Noise in thermocouple hanging up PID

Projects General Guidance
1

I am using the MAX31855 breakout board with the Arduino Nano.

I have used the PID Library by Brett Beauregard to implement a ramp - soak controller for an oven.
The themocouple have is metal sheathed and K type and has a 2 metre cable and I have a 0.1uf cap at the thermocouple connector just as mentioned by Adafruit to reduce noise from the sensor.

The issue I have in sometimes the temperature reading (in celcius) suddenly goes to zero (or near zero) and then returns back to its value and after this happens the PID output just stays 0 so the relay just stays off and doesn't recover from there.

Is there Anyone else facing this kind of problem with the MAX31855 or PID, I would be happy to hear how they solved it.

2

I would be happy to hear how they solved it.

Have you tried:

  1. averaging several readings?
  2. not calling compute() with bogus values?
3

PaulS:
Have you tried:

  1. averaging several readings?
  2. not calling compute() with bogus values?

I am doing neither of those.
I have ran the controller on 2 locations.
At home the readings are very stable the problem occurs once in an hour or maybe 2 hours where as on site the problem is more severe 1 out of every 10 - 20 readings is near zero.

My power supply is also transformer based and but there is a cable connected to the pc usb port where data is being sent to the PC.

Would you still recommend averaging in this case?
When you say bogus values do you mean out of range values, do I have to check if my input is in range before calling compute?

4

Would you still recommend averaging in this case?

It could not hurt.

When you say bogus values do you mean out of range values

Yes.

do I have to check if my input is in range before calling compute?

What do you think? I mean, considering that bad things happen when you don't.

5

From your description of the thermocouple as metal sheathed, I suspect that it is a grounded thermcouple. Grounded thermocouples do not work with the MAX31855. You need one like this: Thermocouple Type-K Glass Braid Insulated [K] : ID 270 : $9.95 : Adafruit Industries, Unique & fun DIY electronics and kits. Adafruit also has a good tutorial and library on using the MAX31855.

Bob

6

bobcorr:
From your description of the thermocouple as metal sheathed, I suspect that it is a grounded thermcouple. Grounded thermocouples do not work with the MAX31855. You need one like this: Thermocouple Type-K Glass Braid Insulated [K] : ID 270 : $9.95 : Adafruit Industries, Unique & fun DIY electronics and kits. Adafruit also has a good tutorial and library on using the MAX31855.

Bob

I had ordered a ungrounded theromocouple. Don't know if the manufacturer made some mistake. I also rechecked again by checking each wire of the thermocouple for connection against the metal body however it's not showing any connectivity on the multimeter.