Getting bad signals from thermocouple. Any suggestions?

[diegoterc3]

Hello I am getting weird temperature results from my thermocouple. I am designing a new circuit with a MAX6675 and a PIC18F4550 on a breadboard. At first at room temperature I get 33 or 34°C. Exactly the same as my multimeter. Then I put the thermocouple on a hot environment and I get 150°C from the circuit and 250°C from the multimeter probe. Is this because of the breadboard? I know that when some metals interact it could cause a different seebek coefficient and mess with my meassurements but is it enough? Also I have attached the thermocouple to a metal surface with a nut since the sensor came with a threaded screw. Anyone got eny ideas?

Than kyou in advance.

[TiN]

Post a photo of your thermocouple connection and MAX, then we can see if it's good enough.
As fail-check you can just attach thermocouple wires directly to MAX6675 pins, and if problem resolved, you know it's the board

[KM4FER]

Just like the "Is the unit plugged in" question in this case one has to ask "Are you sure you have a type K thermocouple?"  Any other type will give you erroneous readings such as you describe.

I doubt this is your problem but I run type K thermocouples up to 1200C which is near their limit and as they age due to oxidation of the junction the readings will start to decrease.

[dom0]

Thermal voltages (EMF) only occur with a temperature gradient. Unless you got some really, really nasty combination of some metals on your breadboard the small temperature differences will only account for a small difference in measured temperature, but not something like 80 % off. So my guess would be wrong thermocouple, too.

[Christopher]

I had a similar problem at the beginning of the year.

Thought I blew the chips up but it turns out putting a 10n x7r 0603 cap across the inputs sorted it.

[mzzj]

Cheapest thermocouple from eBay? Have seen some useless junk and also have sen k-type thermocouples made of n- type material!

[Neilm]

I have had similar results with a damaged thermocouple - the wires were touching at multiple points.

[dexters_lab]

i have used the max6675 before on a three channel datalogger, i have seen issues if there is an electrical connection through a non-insulated probe to the thing your measuring. Try using a probe that is electrically insulated

[wraper]

Likely you either have thermocouple type other than K or ordered your MAX6675 from ebay or other dodgy place.

[charlespax]

It could be a crappy weld on the thermocouple tip. If that has high resistance, you would see a lower than normal temperature.

As Neilm suggests, your wires could be touching somewhere else. With thermocouples you are measuring the temperature where the two wire touch.