Ungrounded; Thermocouple - Spectrum Controls 1769 Series User Manual

B-2 Compact™ IO Isolated Thermocouple Module
Section B.2
Using an

Ungrounded

(Isolated) Junction

Thermocouple

User's Manual Pub. 0300244-02 Rev. A
isolation of the module. In addition, if multiple grounded junction
thermocouples are used, the module 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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