Published: July 12, 2014
When you arrive at Skydive the Wasatch for your tandem skydiving debut, you'll notice that we're all wearing funny watches. If you take a closer look, it'll become obvious that they're not a watch at all - they're visual altimeters, and we use them to keep us (and you!) safe. Here's what they are and what they do.
Types of Skydiving Altimeters
There are three different kinds of altimeter that you'll see in skydiving: analog, digital and audible. They look different, but they use the same method to function and do the same thing: keep the skydiver aware of his or her altitude.
- Analog visual altimeters are the form of altitude indicator you'll be introduced to on your tandem skydive. These gadgets show the altitude on a large, round dial (much like an oversized watch). They're typically worn either on the wrist or on a triangular "pillow" in front of the torso that allows the jumper to reference it while the arms are extended. The altimeter's dial spins to 12,000 feet, but it works past that point: once the altitude advances past 12,000', one simply adds its indicated number to 12,000' to determine the altitude. The yellow area of the dial represents the "danger zone," and red indicates the altitudes at which it is absolutely necessary to have a parachute overhead.
- Digital visual altimeters are more advanced, and are very common among dedicated sport skydivers for several reasons. Firstly, they're much smaller and more streamlined. Second, they're multi-purpose: as well as showing the altitude, they record skydive data in the form of an electronic logbook, storing information about the last 200 jumps and allowing for download to a computer. Many are also backlit, making them suitable for night jumps.
- You probably won't actually see an audible altimeter, as they're worn inside the skydiver's helmet. Audible altimeters are used as a backup indicator, together with a visual version. They make a loud noise when the jumper reaches a pre-set breakoff altitude, then sound again when the jumper reaches a pre-set deployment altitude. They're worn in the helmet in order to make sure the jumper can hear them during while in freefall, so the data display doesn't need to be large and easily readable.
How a Skydiving Altimeter Works
Skydiving altimeters work barometrically, determining altitude above a pre-set point using changes in atmospheric pressure. (This is why they're also called "pressure altimeters.") The relationship is very simple: the greater the altitude, the lower the pressure.
You may see your tandem master set his or her altimeter to recognize the landing area as "zero." (In Salt Lake City, skydiving starts high up: our starting altitude is higher above sea level than most dropzones, so we "tell" our altimeters where the ground is.) As you gain altitude, you can look over and watch the dial spin or the number on the digital readout rise along with you. After you exit the aircraft, the increase in pressure causes the dial to drop.