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I am building an electronic controller that measures the temperature on a metal chamber. Type K thermocouple sensors are placed on the metal chamber, and type k thermocouple extension cable connects the thermocouple junction at the measurement point to the electronic controller that processes the signal. The type k thermocouple extension cable we have has a foil shield because it is a noisy electrical environment. I'm confused about how to connect this shield on the thermocouple cable though, because when I search for type k connectors, they only have two prongs so I don't see a way for the extension cable shield to connect to ground at either the metal chamber or at the electronic controller. What am I missing? What do people usually do?

It seems like most people don't buy thermocouple cable that has a shield, but we did from Omega (link here). Is there a type of thermocouple connector that isn't just all plastic? If so I don't see it. Or do people manually tie the shield to a screw on the thermocouple port on the chassis? Just wondering what is typical here that I'm missing. Thanks!

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As Andy says, you should ground it at the cold end and leave the other end open. I have never seen a common thermocouple plug/jack that supports a third connection for the shield. The two common types have the requisite alloy types for the +/- and nothing else. Maybe you can integrate a grounding connection with a strain relief. Or don't use a connector at all and use a terminal strip. Shielded thermocouple wire is not that common IME.

That said, it's far more common to have (often exposed) braid or spiral armor that is connected right to the junction- so probably grounded- at the hot end and runs all the way back to a connector or terminal strip at the amplifier or controller and is left floating at that end. Most commercial controllers and data acquisition systems accommodate that arrangement (there is expected to be some common mode voltage noise relative to ground, and a good system can even handle mains voltage without too much error). The shield won't do anything to help with induced noise from nearby wiring so keep the sensor wires well away from anything carrying high currents.

Thermocouples have very low differential impedance so the purpose of a foil shield would mostly be to reduce common mode noise for a floating junction. For a grounded junction it would not do much at all.

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You ground the shield at the input of the amplifier. That's best recommended practice. This keeps common mode interference (that might disturb the amplifier) at a minimum. No need to ground or earth at the thermocouple.

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  • \$\begingroup\$ Thanks! I'm wondering specifically about how to make the ground connection though. I have the thermocouple cable that has a foil/drain wire shield. On either end I need a connector. When I search for thermocouple connectors all I see are the plastic thermocouple connectors with a couple prongs for the signal. There is no way for the shield to actually connect to anything. Is there a type of connector I'm unfamiliar with? Or do people pull the drain wire from the cable and run it to a screw on the chassis? \$\endgroup\$ Commented Mar 4 at 23:53
  • \$\begingroup\$ I can't help you with how your equipment works with a ground wire. All I can say is that if you have a ground shield on the cable and you want to ground it then, to avoid earth/ground loops you either leave it unconnected or, you ground at the amplifier on the chassis. \$\endgroup\$
    – Andy aka
    Commented Mar 5 at 8:43
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It turns out there is an easy common solution I wasn't aware of. You can buy 3 pin type K thermocouple connectors. The third pin is meant for a ground reference to the shield drain wire! I would have to switch my thermocouple panel mount ports to 3 pin, but that's doable. I just didn't even realize this was something that was an option.

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