Engine Instrumentation — Page 530, Lesson 618

Let's start with the big picture. Piston and gas turbine aero engines are heat engines. That's the foundational idea. The power they produce is directly proportional to the heat released during combustion of fuel. So the hotter the combustion, the more power you get. But here's the catch — engine components and systems are designed to withstand certain temperatures. If their limits are exceeded, they may fail. So to allow safe operation, the engine temperatures must be monitored. And you can't just watch the combustion itself. The effect of ambient temperatures, as well as combustion, must be considered. So on piston and gas turbine engines, we monitor a whole list of temperatures: air inlet, piston cylinder heads, piston exhaust gas, gas turbine compressor outlets, turbine oil and fuel systems, and the internal air system. Each of those is a specific place where temperature matters for safety or performance. Now, the range we're dealing with is huge. The temperatures monitored may range from -56°C to +1200°C. That's a massive span. So different sensors are used depending on the temperature range to be monitored. They fall broadly into two categories — high temperatures and low temperatures. That's the first big split you need to remember. Within those categories, there are four major types of measuring devices. Let me walk you through each one. First, the Expansion Type. This relies on the principle that most solids, liquids and gases expand and contract with temperature changes. The classic examples are the mercury thermometer and the bimetallic strip. So you're literally measuring how much something grows or shrinks as it heats or cools. Second, the Vapour Pressure Type. Liquids, when subjected to a rise in temperature, change their state from liquid to vapour. So by measuring the pressure of that vapour, you can gain an indication of temperature. The pressure tells you how much has vaporised, which tells you how hot it is. Third, the Electrical Type. A change in temperature of an electrical conductor can cause a change in resistance of that conductor. So by measuring the change in resistance, you can indicate the temperature of the conductor. This sensor is called the Resistance Type — also known as the temperature bulb. That's a key term to hold onto. And fourth, there's a second electrical principle. Two dissimilar metals, when joined together at their ends — that junction — can produce an electrical potential called a thermo EMF. That's the thermocouple principle. The voltage generated at that junction is directly related to temperature. So you have four families: expansion, vapour pressure, resistance, and thermo EMF. Each one is suited to a different part of that -56°C to +1200°C range. The low-temperature end tends to use the expansion and vapour pressure types, while the high-temperature end — like turbine gas paths — relies on the electrical types, particularly the thermocouple. Let me show you how the gas flow interacts with these probes. That figure shows the gas flow over the probes and their electrical connections. The key point is that the probe sits in the gas stream, and its electrical connection carries the signal back to the instrument. The gas flow is what heats the probe, and the probe's response — whether it's expansion, vapour pressure, resistance change, or thermo EMF — is what gets converted into a temperature reading. So the whole system is about matching the right sensor to the right temperature range, and understanding that every one of these devices is ultimately measuring the effect of heat on a physical property. That's the core of temperature sensing in engine instrumentation.