3. MEASURING JUNCTION
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The hot junction is the junction which is subjected to the process
or medium that is being measured or controlled.
Three common types of junctions:
1. Grounded
2. Insulated
3. Exposed (fast response)
Of these three the one which has the highest application rate
in greatest use, is the grounded junction. As will be seen, its
characteristics meet most requirements.
Grounded junction
In this construction the mineral insulation is completely sealed
from containments and the measuring junction becomes an in-
tegral part of the tip of the thermocouple. The response time,
as we will see later, approaches that of an exposed loop ther-
mocouple. In addition, the junction conductors are completely
protected from harsh environmental conditions. Small diameter
thermocouples may be selected to match or better the response
time of exposed loop thermocouples, yet the operational lift and
upper temperature limit of the junction will be extended due to
protection offered by the sheath.
Insulated measuring junction
In this construction the thermocouple conductors are welded
together to form the junction which is insulated from the exter-
nal sheath with the mineral insulation. The response time for an
insulated junction is longer than it is for a grounded junction
thermocouple of the same outside diameter. In insulated junc-
tion thermocouples, however, conductors are electrically insu-
lated from the sheath. A feature advantageous in applications
where thermocouples are used:
- in conductive solutions;
- for differential averaging (paralleled);
- for additive (series) applications;
- wherever isolation of the measuring circuitry is required.
Exposed loop junction
The exposed loop junction offers a faster thermal response time
than the other two types of junctions. However this type of junction
is limited to mild environmental conditions or one time usage
under more severe conditions.
COMMON MEASURING JUNCTION TYPES
Grounded
Insulated
Reduced Tip