Arterial gas embolism is the leading causing of non-accidental injury or death to a diver. This is the result of a too fast ascent, i.e. an ascent that exceeds the safe limit of 60 feet per minute. Because this rate happens to be approximately equal to the ascent-rate of the smallest exhaust air bubbles, the diver uses them as a rough indicator. Air bubbles however, expand while ascending (which increases their ascent rate) and then burst into smaller ones, which makes this technique less reliable.
To achieve a more accurate and safe ascent, a simple apparatus is required which does not measure ascent-rate in vertical feet/second. but is sensitive to pressure change/time. This latter unit is more relevent to the law of physics that applies to the diver with respect to the pressure/volume relationship as stated by Boyle's law. For example, the air volume increase during a rise from 90 to 60 feet depth is 8%, whereas from 30 feet to the surface (0 feet) the increase is 100%. Thus, it is more important to measure pressure-change/time than ascent-rate/time, because even when the latter is constant, the pressure-change/time is not.