If you are into watches with altimeters you need to know the basics of how they function. They're not all that complicated, but it is good to understand how they work so you can draw inference from the data they give you.
The altimeter readout on the timepiece is simply a translation of barometric pressure. The barometric pressure technology is what drives much of these watches' altitude and weather predicting capabilities. Thus, without a barometer, these watches would be just another watch. The small aneroid wafer barometer technology that is housed inside the watch case is really the brains of the entire device.
But, how does the watch read barometric pressure? I mean, the darned thing is so small - how can it possibly contain something as bulky as a barometer? What the devices were in the beginning there was no way that these things were going to be functional on such a small scale. However, with miniaturization of electronics, there are ways of sensing air pressure without having to actually have a column of mercury involved.
Well, there are 2 basic types of barometers - mercury and aneroid. Both measure outside air pressure and both measure that outside air pressure in terms of "inches of mercury".
Here are the 2 types described in a little more detail:
1. Mercury barometer - this is the original design and the one that really defined the science of weather prediction. The mercury serves as a translator of atmospheric pressure. That is, the pressure on the mercury by the air causes it to depress downward and make the level of the fluid rise in the tube.
2. Aneroid - this is the type of system that drives the altimeter in an airplane. It is very small and senses the air pressure and aneroid wafer meaning a wafer that has a vacuum inside. It is very sensitive to changes in pressure from the outside air. This is really what makes it possible to have watch altimeters in the first place. This is the only way you could get this type of information in the space so small.
Watches with altimeters couldn't function without barometers. The precision of the barometer in the timepiece will determine how much good you get out of the altimeter function on the watch. Without the miniaturization of electronics that preceded these watches with altimeters, you would have to keep each one of these devices separately maintained - and altimeter, a compos, a barometer, a title chart. All these things would have to be housed in separate devices.
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