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Temperature and pressure represent two of the major parameters meas-ured and evaluated in a monitoring system. All gas turbine engines areequipped with sensors of this type; however， the exact number as well as their location varies considerably among manufacturers.
At each of the measurement locations， pressure probes may be attached to aharness， and these probes will direct the air flow to external pressure transducers for measurement while serving as a sheath for the appropriate thermocouple at that location (each thermocouple will be seated inside a pressure probe).
The electrical output of the thermocouple varies with temperature. This output is fed through a flexible cable to an external signal-conditioner circuit to amplify and condition the signal for interfacing to the moni-toring system.
Temperature .easurement
Temperature measurement is important to gas turbine performance. Exhaust gas temperature should be monitored to avoid overheating of turbine components. Most gas turbines are equipped with a series of ther-mocouples in their exhausts. Measuring turbine inlet temperature directly isvery useful but， because of the turbine damage that results if a thermocouple breaks and passes through the turbineblades， thermocouples are not generally installed upstream of the turbine. Bearing oil temperature is normally monitored at the discharge to ensure proper oil characteristics;however， this temperature is not an accurate indication of bearing condi-tions， since bearings may develop localized hot spots during operation. Tomeasure bearing temperature accurately， transducers should be located in the bearings themselves. The temperature will indicate problems in either journal or thrust-bearings prior to damage. In addition to turbine exhausttemperatures， compressor inlet and discharge temperature measurement is necessary to evaluate compressor performance.
For most points requiring temperature monitoring， either thermocouples or resistive thermal detectors (.TDs) can be used. Each type of temperaturetransducer has its own advantages and disadvantages， and both should be considered when temperature is to be measured. Since there is considerableconfusion in thisarea， a short discussion of the two types of transducers is necessary.
Thermocouples
The various types of thermocouples provide transducers suitable for measuring temperatures from -330 to 5000 0F(-201 to 2760 0C). The useful ranges for the various types are shown in Figure 19-12. Thermocouples function by producing a voltage proportional to the temperature differencebetween two junctions of dissimilar metals. By measuring this voltage， the temperature difference can be determined. It is assumed that the temperatureis known at one of the junctions; therefore， the temperature at the otherjunction can be determined. Since the thermocouples produce a voltage， noexternal power supply is required to the test junction; however， for accuratemeasurement， a reference junction is required. For a temperature monitor-ing system， reference junctions must be placed at each thermocouple or similar thermocouple wire installed from the thermocouple to the monitor where there is a reference junction. Properly designed thermocouple systems can be accurate to approximately土2 0F(土1 0C).
copper/constantan
iron/constantan chromel/alumel chromel/constantan platinum, 10% rhodium/platinum
platinum, 13% rhodium/platinum
platinum, 30% rhodium/ platinum, 6% rhodium platinel 1813/platinel 1503
iridium/iridium 60%, rhodium 40%
Figure 19-1.. .anges of .arious t.ermocouples.
.esistive Thermal .etectors
.TDs determine temperature by measuring the change in resistance of an element due to temperature. Platinum is generally utilized in .TDs because it is mechanicallyand electrically stable， resistscontamination， and can be highly refined. The useful range of platinum .TDs is -454-1832 0F (-270-1000 0C). Since the temperature is determined by the resistance intheelement， any type of electrical conductor can be utilized to connectthe .TDto the indicator; however， an electrical current must be provided to the .TD. A properly designed temperature monitoring system utilizing .TDs can be accurate to土0.02 0F(土0.01 0C).
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