are composed of the metals Chromel and
These modules convert the ohmic value of
Alumel.
the RTE to an output range of 4 to 20
milliamperes (mA) direct current (dc). How-
ever, most RTEs read their value directly
L M 2 5 0 0 THERMOCOUPLES.--The
into the propulsion electronics as an ohmic
LM2500 uses eleven thermocouples to monitor
value.
T5.4. Individual thermocouples cannot be re-
placed though, as they are arranged in four
The 0 to + 400F and the -- 60 to +500F
groups. These groups, or harnesses, are the
range RTEs are commonly mounted in a
components you must change if you have a faulty
thermowell. Since you can change the RTE
thermocouple. Figure 5-8 shows the four
without securing the equipment it serves, this also
harnesses. You should note that the two upper
simplifies maintenance.
harnesses and the lower right harness each
contains three thermocouples. The lower left
harness has only two thermocouples. Be very
Thermocouples
careful when you replace a harness. Take care not
to damage a probe or bend the harness. For
Thermocouple sensors are used to monitor
complete detailed information on replacement of
LM2500 power turbine inlet temperature (T5.4)
thermocouple harnesses, refer to the Propulsion
and Allison 501-K17 turbine inlet temperature
Gas Turbine Module LM2500, Volume 2, Part 3,
(TIT). Although each engine uses a different type
NAVSEA S9234-AD-MMO-050.
of thermocouple, the theory of operation is the
same. These sensors work on the principle that
ALLISON 501-K17 THERMPCOUPLES.--
when two dissimilar metals are fused together at
Eighteen dual-element, Chromel-Alumel thermo-
a junction by heat, a small voltage is produced.
couples are used on the Allison 501-K17 to
The amount of this voltage is directly proportional
monitor TIT. Unlike the thermocouples of the
to the T5.4 or TIT.
LM2500, you can change these thermocouples
individually. Each of these sensors has two
Figure 5-7 shows the LM2500 and the Allison
independent elements that allow two sampling
501-K17 thermocouples. You should note that
circuits per thermocouple. Only one circuit is
although they are physically different, their
used for monitoring TIT; the other circuit is not
operation is the same. Both thermocouples
Figure 5-7.--Thermocouples.
5-6
