Keep the probes away from any potential EMI sources.

Observe the change in output with changes in temperature. (cold/hot water are easy ways to test this). If the response is in the wrong direction, reverse the probe leads. We did have a bunch of them that were marked in reverse of the normal color codes.
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