grounded and ungrounded thermocouples

The input modules on the system that are going to be monitoring your T/C's is the biggest factor. Most high level systems are not compatable with grounded T/C's. In most cases if a ungrounded T/C grounds out the system will display bad readings.
If the TE happens to be on a cathodically protected line, it needs to be ungrounded to prevent bleeding off the impressed current.

Gil Pinheiro

If you need fast response use grounded thermocouples. But pay attention to not get more noise than signal. Grounded thermocouple is no problem if you are measuring high temperatures and, you have high output voltages from your thermocouple. But if your environment is very noisy and/or you have low temperatures. You can use grounded thermocouples with signal isolators and grounded shielding.

Derek Appleton

I'm not sure if any of the other answers address your exact question but, here goes. . .
Select a grounded Thermocouple if you wish to sense the temperature as accuratly as possible i.e. the junction is in physical contact with the metal of the sheath which, in turn is in physical contact with the Thermowell ( or should be ).

A un-grounded unit will have losses based upon the insulating material usually a thermally conductive material that is a electrical insulator. If the piping and Thermowell are well grounded noise shouldn't be a problem ( assuming ground potentials at the source & measuring points are very close ).

Induced noise is always a concern in electrically floating systems which un-grounded systems are. However most noise problems if within cirtain limits are taken care of by the measuring device.