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The first variometers used a vessel or flask that was sealed from ambient air. The flask had a tap on it to attach a piece of tubing. If altitude was constant the pressure inside the flask would equal the ambient pressure outside the flask. As a change in altitude would occur air would have to either flow in or out of the flask as it constantly tried to maintain pressure equilibrium. By measuring the amount of flow it was possible to derive a rate of altitude change and by measuring the direction of flow you could determine ascent or descent. Many years later piezo resistive pressure sensors were developed which are basically very small thin diaphragms with strain gauges mounted on the surface. The diaphragm separates two chambers. One being a vacuum and the other exposed to atmosphere. As ambient pressure changes it deflects the diaphragm "straining" the gauges which change the electrical resistance of the sensor. Piezo resistive sensors unfortunately tend to require a fair amount of battery power. Approximately 20% of any vario goes to power just the sensor. There are ways to decrease the amount of power they take but other sacrifices are made to do so. A general rule of thumb in any electronic device that is battery powered is the longer the battery life the more thorough the engineering. Response time is another key element in designing a vario. If it responds to fast to the median in which we are measuring it becomes incomprehensible and annoying. If its to slow we lose valuable information. Response time is defined as the time it will take for an instrument to report 63% of a change. If an instrument reads 0 to 100, its median is stable and the response time is 1 second then if the median goes from 0 instantly to 100 the instrument will read 63 after 1 second. Response time often gets confused with sensitivity which is something entirely different. Sensitivity of an instrument is how little of a change of the median being measured can be reported. A watch with a second hand has better sensitivity than one with only two hands. If a vario only reported changes every 100 feet it wouldn't be near as sensitive as one that reports every 10 feet. With audio variometers sensitivity becomes an important issue. With a visual gauge everyone sees the same thing 100=100. With audio 3khz might sound like 3.2khz to a different person. To the human ear, being not as accurate and repeatable as the eyes, its smarter to use 2 things to help differentiate between 100 feet and 110 feet rate of climb. This is why the better audio varios "chop" the frequency so that the number beeps increase as well as the tone of the beeps as climb rates increase. Zeroing or zero adjust refers to what the instrument reports at rest. All altimeters come with a zero adjustment due to changes in barometric pressure. With varios however its not necessary because barometric pressure changes over a much slower and longer time frame compared to flying. High end microprocessor based variometers are able to include automatic zero settings in their programming. With smaller analog varios its necessary to have a zero adjustment to accurately set a zero due to the extreme sensitivity of the instrument. Zero adjust and auto zeroing are two different things. Zero adjust is the ability to set your zero at rest at any altitude at any battery level. Auto zeroing means that once you've set your zero it will be the same at any altitude and any battery level even if you change the batteries from nearly dead to brand new. There are several varios on the market that have zero adjust but NOT auto zero. Its very unacceptable to set your zero at takeoff then have to reset it at 10000 ft.! |
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