| By :
Roger Brown
Owning and using watches with altimeters is a great way to stay on top of weather conditions on your outdoor treks. They also supply a lot of other information than altitude that can make your outdoor experience safer and more relaxing. But, how do they do what they do? What is the mechanism that allows them to tell altitude? First of all you need to understand that all an altimeter watch does is give a different readout for barometric pressure. It is no more complicated than that. All watches with altimeters operate on outside air pressure. The operative device is actually just a barometer. This is how a barometer works: First, what a barometer does is sense air pressure and makes certain correlations between that and the altitude that you are. That is not a linear relationship however. The outside air pressure where you are is a function of other variables like temperature and humidity too. The barometer is an instrument that measures outside air pressure in relation to some other fluid media, usually mercury. Mercury is usually chosen as the media because it is a heavy fluid. The standard barometer takes a relative pressure reading. That is, it senses the pressure of the air on some other media. The barometer in a watch is an aneroid barometer meaning it contains an aneroid wafer that senses the air pressure. The aneroid barometer style is chosen because it can be more easily miniaturized and placed in a housing small as a watch. Temperature is not calculated in with barometric pressure to figure altitude in a watch. Instead only barometric pressure is considered. That is why these devices need constant calibration. To calibrate and altimeter you need to be at a static reference point - like a certain height that you know for sure is right. A common way to do it is to find a place of known height - like for example somewhere that you know for sure on a terrain map is absolutely accurate and use that is your place to calibrate. You need to probably do that several times during the course of the trip or maybe even several times a day if the weather is changing rapidly. The reason for that is so many different things you can influence atmospheric pressure that is rapidly changing nearby you in order to get an accurate altimeter reading. It is amazing that watches with altimeters can be so small and contain so much data and be so helpful to hikers and outdoorsmen.
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