Editing Variometer (section)

==Netto variometer==

A second type of compensated variometer is the '''Netto''' or '''airmass''' variometer. In addition to TE compensation, the Netto variometer adjusts for the intrinsic sink rate of the glider at a given speed (the [[polar curve (aviation)|polar curve]]) adjusted for the [[wing loading]] due to water ballast.  The Netto variometer will always read zero in still air. This provides the pilot with the accurate measurement of air mass vertical movement critical for final glides (the last glide to the ultimate destination location).

In 1954, [[Paul MacCready]] wrote about a sinking speed correction for a total energy venturi. MacCready stated, "In still air...a glider has a different sinking speed at each airspeed...it would be nicer if the variometer automatically added the sink rate, and thus showed the vertical air motion instead of the vertical glider motion.  The correction can be made by a variety of methods.  Probably the nicest is to utilize the total energy venturi and the dynamic pressure from the pitot tube."<ref name=pm/>  As Reichmann explained, a "Netto variometer shows the climb and sink of airmass (not of the sailplane!)...In order to achieve a 'net' indication, the always present polar sink of the sailplane must be 'compensated out' of the indication. To do this, one makes use of the fact that above the speed for best glide the polar sink speed of the sailplane increases roughly with the square of the airspeed. Since the pitot pressure also increases with the square of the speed, one can use it to 'compensate away' the effect of sailplane polar sink over virtually the entire speed range."<ref name=hr/>  Tom Brandes states, "Netto is simply the German way of saying 'net,' and a Netto Variometer System (or polar compensator) is simply one that tells you the net vertical air movement with the sailplane movement or sink taken out of the usual variometer reading."<ref>{{cite journal |last1=Brandes |first1=Tom |title=The Netto System |journal=Soaring |date=1975 |volume=39 |issue=3 |pages=37–39 |publisher=Soaring Society of America}}</ref>

The '''Relative Netto Variometer''' indicates the vertical speed the glider would achieve IF it flies at thermalling speed - independent of current air speed and attitude. This reading is calculated as the Netto reading minus the glider's minimum sink. When the glider circles to thermal, the pilot needs to know the glider's vertical speed instead of that of the air mass. The '''Relative Netto Variometer''' (or sometimes the '''super Netto''') includes a g-sensor to detect thermalling. When thermalling, the sensor will detect acceleration (gravity plus centrifugal) above 1 g and tell the relative netto variometer to stop subtracting the sailplane's wing load-adjusted polar sink rate for the duration. Some earlier nettos used a manual switch instead of the g sensor.