Probably the most important instrument to a glider pilot, the vario is the ultimate sensitive instrument! Measuring the difference between static pressure and a reference chamber, minute differences in pressure indicate the rising or sinking of the glider. The speed and rate of the reaction give the pilot the information he needs to stop and climb or speed up and move on to find lift. Winter have been making vario’s since the 1930’s and both materials and methods of construction have changed. These super sensitive instruments now employ shock proof jewel bearings and ultra-fine springs to centre the measuring vane.
The science bit, how does it work?
Vane type variometers measure the change in air pressure inherent to changes in altitude. The instrument consists of a cylindrical chamber with a precision-fit baffle (vane) rotating on shockproof jewel bearings and centered by a coil spring. The vane divides the chamber in two: one section is open to static pressure, while the other is connected to an expansion tank, in which a volume of air is insulated against the thermal effects. Differences in pressure are compensated by the narrow gap between vane and chamber wall. There is a change in static pressure when an aircraft climbs or descends, and a differential pressure is established between the two sections of the chamber. The resultant deflection of the vane provides a measure of the vertical speed and these deflections are transferred to the pointer of the instrument.
The response rate of a variometer is important. In high performance gliding, up-currents can be identified all the quicker and used all the more efficiently if the variometer responds without delay. Defined as the length of time the instrument takes to reach 65 % of its final reading in response to a sudden change in vertical speed, the time constant serves as a standard for gauging speed of response. The faster a variometer responds, the smaller the time constant.