Thermocouple isolation

Other Hardware Sensors
1

I've read through a few existing threads on this subject, but none of the solutions and work-arounds are possible in my application. So, let's try it again :lol:

I need to measure the temperature in the brake pads of a car, close to the rubbing surface. Brake temp measurement is a standard practice in our lab, and we routinely use K-type thermocouples embedded in the pad (as is standard across the industry). We make and destroy many thermocouples every week.

Some friction materials are electrically conductive. So, at least some of the time, my hot junctions are going to be earthed, through the friction material, caliper, suspension, to the chassis of the car. Or through the friction material, rotor, hub bearing, suspension etc..There is no way around this. I can't insulate my hot junctions from the vehicle's chassis.

Looking at the various Arduino-friendly thermocouple amplifiers, based on the MAX31855 (or older cheaper MAX6675) chip, they all specify that the hot junction must not be earthed. Problem.

Ordinarily, we power our vehicle test instrumentation off the car's 12V system (chassis earth). My best solution so far is to run my new Arduino-based temperature monitor off an isolated power supply (or a battery), so the processor itself will be floating, with no connection to earth except through the thermocouple.

But that leaves the problem that I need to monitor (at least) two brake temperatures, one front and one rear. Both using earthed thermocouples. And I can't see how that's going to be possible without messy earth loops. My only idea so far is to run each temperature channel through a separate Arduino processor, electrically isolated from one another. That's a pretty clumsy way of doing things, but Arduinos are cheap enough that it might be the best, simplest solution.

I'm a mechanical / noise / vibration guy, with only a basic knowledge of electronics. I've heard of opto-couplers, and can see that they could be helpful here, but I've never used them and I don't know how to implement them in my design.

Any suggestions from the amassed wisdom here would be appreciated.

2

My only idea so far is to run each temperature channel through a separate Arduino processor, electrically isolated from one another.

That is perfectly reasonable for the input side, where the signals are analog voltages. For the output side you have a similar problem: how to get the data out of the Arduino(s), e.g. to a PC, without introducing ground loops. For that, you can use optocoupler isolated TTL serial converters, or radio connections. The only opto-isolated TTL serial converters I know about are industrial products and tend to be expensive, but it would not be difficult to make your own.

For radio, you might consider something like the Pololu Wixel, for which a pre-programmed lossless serial repeater app is available, or cheap 433 MHz modules and the VirtualWire library.

3

Hello there, Sorry for posting on this old topic but I have a quite similar problem: I need to measure high temp. (200-300°C) of a hot grounded block of metal. Is there a good alternative to MAX31855 with K-type thermocouple for this purpose? Or is there an easy circuit to connect between arduino and MAX31855 which will isolate the ground and all other connections of these two? Thank you for hints!