Thermocouples - grounded vs. ungrounded?


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Can any one explain me what is the difference between grounded and ungrounded thermocouples. People say that ungrounded thermocouples are best suited in flameproof environment. How is it so? In what what way it is related with plant safety of refineries and mines. with thanks, vedachalam. india. [email protected]
Hello A grounded T/C will give a faster response - as the wires are grounded to or in other words touching the inside metal wall of the well or sheath. The good thing about “grounded” is that it has a faster response time. The bad thing is if – say you have a heater running inside the tank as well and a voltage happens to get into the fluid or slight voltage leak – then the grounded T/C offers a direct current path to it’s power supply and can short something out, make the tank live or burn out the equipment. A grounded T/C in a tank with no other power insertion is good. A non-grounded T/C is safe or isolated from the circuit, if any – it does have a slightly slower response time as more of the tube or well needs be heated to get to the connection. That is the basic difference in the two. Bob Hogg [email protected]


Does an ungrounded thermocouple works the same as an isolated thermocouple?

In my project, I'm reading a grounded pot.
> Does an ungrounded thermocouple works the same as an isolated thermocouple?


> In my project, I'm reading a grounded pot.

If "pot" is a potentiometer, that is a variable resistor, not a thermocouple. It is likely isolated, but will not generate an EMF on its own.

If "pot" is a vessel, then the use of a grounded thermocouple might create a ground loop between the vessel and the analog input. An ungrounded thermocouple will not do so.
Hi Ruben,

Ungrounded thermocouples is the same with isolated thermocouples, in grounded or isolated system, the thermocouple bead is in direct contact with the object being measured, or it is in electrical contact with the outer and electrically conductive sheath that is in contact with the object.

If there is any chance that there may be a reference to ground (common in controllers with nonisolated inputs) or If the object being measured is electrically conductive and at an elevated potential so it is needed to prevent an electric shock to the operator or damage to the measuring instrument, then an ungrounded/isolated probe is required.

In your case, I believe you will need isolated thermocouples.