Or do I?
It is still very common to see mechanical varios in the instrument panels of modern sailplanes, yet when questioned, pilots can often give only vague reasons for this.
One reason given is that you still have a vario when the glider electrical power system fails completely.
This problem can be overcome by proper design of the electrical system in the glider. So much equipment now depends on electrical power being available at all times that one backup mechanical vario may not be much use on its own.
How well will you fly with no audio, no speed command information no GPS, no flight computer, no transponder, no Flight recorder? How much fun will it be?
By using two avionics batteries, either of which is capable of running all the electrical loads for the longest flight, you are unlikely to suffer complete electrical failure. Proper fusing of the circuits - a 5 amp fuse at the battery, lower rated fuses in the individual service circuits will prevent a failure in one piece of equipment blowing the battery fuse. Alternatively, resettable circuit breakers may be used and a fault isolated.
In the unlikely event that, despite these precautions, electrical power is unavailable, a B400 variometer can be run from the 4*AA 6 volt alkaline battery pack mounted on the back of the instrument. This will provide not only a vario but also audio and averager for about 10 hours.
This variometer also has the following advantages -
Altitude compensation to 35000 feet - the calibration is correct at all altitudes from sea level to 35000 feet.
Accurate linear calibration (not always true of mechanical variometers)
Stable zero point which does not shift with time, temperature or altitude.
No flask - installation is easier and the vario does not have any effect on other varios in the Total Energy circuit.
It is available in both 57mm and 80mm sizes at the same price.
The response is fast and smooth and the speed of response can be adjusted electrically on the instrument without restrictors, filter bottles and such.
The price is comparable to that of mechanical varios which do not have audio or an averager.
Much more rugged than a mechanical variometer.
A digital averager may be fitted in addition to the built in, on demand analog averager.
So perhaps a mechanical vario is not really required in a modern glider cockpit ?