Having trouble with an unstable/noisy  ?

Having trouble with an unstable/noisy #thermocouple ?

I tried alot of things from shielding my MAX6675 converter pcb to using ferrite beads on near by cables. I almost gave up and started looking at using a thermistor… blaeh…

Then I stumbled upon this at Adafruit;
https://learn.adafruit.com/thermocouple/f-dot-a-q

“The MAX31855 is surprisingly sensitive, we’ve found a good way to fix this is to place a 0.01uF to 0.1uF capacitor across the thermocouple leads (that is, place the capacitor into the blue terminal block”

I’m aware of decoupling capacitors in general but putting one straight across the thermocouple leads came as a small surprise. I thought that would mess with the readings but instead it does indeed get rid of the noise just as it would for an integrated circuit. No weird/unstable readings so far!

Just make sure to use a low-leakage cap (basically anything that’s not an electrolytic) or you’ll risk introducing a fairly large offset in the reading!

@Thomas_Sanladerer indeed. A ceramic disc capacitor works well.

what’s wrong with thermistors?

@Brad_Hopper nothing wrong per say but I personally prefer thermocouples where possible. Thermocouples can stand much higher temperatures and are simple to mount. Just isolate them electrically from other metal with a piece of kapton tape, and fix them to the heater block with a screw and washer. They can stand pretty tough handling.

http://www.controleng.com/search/search-single-display/temperature-tutorial-thermocouple-vs-rtd-vs-thermistor/aae906fe91.html

A side note, be sure to verify the function of your thermocouple if buying from the China lottery - or simply get a good one from a known reseller. I tried out some from an eBay bargain which proved to be junk. Room temp was sensible but way off at extruding temp.

I have a meter with a thermocouple and have found a couple of problematic aspects. Temp takes much longer to stabilize I suppose that’s due to the jacket? Also very sensitive I.e. bad readings when the reference temp changes. If you bring the meter in from cold outside the temp will be off by 10 degrees until the meter side of the junction warms up. I guess you have to calibrate to your ambient temp but if that’s changing like the heater or A/C cycling, all bets are off. Thermistors seem more robust in that respect but I guess I had lower standards for the thermistor and havent scrutinized it in the same way.