Let me pick up where we left off with temperature measurement — Page 530, Lesson 619

Let me pick up where we left off with temperature measurement. We've covered expansion and vapour pressure sensors, and now I want to introduce you to two more types that work on completely different principles. First, the thermo-electric type, which is also called a thermo-couple. Here's the principle: when two dissimilar metals are joined together at their ends — and that junction is the key part — they can produce an electrical potential. That potential is called a thermo EMF. EMF stands for electromotive force, which is essentially voltage. This effect has a proper name: the Seebeck Effect. The magnitude of that thermo EMF depends on the temperature difference between the junctions. So you have two junctions, and the voltage you get out is proportional to how different their temperatures are. I'll explain this system in detail later in the book, but for now, just hold onto the core idea: two different metals joined at a junction produce a voltage that varies with temperature difference. Now the second type: the radiation type. Every body emits radiation at any wavelength, and the amount of radiation emitted depends on the temperature of that body. That property — how much radiation a body emits relative to its temperature — is called its emissivity. So the measuring technique works like this: you measure the radiation coming off a body, and if you already know its emissivity, you can work out its temperature. That technique has a name: Pyrometry. Now, here's how these sensors fit into the bigger picture of temperature measurement. The expansion and vapour pressure sensors we covered earlier are used for lower temperatures. They're direct reading — think of a simple thermometer. The electrical and radiation sensors, on the other hand, are used for higher temperatures. They can also be direct reading, feeding a moving coil indicator — for example, piston engine exhaust gas temperature. But here's the important modern development: most systems today use remote sensors. The signal from the sensor gets amplified, and then it feeds a servo-operated indicator. And when you need really high accuracy — for example, indicating the temperature of a critical component like a turbine — you use a Ratiometer-type indicating system. Now, why does all this matter so much? Let me tell you about the most critical temperature of all. In a gas turbine engine, the temperature of the gas passing through the turbine is the single most important parameter displayed on the engine instruments. Here's the danger: if you operate the engine beyond the limits of turbine temperature, even for just a moment, you risk excessive turbine blade creep. And creep is the slow deformation of metal under heat and stress. If the rotating blades creep enough to touch the casing of the engine, that's catastrophic. So this is why we need accurate, high-temperature measurement — the turbine temperature is the parameter that can destroy the engine if it's exceeded, even briefly. Let me make sure you've got the key terms straight. Thermo-couple: two dissimilar metals joined at a junction, producing a thermo EMF via the Seebeck Effect, dependent on temperature difference between junctions. Emissivity: a body's radiation emission property dependent on its temperature. Pyrometry: measuring radiation and using known emissivity to determine temperature. And the hierarchy: expansion and vapour pressure for low temperatures, direct reading; electrical and radiation for high temperatures, possibly direct reading to a moving coil, but increasingly remote sensors feeding amplified signals to servo indicators, with Ratiometer systems for critical components like turbines. And the crown jewel: turbine gas temperature, the most important engine parameter, where exceeding limits even momentarily causes blade creep that can be catastrophic. That's the full picture of how these temperature sensors work and why they matter.