Engine Instrumentation — Page 530, Lesson 623

Right, let's pick this up with the temperature probes themselves. We're looking at how we actually measure the hot gas temperatures inside a gas turbine engine, and the first thing to know is the material these probes are made from. The industry standard for the material used in the temperature probes in gas turbine engines is chromel and alumel. Chromel is a nickel-chromium alloy, and alumel is a nickel-aluminium alloy. Now, these two materials may not have the highest milli-voltage output of the materials available to us, but their ability to withstand very high temperatures, coupled with a reasonable volts-per-degree ratio, makes them ideal for the job. So we're trading a bit of electrical output for durability and a usable, consistent signal. Now, the system that uses these probes is called a thermocouple indicating system. And here's a key point: the system requires no power supply to indicate temperature. The thermocouple generates its own tiny voltage from the heat. However, if that signal is to be used to supply a temperature limiting system, the voltage will need to be amplified. And that amplification is supplied by the aircraft's electrical system. So the indicator is self-powered, but the limiter needs aircraft power. Now, in the case of a system that supplies both a temperature limiting system—which we call top temperature control—and a temperature indicating system, the probes will contain two hot junctions. One junction feeds the limiter, and one feeds the indicator. So a single probe physically carries two sensing points, each dedicated to its own job. That's illustrated in Figure 38.11, which shows the gas flow over the probes and their electrical connections. Now, where do we put these probes? The positioning of the probes within the engine depends on two things: the temperature of the gas, and the ability of the metal they're made of to withstand it. On engines where the temperature of the gas within the turbine is too high for the metal of the probes to withstand, they may be positioned after the turbine, and the gauge is then calibrated to read 'exhaust gas temperature'—EGT. On other engines, it may be found convenient to combine the temperature probes with the pitot probes which measure exhaust gas pressure, which is called P7. In that case, the gauges will read 'jet pipe temperature'—JPT. Obviously, it would be ideal if the temperature could be sampled either before the turbine, which is called either 'turbine inlet temperature'—TIT—or 'turbine entry temperature'—TET—or inside the turbine, which is called 'turbine gas temperature'—TGT. But in every case, the position of the probes is dependent upon their ability to withstand the temperatures they encounter. So the ideal location is limited by the material's heat tolerance. Finally, there's one more method for measuring actual blade temperature: the radiation method, using an optical pyrometer. So instead of a probe in contact with the gas, we measure the radiant heat from the blades themselves. So to tie it together: we have chromel and alumel thermocouples, which are self-generating for indication but need amplification for limiting. We have the naming conventions—EGT, JPT, TIT, TET, TGT—each tied to probe position, and the position is always dictated by the metal's ability to survive the heat. And we have the optical pyrometer as the radiation-based alternative for blade temperature.