Some dive computers have the ability to monitor a sensor that allows them to provide a digital display of the gas pressure in the dive cylinder. This feature is often referred to as 'air integration' (AI) but since nearly all modern dive computers are capable of handing a variety of breathing gases the term air is a misnomer. A few of the AI computers go a step further and use the tank pressure to calculate an estimate of the breathing gas time remaining (GTR). There are a variety of different methods and formulas for GTR calculations but they all share limitations such that, for specific divers and specific dives, the GTR displayed by your computer can be wildly inaccurate.
Just to be clear, the accuracy of the GTR is not a limitation of the computer or the transmitter but because there are simply too many unknowns to make the GTR anything other than a gross approximation. Beyond the obvious factors such as environment, depth variations in a multi-level dive profile, ascent rates, and stop times; the divers gas consumption rate will vary greatly depending on changes in equipment configuration and exertion. For this reason, GTR estimates are meaningless on most technical dives and even some deeper sport dives, particularly if hunting.
Another factor that can affect the GTR depends on the algorithm used for making the estimate. Some methods require the diver to know and enter an RMV consumption rate along with the volume of gas available (i.e. 'tank size'). Other methods use an observed SAC rate to develop a prediction based on consumption history and may optionally require the cylinder rated service pressure be entered. Computers that calculate GTR based on observed SAC rate history are particularly problematic when the diver uses different capacity cylinders on different dives.
While the GTR can be useful in warning inexperienced sport divers when gas consumption is abnormally high, Dive Gear Express does not recommend relying upon dive computer GTR estimates to control or extend the dive time. Never allow the computer GTR estimate to become part of your dive plan, always closely monitor your actual tank pressure.