air supply giving the ability to very quickly determine how much time remains for safe diving. Surface supplied divers can use the same to make quick calculations of distance traveled over time. Metered bezels: Many chronographs have a bezel, that is either fixed or can rotate, around the outside of the dial that is marked to specific scales to allow rapid calculations. While any wristwatch can have a bezel, the chronograph stop start feature, as well as the rotation of the bezel, allows more complex calculations or repeated measurements for a series of calculations. The most popular meter is for Tachymeter readings: a simple scale that allows rapid calculations of speed. Other bezels feature Telemeter scale, for distance. The watchmaking company Breitling offers a model with a rotating bezel, in conjunction with another, fixed, meter on the dial, scaled for use as a slide rule for more complex calculations. Flyback chronographs have a timing hand that can be rapidly reset, or flyback, to zero. Ordinarily the sweep second hand is stopped to record the time and started again at that spot on the dial, or reset by spinning the second hand all the way to zero again, clockwise. The flyback allows a reading and a quick reset—a counterclockwise flyback—for the next measurement to start at zero. A rattrapante, sometimes called a double chronograph, has multiple second hands, at least one of which can be stopped and started independently. When not activated, the second hands travel together, one under the other, to appear as just one second hand. A tourbillon, although not strictly limited to chronographs, is an escapement set in a cage and placed in a rotating balance in order to minimize the effects of gravity on the escapement and increase precision. Because chronograph escapements are generally larger and connect with more complications, a tourbillion in a chronograph will differ from a tourbillion in a more simple timepiece. Other types of modern-day chronographs are the automatic chronograph and the digital chronograph. The automatic chronograph depends solely on kinetic energy as its power source, while the digital chronograph is much like the common stopwatch and uses a battery to gain power, as well as quartz for timing. Other, more specific, types of chronographs include split second chronographs, tide chronographs, and asthometer chronographs. Each of these chronographs has an added feature that sets them apart. Telemeter The telemeter chronograph allows the user