Engine Instrumentation — Page 530, Lesson 623

I want to walk you through engine instrumentation for gas turbine engines, starting with the temperature probes themselves. The industry standard materials for these probes are chromel, which is nickel chromium, and alumel, which is nickel aluminium. Now, these two materials don't give the highest milli-voltage output of all the materials available, but what makes them ideal is their ability to withstand very high temperatures, combined with a reasonable volts per degree ratio. So you get a usable electrical signal per degree of temperature change, and the probes survive the harsh environment. Here's the key principle: a thermocouple works by generating a small voltage when two dissimilar metals are joined and heated. And notice something important — this system requires no power supply to indicate temperature. The voltage is generated purely by the heat. However, if that signal is going to be used to feed a temperature limiting system, the voltage will need to be amplified, and that amplification is supplied by the aircraft's electrical system. Now, in a system that supplies both a temperature limiting system, which is called top temperature control, and a temperature indicating system, the probes will contain two hot junctions. One hot junction feeds the limiter, and the other feeds the indicator. That's illustrated in Figure 38.11, which shows the gas flow over the probes and their electrical connections. Now let's talk about where these probes are positioned within the engine. The positioning depends on two things: the temperature of the gas, and the ability of the metal the probes are made of to withstand that temperature. On engines where the gas temperature within the turbine is too high for the probe metal to withstand, the probes may be positioned after the turbine, and the gauge is then calibrated to read exhaust gas temperature, abbreviated EGT. On other engines, it may be convenient to combine the temperature probes with the pitot probes that measure exhaust gas pressure, which is called P7. In that case, the gauges will read jet pipe temperature, abbreviated JPT. Ideally, you'd want to sample the temperature either before the turbine, which is called 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 measurement point is limited by the probe material's heat tolerance. Finally, there's another method for measuring actual blade temperature — the radiation method, which uses an optical pyrometer. So instead of a probe in direct contact with the gas, you measure the temperature by the radiation emitted from the blades themselves. So to tie it all together: chromel and alumel thermocouples generate a self-powered voltage, they're placed where the metal can survive, and depending on that placement you read EGT, JPT, TIT, TET, or TGT — or you use a radiation pyrometer for actual blade temperature.