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OM-222166 Page 25
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5-16. Using Contact Thermocouple Sensors
1 Blanket
2 Contact Thermocouple Sensor
(See Product Literature Sheet)
The welded thermocouples discussed previously can be
used for preheating or stress relieving. Welded thermo-
couples are normally used in stress relieving applications
because of their accuracy and ability to withstand high
temperatures.
As an alternative, in preheating applications a contact
thermocouple sensor can be used. This eliminates the
need to weld thermocouples and the sensor can be
moved during the preheat process to check temperatures
at other locations on the joint. (Contact thermocouple
sensors are limited to 500
°
F [260
°
C ].)
.
If the contact thermocouple sensor is removed, the
temperature recorder (if used) will display a short
duration of heat drop.
The contact thermocouple sensor can be plugged into the
thermocouple extension cable or a Type K 25 ft. armored
extension cable. One of these extensions is required for
each sensor.
In preheating applications, the sensor must be placed un-
der the induction coil. To be accurate, the sensor flat end
must be held against the heated surface. If liquid-cooled
or air-cooled coils overlap at any location in your installa-
tion, it is recommended that a second sensor be placed at
that location.
NOTICE − Failure to follow these recommendations could
cause heat damage in the blanket or coil.
Temperatures at the weld joint can be checked with tem-
perature sensitive crayons to verify the preheat tempera-
ture.
Placing Contact Thermocouple Sensor
Place sensor between blanket and metal material. The
sensor must be flat and in firm contact with the material
being heated. The tip of the sensor should be positioned
under the center of the wires of the blanket anywhere
along the blanket length.
To prevent thermocouple sensor from pulling out of
position, contact TC sensor securing band 301 073 is
available.
NOTICE − When using a contact thermocouple with air
cooled blankets, position is critical. If any part of the blan-
ket exceeds 400
F (200 C) for extended periods of time,
damage to the blanket will occur.
Additional sensors may be placed at the ends of the part
to protect from overheating. If a blanket is hanging off the
end of the part or goes over a cutout, insulation may be
added between the blanket and metal, slightly decoupling
the coil, which causes the area to receive less eddy
currents and generate less heat.
NOTICE − When heating next to a joint, control sensor
(TC#1) must be under the heating coil. A separate sensor
may be placed at the joint to monitor joint temperature.
When positioning blankets in vertical or overhead posi-
tions, ensure the non-insulated side of the blanket is able
to radiate heat. Do not cover blanket with insulation,
wood, etc. Trapping heat in the blanket may damage the
blanket.
1
2
Flat on part under
center of coils
Do not place TC probe
transition under heating
blanket
Not this
Control TC
Add additional TC
locations near edges
Decouple blanket
near edges
5-17. Using Non−contact Temperature Sensors
It is important to locate non-contact temperature sensors so that they are measuring as close to the heat zone as possible to achieve consistent temper-
ature control of the system. The greater the distance between the heat zone and the area read by the sensor, the more th control loop will fluctuate
around the target temperature.
The spot measurement location should be within two inches (50 mm) of the heat zone.