Gas Usage

What it is

The Gas Usage calculator works out how much breathing gas a planned dive will consume and whether the cylinder you packed will get you through it with a reserve to spare. You describe the dive as one or more steps (a depth and a time each), tell it your cylinder and your breathing rate, and it tracks the pressure draining out of the cylinder as the dive runs.

It is a planning tool for the surface. Build the profile before you splash, see where your gas runs low, and adjust depth, time, or cylinder size until the numbers work. In the app this screen is titled Gas Usage.

How to use it

Inputs and outputs

Everything follows your unit settings. Depth reads in metres or feet, pressure in bar or psi, and volume and SRMV in litres or cubic feet, all set from the app’s unit toggle.

Field	Meaning	Notes
Single cylinder / Twin Set	Cylinder configuration	Twin Set doubles the effective volume
Cylinder size	One cylinder’s size, in your sizing language	Metric: water volume (capped at 30 L); imperial: capacity at working pressure, with a hint showing the derived water volume
Start Pressure	Pressure in the cylinder at the start	Minimum 50 bar (725 psi)
SRMV	Your surface breathing rate	Per minute; 5 to 50 L/min (0.18 to 1.77 ft³). Measure it with the SAC/SRMV tool
Depth	Depth of the current step	Drives the ambient pressure; 1 to 500 m (3 to 1640 ft)
Time	Minutes spent at that depth

The tool opens pre-filled with sensible defaults: 12.2 L cylinder, 200 bar start pressure, 20 m depth, 20 minutes, and a 20 L/min SRMV. The fields are bounded, so if you type a value outside the ranges above the tool clamps it.

The calculator converts depth to an ambient pressure (about 1 bar of water for every 10 m, set by your Water Density setting), multiplies by your SRMV and the time to get the surface-equivalent litres used, then turns that back into pressure drained from your cylinder. Water Density offers four options (Fresh, EN13319, Salt, or Red Sea), with salt water at the familiar 10 m per bar.

Output	Meaning
Consumed Gas	Surface-equivalent volume (litres or cubic feet) used for the current step, before you save it. The pressure effect shows in the remaining value beside it
Dive Profile	Your saved steps, each with its gas used, the gas remaining after it, and the cumulative runtime
Summary	Total gas consumed, remaining pressure, a used/remaining progress bar, and a reserve warning

Turn pressure and your reserve rule

Two different numbers matter here, and the card prints both side by side under the cylinder inputs: Turn pressure and Reserve. Both follow your pressure unit.

Turn pressure is where you turn the dive around. It is the pressure at which half of your usable gas is gone, so the same amount is left to get back out. On a 200 bar (2900 psi) single with the rule of thirds that is about 131 bar (1900 psi), not 67.

Your reserve is the floor you want to surface above: 67 bar (970 psi) on that same fill. It is what the warning fires on, at the end of the plan rather than at the halfway point. The warning names the number and the rule it came from, so it reads “Remaining gas is below your reserve (67 bar, Thirds)” and you can tell a rule-of-thirds nudge from your own fixed 50 bar target without opening Settings.

Pick the rule in Settings → Diving. All five apply to the whole cylinder:

Rule	Keeps in reserve	Turn on a 200 bar (2900 psi) fill
Thirds	A third of the fill	About 131 bar (1900 psi)
Fourths	Half the fill	About 148 bar (2150 psi)
Fifths	Three fifths of the fill	About 159 bar (2300 psi)
Sixths	Four sixths of the fill	About 166 bar (2400 psi)
Fixed	The pressure you type	Depends on your target

Thirds is the usual choice for overhead and demanding dives. Fourths and fifths are common in cave diving when flow, distance, or team rules call for more in hand. Fixed suits the open-water habit of “be back on the boat with 50 bar (725 psi)”. A fixed target larger than the fill you enter is capped at that fill, and the card then shows the same number twice: you have no usable gas on that fill.

The turn pressures above are slightly under the arithmetic you may expect, because Dive Kit works in real litres rather than treating pressure as volume. A cylinder’s top 50 bar holds a little less gas than its bottom 50 bar.

Watch the step in your profile where the remaining gas first crosses your turn pressure: that is the point in the dive where you should already be heading up.

SAC vs SRMV

You enter a breathing rate here. The field is labelled SRMV in the app, and there are two ways divers express a breathing rate.

SRMV (surface respiratory minute volume) is a true volume per minute at the surface, in litres or cubic feet per minute. It belongs to you, not your cylinder, so the same SRMV works whatever cylinder you pick. This is the value Gas Usage asks for.

SAC (surface air consumption) is a pressure drop per minute for one specific cylinder, in bar or psi per minute. It is handy at the fill station but it changes the moment you switch cylinder sizes, so it is not portable between cylinders.

Measure either one from a real dive with the SAC/SRMV calculator, then feed the SRMV into this planner.

Behaviour and limits

• The plan is only as good as the SRMV you give it. Your real breathing rate climbs with cold, current, exertion, and stress, so plan with a rate from a comparable dive, not your calmest ever.
• It models gas for an open-circuit recreational profile. It does not build decompression schedules or auto-switch deco gases. For staged deco dives use the Deco Planner, which estimates gas required per cylinder as part of the schedule.
• Numbers reflect exactly what you typed. Confirm your actual fill pressure and analysis before the dive.
• Remaining gas uses Dive Kit’s real-gas (compressibility) model, the same one behind the cylinder sizes. At a full cylinder it reads a little lower than the simple “pressure times volume” rule, which is the safe direction: it never tells you that you have more gas left than you really do.

Related

• SAC/SRMV calculator measures the SRMV this planner needs.
• Deco Planner handles gas planning for staged decompression dives.