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I have a system with redundancy on the control unit using thermocouples. My question is : is it possible to get from different devices the measurement from a single thermocouple ?

I am thinking about sending the signal after the cold junction is applied into 2 different ADC, but as the thermocouples involves very little voltage, I'm not sure if this would screw up my measurement.

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4 Answers 4

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Thermocouple voltages are so low it's unlikely you'd connect one directly to an ADC in practice. Instead there's going to be some kind of buffer amplifier between the thermocouple and the ADC. Since the buffer amplifier has a low output impedance, connecting multiple ADCs in parallel shouldn't present much of a problem.

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  • \$\begingroup\$ Ok, and what if I get the fork at the input of the amplifier buffer, instead of at the output ?? \$\endgroup\$
    – dudu721
    Sep 8, 2016 at 16:09
  • \$\begingroup\$ You should be OK because the output resistance of the thermocouple itself is very low (10s of Ohms) and the input impedance of amplifier(s) should be much higher than that. \$\endgroup\$
    – BobU
    Sep 8, 2016 at 16:20
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Yes, there is no problem at all- except the devices must accept a T/C input that has some arbitrary common mode voltage on it.

Thermocouples are very low impedance devices (less than 100 ohms) and one instrument will hardly affect another at all.

You are really getting into diminishing returns looking for redundancy after the CJC. I would suggest your #1 choice is two completely independent sensors and signal conditioning circuitry, followed by two completely galvanically isolated front ends. There will be a small effect from the burnout current from each input, but you can design the front ends so that any likely failure (for example, put two resistors in series) will not unduly shunt the sensor and an open will be detected as an unsafe condition.

The most likely failure mode for a properly installed and functioning thermocouple system would be the wiring (and any connectors) to the sensor, possibly the sensor itself if abuse is possible, followed by damage due to lightning or EMP type events, which would likely affect the front end itself.

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Thermocouples are cheap; install several.

There are a LOT of different devices that accept thermocouple input, and some of them ground the (-) lead. Some ground the (+) lead. Some apply a test current through the wiring periodically, to check for continuity.

Lacking complete knowledge of the controllers, one safe way to mount a single thermocouple, but give all the controllers a separate thermocouple circuit: you could make a heat-bath, with N thermocouple hot junctions and one cold junction. Each of your N controllers connects to one of the hot junctions, your 'input' thermocouple connects to the cold junction, and a heat-bath-control thermostat heats the bath until the 'input' thermocouple circuit reads zero temperature difference.

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If you want redundancy, you need several controllers and several thermoelements. To keep things simple, each controller should be connected to its own thermoelement. If you connect one thermoelement to several controllers, you might get additional noise and errors.

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