Air Temperature Measurement — Page 32, Lesson 35

I want to walk you through air temperature measurement, and the very first thing to understand is that air temperature thermometers come in two basic types: Direct Reading and Remote Reading. Let's start with the Direct Reading type, because that's the foundation. The Direct Reading thermometer works on a principle called differential coefficients of expansion with temperature. Let me unpack that. Every material expands when you heat it, but they don't all expand at the same rate. That rate is the coefficient of expansion. So "differential" just means we're exploiting the difference between two materials' expansion rates. The key material here is Invar. Invar is a nickel steel alloy, and its defining feature is a uniquely low coefficient of thermal expansion — it barely expands when heated. Now, we take Invar and bond it together with brass, which has a much higher coefficient of expansion. That bonded pair is a bimetallic strip, and you can see it in Figure 3.1. Here's the clever part. When this strip is heated, the brass expands more than the Invar. Since they're bonded together, the brass side gets longer and the Invar side stays short, so the whole strip curls. The amount it curls depends directly on the temperature rise the strip is subjected to — more heat, more curl. Now, a flat strip curling is fine, but the movement is small. So the strip is drawn out into a helix — a spiral shape. That gives greater pointer movement for a given temperature rise. The helix is incorporated into an instrument with a dial, and that's Figure 3.2 — a bimetallic helix thermometer. Physically, this thermometer is mounted on the windscreen or the fuselage. The tube protrudes out into the air stream, and the dial is visible to the pilot. Figure 3.3 shows that placement on a PA28 Warrior. Now, here's the limitation. The Direct Reading thermometer is not a workable solution for a larger aircraft, and there are two specific reasons. First, a device that penetrates the windscreens would weaken the structure to an unacceptable extent at the pressures associated with higher speeds. Second, it's desirable to have the temperature information in an electrical form, so it can be fed to other instruments and systems. That's exactly what leads us to the Remote Reading type — but that's where we'll go next.