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Variometer

A variometer is sensitive rate of climb indicators. It shows the rate of change of static pressure. In doing this it compares the static pressure to that in a insulated reservoir (thermos bottle). As the aircraft climbs the air pressure is less and the air flows out of the reservoir and pushes out across the diaphragm via and moves air on the other side out the static port. The reverse procedure happens when the airplane descends. A calibrated 'leak' to the reservoir ensures that only changes in the rate of climb are indicated. The basic instrument is uncompensated for motion due to control movements of the aircraft.

Ideally we would want an instrument that only reflects air pressure changes due to the lifting of the air. To accomplish this we add a calibrated pressure coming from the forward motion of the aircraft to counteract the static pressure changes due to the aircraft control movements to create what is known as a total energy variometer. In the diagram, this pressure is obtained by putting a bend in a tube called the total energy probe that samples the air flowing past the glider. If the gliders speed is higher a suction will be created by this tube and counteract the increased pressure of the aircraft descending due to its motion. Conversely if the glider slows then less suction will be generated by this tube and counteract the decreased pressure seen by the aircraft when it climbs due to its motion.