First, the thermo-electric type — Page 530, Lesson 619

Let’s pick this up right where the temperature-measuring systems are being classified, because we’ve just moved into the electrical and radiation families. I want to walk you through the two new types, then how they fit into the bigger picture of engine instrumentation. First, the thermo-electric type. Here’s the principle: when you take two dissimilar metals and join them together at their ends, you create what we call a junction. That junction can produce an electrical potential — a voltage — and that voltage is called a thermo EMF. EMF stands for electromotive force, which is just the technical name for the voltage that drives current. This effect is named the Seebeck Effect, after the physicist who discovered it. The key point is that this thermo EMF is dependent on the temperature difference between the junctions. So you don’t get a voltage from a single junction alone; you get it from the difference in temperature between two junctions. This whole arrangement is known as a ‘Thermo-electric Type’ or simply a ‘Thermo-couple’. I’ll explain that system in detail later in the course, but for now, hold onto the core idea: dissimilar metals, a junction, and a voltage that depends on a temperature difference. Now the second type — the radiation type. Every body emits radiation at any wavelength, and the amount of radiation emitted is dependent upon the temperature of that body. That property is termed its ‘emissivity’. So emissivity is essentially how much radiation a body gives off at a given temperature. If you measure the radiation coming off a body, and you already know its emissivity, then you can determine the temperature of that body. That measuring technique is known as Pyrometry. So a pyrometer isn’t touching the hot object; it’s reading the radiation it emits. Now let’s put these into context with the other sensor families, because there’s a clear division of labour here. Generally, the expansion and vapour pressure sensors are used to indicate lower temperatures. They are direct reading — think of a simple thermometer. The electrical and radiation sensors, on the other hand, are used to measure higher temperatures. They can be direct reading to a moving coil indicator — for example, piston engine exhaust gas temperature. But here’s the important modern twist: most systems today use remote sensors that feed to servo-operated indicators, after the signal from the sensor has been amplified. So the sensor sits at the hot location, sends a small signal, that signal gets amplified, and then it drives a servo-operated indicator — a needle that’s moved by a servo motor rather than directly by the sensor’s own force. There’s also a special type of indicating system called a Ratiometer-type indicating system. This is used when you need greater accuracy — specifically when indicating the temperature of a critical component, such as turbines. So when the temperature really matters, you go to the ratiometer for the extra precision. And that brings us to why all of this matters so much in a gas turbine engine. The temperature of the gas passing through the turbine is the most important parameter of all those displayed on the engine instruments. Here’s the danger: if you operate the engine beyond the limits of turbine temperature, even for only a moment, you’re liable to cause excessive turbine blade creep. Creep is the slow, permanent deformation of a metal under sustained heat and stress. If the blades creep too much, they can grow and touch the casing of the engine — and that contact can be catastrophic. So the whole point of measuring turbine temperature accurately is to keep the blades inside their safe envelope, because exceeding that limit, even briefly, risks blade growth and a blade-to-casing contact that can destroy the engine. So to tie it together: we’ve got the thermo-couple, which uses the Seebeck Effect to turn a temperature difference into a voltage; we’ve got pyrometry, which reads radiation to infer temperature from emissivity; and we’ve got the system architecture — expansion and vapour pressure for low temperatures, electrical and radiation for high temperatures, with remote sensors feeding amplified signals to servo indicators, and ratiometers for the critical turbine temperature that must never be exceeded.