Getting bad signals from thermocouple. Any suggestions? - Page 1
Forum section: electronics / projects

author: diegoterc3
author_rank: Regular Contributor
timestamp: March 27, 2015, 12:29:12 am
content: 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.

author: TiN
author_rank: Super Contributor
timestamp: March 27, 2015, 04:15:03 am
content: 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

author: KM4FER
author_rank: Regular Contributor
timestamp: March 27, 2015, 04:25:30 pm
content: 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.

author: dom0
author_rank: Super Contributor
timestamp: March 27, 2015, 04:29:09 pm
content: 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.

author: Christopher
author_rank: Frequent Contributor
timestamp: March 28, 2015, 09:39:35 pm
content: 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.

author: mzzj
author_rank: Super Contributor
timestamp: March 28, 2015, 11:21:04 pm
content: Cheapest thermocouple from eBay? Have seen some useless junk and also have sen k-type thermocouples made of n- type material!

author: Neilm
author_rank: Super Contributor
timestamp: March 29, 2015, 04:03:02 pm
content: I have had similar results with a damaged thermocouple - the wires were touching at multiple points.

author: dexters_lab
author_rank: Unknown
timestamp: March 29, 2015, 04:25:14 pm
content: 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

author: wraper
author_rank: Unknown
timestamp: March 29, 2015, 04:40:18 pm
content: Likely you either have thermocouple type other than K or ordered your MAX6675 from ebay or other dodgy place.

author: charlespax
author_rank: Regular Contributor
timestamp: May 27, 2015, 08:35:15 am
content: 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.