Controls and instrumentation - Turbine gas temperature

want to read about :  Controls and instrumentation - Engine speed

Turbine gas temperature
27. The temperature of the exhaust gases is always indicated to ensure that the temperature of the turbine assembly can be checked at any specific operating condition. In addition, an automatic gas temperature control system is usually provided, to ensure that the maximum gas temperature is not exceeded (Part 10).

Fig. 12-7 Turbine thermocouple installation.

28. Turbine gas temperature (T.G.T.) sometimes referred to as exhaust gas temperature (E.G.T.) or jet pipe temperature (J.P.T.), is a critical variable of engine operation and it is essential to provide an indication of this temperature. Ideally, turbine entry temperature (T.E.T.) should be measured; however, because of the high temperatures involved this is not practical, but, as the temperature drop across the turbine varies in a known manner, the temperature at the outlet from the turbine is usually measured by suitably positioned thermocouples. The temperature may alternatively be measured at an intermediate stage of the turbine assembly, as shown in fig. 12-7.
29. The thermocouple probes used to transmit the temperature signal to the indicator consist of two wires of dissimilar metals that are joined together inside a metal guard tube. Transfer holes in the tube allow the exhaust gas to flow across the junction. The materials from which the thermocouples wires are made are usually nickel-chromium and nickel- aluminium alloys.
30. The probes are positioned in the gas stream so as to obtain a good average temperature reading and are normally connected to form a parallel circuit. An  indicator,  which  is  basically  a  millivoltmeter calibrated  to  read  in  degrees  centigrade,  is connected into the circuit (fig. 12-8).