Engine Instrumentation — Page 530, Lesson 621

Right, let's get into engine instrumentation. We're starting with gas temperature measurement on a gas turbine, and the very first thing to understand is why we even bother. The gas temperature must be monitored closely, and automatic temperature limiting equipment is fitted to most gas turbine engines operating today. So the whole point here is that the engine's internal temperature is a critical parameter — it has to be watched, and in most modern engines, there's automatic equipment that will limit it. To enable this monitoring to be achieved, temperature probes are inserted in the gas stream. So we physically put sensors into the hot gas flow. Now, how does one of these probes actually work? It's formed from the junction of two dissimilar metals. When heated, that junction generates a small voltage which is proportional to the actual temperature which produced it. So the hotter the junction gets, the more voltage it makes, and that voltage is directly proportional to the temperature. This is the classic thermocouple principle — two different metals joined together, and the heat at that junction creates a voltage. That voltage can be measured on a milli-voltmeter and displayed in the cockpit as the temperature at the rear of the engine. So we're measuring a small voltage, in millivolts, and converting that into a temperature readout for the pilot. Now, here's an important instrument name: a Galvanometer. A galvanometer is a very sensitive instrument used to indicate these low voltages. It's the device that actually reads that tiny voltage. And here's the clever bit about how it works — the galvanometer uses a basic Wheatstone Bridge Balancing circuit that alters the magnetic field in a coil; this change produces a torque to drive an indicator. So inside the instrument, there's a Wheatstone bridge circuit — that's a balancing circuit — and as the voltage changes, it alters the magnetic field in a coil, and that change in magnetic field produces a torque, a turning force, which drives the needle on the indicator. So the electrical signal becomes a mechanical movement of the gauge. Let me show you the layout. That figure shows how the probes — the hot junctions — are connected, and also how the gas flows over them. So the probes sit in the gas stream, and the output from the probes is sent to the cockpit engine instrument. Now, here's a key term: the cockpit instrument is the cold junction. So we have hot junctions out in the engine, and the cockpit instrument is the cold junction, where the EMF is measured on a very sensitive milli-voltmeter to display the engine gas temperature. EMF — that's electromotive force, which is just the voltage generated. So the hot junctions generate the EMF, and the cold junction, the cockpit instrument, measures it. Now, here's the practical problem. Just one probe would obviously not supply enough information to accurately tell the pilot what was going on in the whole turbine. It could only inform him about the small part of the turbine that it was monitoring. So one probe only samples one small spot — that's not enough to know the temperature across the whole turbine. So it's necessary to place a number of probes, electrically connected in parallel, all around the periphery of the engine or the exhaust system. Periphery means the outer edge, the circumference. So we put multiple probes around the engine, and they're wired in parallel. This means the gas stream is sampled in many more places, and the output is the average of all of the probes. So instead of one reading from one spot, we get an average across the whole turbine. And there's an added advantage to this parallel arrangement: if one probe is damaged, the effect on the reading on the gauge is minimal — a slight drop may occur. So one probe fails, and the gauge barely moves. That's a nice redundancy feature. Finally, the actual position of the probes depends upon two things: the anticipated maximum temperature of the gas, and the ability of the probe material to withstand that temperature. So where we put the probes is a trade-off — we need to place them where the gas is hot enough to be meaningful, but the probe material has to be able to survive that temperature. So the position is dictated by how hot the gas gets and what the probe material can handle. So to sum up: we use thermocouple probes — junctions of two dissimilar metals — in the gas stream, wired in parallel around the engine, averaging the temperature, measured at the cold junction in the cockpit by a sensitive galvanometer using a Wheatstone bridge, and the probe placement is a balance between gas temperature and material limits. That's the foundation of gas temperature measurement on a turbine.