Although there are applications where thermocouples can be used without protection, in most they must be protected from the environment and media in which they are being asked to measure by the use of insulation materials often with protective sheaths. These latter, provided in the form of tubes or whatever, also serve to protect the thermocouple from mechanical damage.
As a general rule of thumb, engineering practice has it that an exposed thermocouple junction is only recommended for the measurement of static or flowing non-corrosive gas temperatures where fast response is a key issue. Beyond this, insulated thermocouple junctions are more suitable, certainly for corrosive gases and liquids, accepting that thermal response is slower whether an outer sheath is involved or not. Incidentally, earthed thermocouple junctions (grounded, where the thermocouple is welded to the sheath tip) are regarded as best for corrosive gases and liquids and for high pressure applications where faster thermal response is required.
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