Gas Turbines - The Turbine Assembly — Page 272, Lesson 359

I want to walk you through the temperature measurement system on a gas turbine, because this is one of the most safety-critical monitoring functions on the whole engine. Let's start with the fundamental limit. The maximum temperature that the turbine assembly can withstand is what limits the thrust or power available from the engine. Think about that — the turbine isn't limited by how much fuel you can burn, it's limited by how much heat the turbine metal can survive. If you exceed that maximum temperature, you cause irreparable damage to the engine. That's why monitoring turbine temperature is imperative — it's not optional, it's essential for the engine's survival. So how do we measure it? We use thermocouples. A thermocouple is a temperature sensor that produces a small electrical voltage proportional to temperature. These probes are placed in the gas flow somewhere in the turbine assembly, typically after the high pressure turbine or after the low pressure turbine. And when the probes are positioned there, the reading is termed Exhaust Gas Temperature — EGT. That's the term you'll see on modern engine instruments. Now here's an important design detail. The thermocouples are connected electrically in parallel. Let me explain why that matters. If you connect them in parallel, and one probe gets damaged, the temperature reading on the gauge only shows a slight drop — it's virtually unaffected. That's the added advantage of the parallel connection. If they were in series, losing one probe would kill the whole reading. So parallel wiring gives you redundancy and resilience. Now, on older engines you'll come across other terms for gas temperature, and they're named according to where the thermocouples are positioned. There's Turbine Inlet Temperature — TIT — that's measured at the inlet to the turbine. There's Turbine Gas Temperature — TGT. And there's Jet Pipe Temperature — JPT, measured in the jet pipe. So the same basic measurement, different names depending on probe location. On modern engines, EGT is the standard term. Here's a clever modern refinement. In modern engines, the thermocouple probes are fitted inside selected fixed nozzle guide vanes. The nozzle guide vanes are the stationary vanes that direct the gas flow onto the turbine blades. By embedding the probe inside one of those vanes, the temperature is sensed without the probe being battered by the high velocity gas flow. So the probe survives longer because it's protected inside the vane structure. Finally, let's look at the relationship between thrust and temperature. As the engine is accelerated to produce more thrust — or more SHP, shaft horsepower, on a turboprop or turboshaft — the EGT will increase in proportion with the extra fuel flow. More fuel, more heat, higher EGT. And vice-versa — when you reduce thrust, fuel flow drops and EGT drops with it. So EGT is your direct indicator of how hard the engine is working, and your guard against exceeding that temperature limit that would cause irreparable damage.