Does anyone tell why we need to ground thermocouple element head and which process application we will use grounded and ungrounded thermocouple
I think the question should be why one uses grounded vs ungrounded thermocouples. Heads are typically grounded by nature of the process connection.
A grounded thermocouple has a faster response than an ungrounded thermocouple.
But an ungrounded thermocouple also has the electrical potential of its protection sheath, which can easily create a common mode ground loop situation for either direct wiring a thermocouple input or wiring to a temperature transmitter.
Temperature transmitters are sold as Isolated or if isolation is not mentioned (silence on the topic) non-isolated.
Non-isolated transmitters or thermocouple inputs can saturate, produce offsets, produce common mode noise or even burn out in the presence of ground loop common mode voltage (depending on the polarity and magnitude).
In my opinion, ungrounded thermocouples should only be used in those applications where speed of response is critical.
In my experience only about 3 or 4 out of 100 temperature loops require speed of response that warrants a grounded thermocouple.
Grounded Thermocouples has faster response and sensitivity than ungrounded thermocouples but they can cause ground loop (conductive path for electric current between the thermocouple sheath and some other points in the thermocouple circuitry ) thus causing measurement errors .
For grounded thermocouples ,the thermocouple voltage signal is measured using Analog DAQ module equipped with differential input channels , so that common mode noise generated from ground loop (any noise voltage existing between the signal ground and the DAQ ground will be common to both channels) should cancel when the two channels’ signals are mathematically subtracted.
(Grounded thermocouples often have thermal time constant values less than half that of comparable ungrounded thermocouples)
Grounded thermocouple has faster response. But also has the noise issues as spelt out by denizen. However, if we use screened thermocouple extension cable, the noise effect can be reduced to a lot extent