It's amazing to see the number of features packed into the watches of today. You will find watches with a built-in perpetual calendar, tachometer (to measure the speed of a moving object), asthometer (to measure rate of respiration), pulsometer (to measure the heart rate), and much more. You can even find watches that tell you the lunar time, i.e. the time of high tide and low tide in your region! An altimeter watch displays the altitude of a place i.e. the height of the place above sea level. The purpose of such a watch is to show an approximate value of the altitude of a place. Say you are out on a hiking trip, and wish to know how far you have climbed above the sea level. In such a scenario, an altimeter watch is exactly what you would need. Just one look at your altimeter watch, and you'd be aware of the altitude of the point where you are standing!
If you go looking for an altimeter watch, you'd see that not all of them are the same. They vary in their strengths and to some extent, in their accuracy. Also, most altimeter watches do more than just displaying the altitude of a place. These devices come with a built-in compass to help you find the directions correctly, and sometimes are designed to measure temperature and your heart rate, or even to keep a track of the changing weather pattern in your region. Mostly water-resistant, some altimeter watches can even measure underwater depth. A standard altimeter watch weighs between 1.5 to 2.5 ounces.
Working of an Altimeter Watch
Knowing what factors go into calculating altitude and how environmental conditions affect that output, is important when you need to quickly determine if the altitude given is accurate or not. There is a relationship between atmospheric pressure and altitude. The higher you go above the sea level, the lower is the atmospheric pressure. In other words, there is a decrease in the atmospheric pressure with increase in altitude. Atmospheric pressure is nothing but the force exerted by the weight of air above a surface, per unit area of the surface. So, we can safely conclude that lesser the volume of air above a surface, lesser the atmospheric pressure. Now, we know that the thickness of the Earth's atmosphere is constant (close to 400 km). So, as you move away from the sea level, the volume of air above you goes on decreasing, which is the reason for the decrease in atmospheric pressure.
Amazing as it may sound, the principle behind an altimeter watch is quite simple. All that an altimeter does is convert the barometric pressure reading to an altitude reading. It uses the changes in atmospheric pressure to determine the changes in altitude. An altimeter watch has a barometer built within, which measures the atmospheric pressure. The first thing the watch does is finding the value of the atmospheric pressure in bars. Once the value of atmospheric pressure has been determined, it is used to calculate the altitude, which is then displayed in meters or feet. However, it is necessary to note that the value displayed by the altimeter is not always accurate. Most of the time, the altitude reading shown by the altimeter varies from the real altitude, by about 3 feet or so. In spite of the fact that altimeter watches do not always show the correct readings, they are popular just because they are portable and easy to use.
Effect of the Weather on an Altimeter Watch
Even at a particular altitude, the reading of an altimeter is not always constant. This is because the atmospheric pressure is affected by changes in temperature, and changes in the atmospheric pressure, in turn, affect the reading on an altimeter. When the temperature is high, the air above the region rises, as a result of which there is a drop in the atmospheric pressure. Similarly, when the temperature drops, there is an increase in atmospheric pressure. A change in atmospheric pressure equal to 1 millibar can result in a difference of 7-8 meters in the reading of an altimeter. Greater the difference in pressure, greater the variation of the altimeter reading. So, sitting at the same spot, you will find the altimeter showing different readings at different times!
Altimeter watches are sensitive to the slightest change in atmospheric pressure. So, in spite of all the ways in which an altimeter can be of help to you, it remains a fact that no such watch can give you accurate results all the time. Does this mean that you stop using one? Well, not exactly. An effective way to tackle the problem of inaccurate or fluctuating readings is to calibrate the altimeter at regular intervals, and this is simple: just set your altimeter watch to a known altitude. One look at a topographical chart, and you'll find locations with the respective altitudes mentioned. Just walk to any of these locations and set your altimeter watch accordingly. While calibrating your watch frequently can seem irksome, doing so will ensure your watch displays the correct readings. The instructions for calibrating your watch can be found in the manual that comes with the watch itself. So, it is essential that you calibrate it at least once a day, or even as frequently as every few hours when the weather changes.
Uses of an Altimeter Watch
Not only does an altimeter watch display the altitude, but also a plethora of other parameters such as the temperature, direction and speed of ascent, distance of elevation, and so on. The primary and the most popular use of altimeter watches is in adventure sports, which includes rock-climbing, hiking, mountaineering, skiing, skydiving, ballooning, surfing, snowboarding, and aviation, among others. Some altimeter watches come with auto-calibration, which means that the watch calibrates itself with reference to a built-in GPS map. Choosing such a watch will save you from calibrating it manually. However, even GPS elevations are not always accurate, and so even with an auto-calibrated watch you will end up with an error in the 30-60 feet range. Thus, calibrating the device at regular intervals is the key, which can either be done using a known altitude or a known atmospheric pressure value. This way, you can ensure that the reading on your altimeter doesn't vary widely from the real altitude of the place in question.
This was a brief description about altimeter watches and their function. What you need to remember is that your altimeter watch should in no way be the only thing you rely on for determining the altitude. It can, at best, be used as a supplement with topographical charts and other more precise instruments.