b. Indirect.
As shown in Figure 1-6, there are two different types of
indirectly heated thermocouples, isolated and noncontact.
These are both similar
to the direct type in that they are enclosed in glass tubes.
The obvious
difference is that the thermocouple and heater do not make electrical contact.
(1) Isolated.
This type of thermocouple has the element junction and
heater separated by a ceramic or glass insulating bead.
This bead maintains the
thermal contact but no electrical contact is made.
The electrical insulation
prevents the signals on the heater from being felt on the thermocouple.
The
isolation prevents erroneous indications on the monitoring device.
This type is
recommended for high frequency applications, although it is less sensitive than the
contact type.
This is because heat must be transferred from the heater, through
the bead, and then to the measuring junction, rather than from the heater directly
to the junction.
(2) Noncontact.
In this type of thermocouple, the element is separated
from the heater using air spacing in a draft-free enclosure.
The sensitivity of
the thermocouple is varied by movement of the junction; the closer to the heater it
is placed, the more sensitive to changes in temperature of the heater it becomes.
The noncontact type thermocouple is normally used when monitoring current over 150
milliammperes.
Figure 1-6.
Indirectly heated thermocouples
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