BruControl: Brewery control & automation software

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I should be grateful for some help with my BruControl Setup for fermenting please.

I am using an Arduino Mega, 2 x PT100 with Adafruit Max31865 shields, and standard 6 x Arduino type relay board, and the issue is that during my first trial run.

Everything is stable when idling, as well as when cooling is called for, but when the heat is activated my temperature sensor / reading then goes haywire (generally in the minus range) for about 10 to 30 seconds or so (see photo) before it re-stabilises.

The situation appears to be an issue that just happens on the FV1 side, as the FV2 side appears stable.

Therefore, I should be grateful for your thought on what could be the issue with my system please? Thanks.
Is the heat source common for both FV1 and FV2 or individual ? If individual maybe try swapping them both (heat sources) and see if the problem changes with the exchange.
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Do you have good separation of wires/cables between your relay and PT100 filter, I'm wondering if this is an EMI issue? When heating is called for it may be causing a spike which translates over to one of your PT cables/leads thus causing the negative reading.
Hi Tartan1, Thanks for your prompt reply, and although there is some separation between the wires, but you could be right with an EMI issue. I will look at move the MAX31865’ away from some nearby cabling. Thanks again.
Doing my monthly backuo to dropbox. If you do NOT have a good backup plan, you will not be happy one day. There is an automated backup in BruControl ( to your local drive) which is great, but one day that might not be enough!
Great bit of advice Oakbarn, and I have got into a habit of backing up my data, with the use of a dedicated software to send my data to a Qnap Server.