Troubleshooting
found on gas turbine-powered ships are usually the
chromel-alumel type.
After calibrating the temperature switch, check its
Due to the high cost of thermocouple wire, a less
operation through at least one complete cycle. If you
expensive lead wire is often used to connect the
find a variation from the desired operating values, have
t h e r m o c o u p l e wires to the temperature-indicating
the switch recalibrated. In an emergency, you can adjust
instrument. However, to reduce errors, make sure you
the temperature switch while it is in the system instead
get lead wires with the same thermoelectric
of sending it for calibration. You should, however, send
characteristics as the thermocouple wire. For
the switch to the calibration lab as soon as possible for
chromel-alumel thermocouples, copper and constantan
a thorough adjustment. If the switch cannot be adjusted,
lead wires are often used. These lead wires are more
you must troubleshoot the switch or replace it.
rugged than thermocouple wires and may be solid or
T e m p e r a t u r e switches, like pressure switches,
stranded.
normally require replacement if they cannot be adjusted.
When you adjust temperature control devices, allow
Maintenance
several minutes for the thermal unit to reach the
temperatures of the surrounding air, gas, or liquid before
Routine maintenance performed on thermocouples
s e t t i n g the operating range. If replacement of a
s h o u l d be accomplished by PMS. Thermocouples
temperature switch is necessary, refer to the
should be checked regularly. If possible, they should be
manufacturer's technical manual to be certain you have
checked when they are in place. If it is necessary for you
the proper replacement.
to remove a thermocouple, be sure to return it to the
same insertion depth or deeper. This will prevent errors
As with pressure switches, remember the
resulting from the irregular heating of different portions
temperature switch is constantly energized even if the
of the wire.
system is secured. The closing and opening of the
contacts energize the entire electrical circuit. You must
One prime requirement for a thermocouple to meet
make sure electrical power to the pressure switch is
tolerance is that it must be unvarying. Remember, when
secured before you replace the switch. Follow the basic
t h e r m o c o u p l e s are handled they can be bent or
troubleshooting techniques and all applicable electrical
contaminated. This is why you must use extreme care.
safety precautions whenever you repair or replace
B o t h bending and contamination will cause the
temperature switches.
thermocouple to become erratic, thereby affecting its
operation and increasing the likelihood of error.
THERMOCOUPLES
Troubleshooting
The temperature of a body may be measured
without any physical contact with it by use of an
Normally, malfunctioning thermocouples are
instrument that senses radiation. Such an instrument is
noticed during equipment operation. A console operator
k n o w n as a radiation pyrometer. This device is
must know the correct operational parameters of the
commonly called a thermocouple on gas
equipment he or she is monitoring. While taking
turbine-powered ships. Thermocouples used aboard
readings on a GTE, if the turbine inlet temperature is
ship measure temperatures in the gas turbine engine.
incorrect or not consistent with previous readings, the
operator should know something is wrong. The problem
Construction
could be a malfunctioning thermocouple or
thermocouple circuit. As a GSE, you will be called to
Thermocouples are composed of two dissimilar
investigate the problem.
metals joined at one end to produce a change in voltage
that is proportional to a change in the temperature at the
When you are troubleshooting a thermocouple,
joined end. A voltmeter (calibrated in degrees) shows
several common faults can occur. Thermocouples, like
the temperature at the thermocouple.
other electrical equipment, can be grounded, open, or
Types of metals or alloys used in combination in
shorted. The thermocouples on most GTEs are
thermocouples are (1) chromel-alumel, (2)
connected in parallel. In troubleshooting this type of
c o p p e r - c o n s t a n t a n , (3) iron-constantan, and (4)
setup, you will have to isolate the thermocouples
platinum-platinum rhodium. Other combinations also
systematically to locate which particular thermocouple
may be used for special purposes. The thermocouples
or thermocouple set is faulty. Some general
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