Spectrum Controls 1769 Series User Manual page 58

B-2 Compact™ IO Isolated Thermocouple Module
Section B.2
Using an
Ungrounded
(Isolated) Junction
Thermocouple
User's Manual Pub. 0300244-01 Rev. A
channel-to-channel isolation is removed, since there is no isolation between signal and
sheath (sheaths are tied together). It should be noted that the isolation is removed even if
the sheaths are connected to chassis ground at a location other than the module, since the
module is connected to chassis ground.
Spectrum Controls recommends that a grounded junction thermocouple have a protective
sheath made of electrically insulated material (for example, ceramic). An alternative is to
float the metal sheath with respect to any path to chassis ground or to another
thermocouple metal sheath. Thus, the metal sheath must be insulated from electrically
conductive process material, and have all connections to chassis ground broken. Note
that a floated sheath can result in a less noise-immune thermocouple signal.
An ungrounded (isolated) junction thermocouple uses a measuring junction that is
electrically isolated from the protective metal sheath. This junction type is often used in
situations when noise will affect readings, as well as situations using frequent or rapid
temperature cycling. For this type of thermocouple junction, the response time is longer
than for the grounded junction.
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