When you attain a large height above the ground, it is impossible to discern whether you are rising or sinking. To assist the pilot in staying in rising air, the variometer was adapted from the sailplane world (conventional cockpit gliders).
A vario system I set up in the early 70s. This is the earliest use of a vario in a modern hang glider that I am aware of. I used it at the Annie Green Springs prelim test meet practice session.
— Dave Cronk via e-mail in 2020 (see Cronk works)
Annie Green Springs (a wine label of the commercial sponsor) was the U.S. national hang gliding championships of 1973.
A main component was a flask, which contains only air. However, because the sensor measured air exiting or entering through a small tube as a result of changes in pressure of the surrounding air, the flask had to be rigid to prevent in bulging or caving in instead. Furthermore, it had to be insulated against temperature changes. (The air generally is colder higher up.) I covered the flask I made, using a length of plastic drainpipe, in layered aluminium foil and transparent plastic. A major problem was that you had to watch the dial (or a couple of pith balls rising in little transparent tubes on mine) to determine whether you were going up or down. In hang gliding, you really need to be looking about you.
That barrier to sensory input was overcome by using a different sense.
More sophisticated variometers were soon manufactured with internal flasks and, critically, with audio tones that indicated lift or sink. You could then maintain a good look out and listen to the variometer to assist in centering on the strongest lift. As far as I know, the first of these was created by USHGA member 007, Frank Colver. Frank’s son Matt used a cobbled together first version (taped to his control bar) in the 1973 Annie Green Springs competition (see Annie Green Springs 1973 briefing photo key). Before the contest was over, pilots were trying to buy it. However, he could not sell it because he needed to make sure he could duplicate it. (*)
See the link to Frank’s web site under External link later on this page.
The early 1980s Flight Designs variometer is similar in shape to the Litek Hummingbird (of which I do not have a photo) but as far as I can discern, the Flight Designs design is more bulky. Although it is electronic, it is (as far as I know) still a flask type, measuring air flowing into and out of the rectangular metal container as the pressure of the surrounding air changes.
The Systek II variometer was about three inches by two by two.
Miniaturization confers some advantages, but it makes the switches harder to use. Some pilots need to change settings in flight with gloved hands.
Gary Dear built this Motronics variometer in 1998 from circuit diagrams and other drawing provided by its designer Kevin Byrne, a former sailmaker at Aerial Arts. (See Southdown Sailwings, Vulturelite, and Aerial Arts of Sussex, England.) I am told that variometers of this type do not contain a ‘flask’ of air, but instead rely on an electronic pressure transducer.
Manufactured initially by Kevin Byrne, hang gliding school Lejair took over production subsequently. (See Lejair (Tony and Rona Webb).) Nowadays Kevin lives in a converted World War 2 airfield control tower near a major hang gliding site in Wiltshire, England.
The Renschler Como is a lightweight and compact multi-function variometer. Very basic and inexpensive by modern standards.
The ‘flight deck’ mounted on the right control frame down-tube in this 1984 photo is fitted with an impeller that protrudes into the air-flow on a stalk to register airspeed. That information is not displayed to the pilot, who flies by the feel of the glider and the sound of the air flow. It is fed instead to the variometer. Because changes in airspeed cause transitory climbs and descents, which impart false impressions of the rise or fall of the surrounding air (a so-called stick thermal) the total energy compensating variometer ignores those transitory vertical motions, so providing the pilot with a truer indication of lift and sink. (For more photos by Jan Kulhavy, see Grouse Mountain invitational 1984.)
Frank Colver, USHGA #7, hang glider designer and creator of the Colver variometer of 1973