Ball Model 400 Circuit Diagram

The Comstock Autopilot is based largely on the output of the Ball Model 400 variometer, which provides a very stable DC voltage that represents the rate of climb.  The variometer includes a stainless steel foil diaphragm between two chambers, one of which is directly open to the atmosphere and the other of which is connected to the atmosphere only through a tiny calibrated capillary tube which restricts the flow of air into and out of that chamber.  Magnetic transducer coils on each side of the diaphragm sense its deflection by the resulting difference in air pressure between the two sides.  The critical feature of this instrument is that it outputs an extremely stable rate of climb signal, which allows acceleration to be calculated from it.  Any instability in the rate of climb signal would otherwise be seen as acceleration.

The autopilot uses both rate of climb and acceleration in determining how much to heat.  More-modern rate of climb instruments simply measure atmospheric pressure and subsequently calculate rate of climb by dividing small altitude differences by small time lapses.  Instability in the altitude can be averaged out by any of a number of methods suitable for displaying a fairly-accurate rate of climb, but even the best of these sensors is unstable enough to prevent usefully calculating acceleration from the resulting rate of climb.  Sadly, the Ball Model 400 went out of production in 1988 and, to my knowledge, no variometer of this nature is produced today.  It is true, however, that there are a still lot of functional, old Model 400 sittings on shelves collecting dust.