Air Temperature Measurement — Page 32, Lesson 35

Let's start with the two basic families of air temperature thermometers. I want you to hold two categories in your head: Direct Reading and Remote Reading. Everything in this lesson hangs off that split. Direct Reading thermometers work on the principle of differential coefficients of expansion with temperature. Let me unpack that phrase, because it's the whole engine of the instrument. Different materials expand by different amounts when you heat them by the same amount. That difference is what we exploit. The key material here is Invar. Invar is a nickel steel alloy, and its defining property is a uniquely low coefficient of thermal expansion — it barely grows when heated. Now, we take Invar and we bond it to brass, which has a much higher coefficient of expansion. The result is a bimetallic strip — two metals fused together, face to face. Here's the physics. When that strip is heated, the brass expands more than the Invar. The brass side wants to get longer, the Invar side resists. So the strip curls — it bends toward the Invar side. How much it curls depends directly on the temperature rise the strip is subjected to. More heat, more curl. That's the entire sensing mechanism. Now, a flat strip curling gives you only a small movement. So the strip is drawn out into a helix — a spiral coil. Why? To give greater pointer movement for a given temperature rise. The helix multiplies the tiny curl into a big rotation, which drives a needle across a dial. That's the instrument you see in Figure 3.2 — a bimetallic helix thermometer. Where does it live? It's mounted on the windscreen or the fuselage, with the tube protruding out into the air stream, and the dial visible to the pilot. On a small aircraft like a PA28 Warrior, that's exactly how it's placed — you can see the placement in Figure 3.3. Now, the second category: Remote Reading. And here's the crucial point — the direct reading thermometer is not a workable solution for a larger aircraft. I want you to understand exactly why, because there are two distinct reasons. First, a device that penetrates the windscreen would weaken the structure to an unacceptable extent at the pressures associated with higher speeds. Think about it — at high speed, the windscreen is under serious load. Punching a hole through it for a thermometer tube compromises that structure. Not acceptable. Second, it's desirable to have the temperature information in an electrical form, so it can be fed to other instruments and systems. A mechanical dial on the windscreen gives you one reading in one place. But modern aircraft need that temperature data to be shared — sent to other instruments, other systems. That requires an electrical signal, not a mechanical curl. So that's the fundamental split. Direct reading — mechanical, bimetallic, windscreen-mounted, fine for small aircraft. Remote reading — electrical, needed for larger aircraft, because of structural integrity at high speeds and the need to distribute the information electrically. We'll pick up the remote reading side next.