The Pressure Altimeter — Page 63, Lesson 70

We're starting a fresh topic now: the servo-assisted altimeter. This is the high-end version of the pressure altimeter, and it's what you'll actually find in modern aircraft, so let's get into it properly. First, a quick note on why this exists. There's a fundamental problem with a simple, directly-driven altimeter. At high altitude, the change in pressure for a given change in height is much smaller than it is at low altitude. Think about that: the air is thinner up high, so the pressure gradient is shallower. That means for a given height change, the capsule movement at high altitude is relatively small. And if the capsule movement is small, then the frictional resistance in the mechanical linkage of an unassisted altimeter causes correspondingly greater errors and more lag. The pointer just doesn't move as crisply or as accurately as it should. So the solution is servo-assistance. This gives the altimeter two big benefits: an increased operating range, and improved accuracy, particularly at high levels. The whole point is that with servo-assistance, the requisite power is available to overcome that frictional resistance, which gives you enhanced instrument accuracy. Now, the principle of operation. The small movements of the capsules are detected by a very sensitive electromagnetic pick-off. That pick-off produces an electric current, which is amplified, and that amplified current drives a motor which rotates the counters and the pointer. So the capsules themselves aren't directly turning the pointer—they're just sensing the pressure, and the motor does the heavy lifting. Let me walk you through the actual mechanism, because it's elegant. Look at the schematic in Figure 5.7. We have an E-shaped iron core, and an I-bar that sits near it. AC is fed to the middle leg of the E bar, which sets up alternating magnetic fields in the outer legs, which we call 'A' and 'B'. The coils on these two outer legs are wound 180° out of phase. So the exciter induces a current in each leg, but because they're 180° out of phase and of equal strength, they cancel each other out when the I bar is equidistant from the legs of the E bar. That's the null condition—when no pressure change is acting on the capsules, everything is balanced, and no current flows to the motor. Now, when a pressure change occurs, the capsules expand or contract. That moves the I bar on its pivot, closing the gap between the I bar and the E bar at one end, and opening it at the other. That unbalances the magnetic fields—one leg now has a stronger coupling than the other—and that produces a net current. That current is amplified and drives the servo motor, which rotates the counters and pointer until the I bar is brought back to its balanced, equidistant position. It's a closed-loop system: the motor moves the indicators, and the mechanism re-centres the pick-off, so the reading is continuously updated and accurate. So to summarise the chain: static pressure into the capsules → capsule movement → electromagnetic pick-off detects it → amplified current → servo motor and gearbox → worm drive mechanism → rotates the counters and pointer. The cam mechanism and the AC exciter are part of that loop, keeping everything synchronised. That's the servo-assisted altimeter. The key takeaway is that the capsules are just the sensor, and the motor is the muscle—that's what gives you the accuracy and the range, especially up high where the pressure changes are so small.