A variometer is a sensitive rate of climb indicator. 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 and moves air on the other side out the static port. The reverse 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 ‘ram’ or pitot pressure to counteract the static pressure changes due to the aircraft control movements to create what is known as a total energy variometer.
The total energy variometer uses a probe that samples the pressure changes due to changes in airspeed to compensate for these errors in measuring the rate of climb or descent of the glider. In our diagram the probe is attached to the static port and is set at roughly right angles to the airflow. The consequence is that as the airspeed increases air will be sucked out of the static port proportionally. The reverse will be true when the airspeed decreases. The variometer then indicates a rate of climb that reflects a true lift and sink rather than the dive or climb of the aircraft.