intrumentation amp thermocouple measurement

[kubeek]

I am working on a thermocouple measurement amplifier, and came across INA126 which seems to work well for this type of measurement.

My thermocouples are the grounded type, so the measuring tip is coonected to the braided steel sheath. In the thermocouple article on page 3 they suggest to ground the thermocouple on one side with a 1Meg reistor for the grounded type thermocouple. Why not use a resistor on both sides to get more symmetrical input impedance?

Also, where would you connect the shield to? Ground it directly,? The thermocouple will be inside an electric kiln in close proximity to the heating elements, so I´d like to tie the whole setup to PE.
So should I tie all grounds together and then to PE?

 

[KeepItSimpleStupid]

Why not use a resistor on both sides to get more symmetrical input impedance?

Because you want the temperature to float to a high value to obtain open TC protection. You man want to use the other side if you were cooling.

Also, where would you connect the shield to? Ground it directly,?

Standard practice dictates that the sensor should be grounded at the sourcing end.

All grounds (references and otherwise) should ideally tie together at one point, That is not always practical. So you have Protective Earth, signal ground, digital ground, analog ground(s), high current grounds all being used as separate entities until they tie together at ONE POINT.

Grounding isn't an easy concept to grasp.

Added stuff.

I your schematics, show the Earth ground separately and I think you will see the paths. The TC normally gets inserted in a ferrule of sorts and is connected to Earth in that case.

Now show your power supply grounds and also Earth, A loop really can't develop because of the high resistances.