Conventions: why Dive Kit's numbers differ from the formulas

What it is

Divers learn a few rules that make pressure maths possible in your head: 10 m of water adds 1 bar, the surface is 1 bar, a cylinder holds its volume times its pressure. They are good rules. They are also approximations, each off by a little. Dive Kit is a computer, so it does not need them: every tool uses the physical model instead. It takes the real air pressure at the surface, the density of the water you picked, and real-gas behaviour for what is inside a cylinder.

The differences are small, usually a few tenths of a metre or one percent of oxygen. When Dive Kit’s number and your mental arithmetic disagree slightly, this page is why. Every value below is defined once in Dive Kit’s calculation core and shared by the app, the website and the Dive Kit assistant tools, so they all print the same number for the same input.

Surface pressure: 1.01325 bar, less at altitude

The textbook anchors every depth at 1 bar of air. Real sea-level air pressure is 1.01325 bar (one standard atmosphere), and Dive Kit uses that. Every conversion between depth and absolute pressure is:

Absolute pressure = surface pressure + depth ÷ metres per bar

At altitude the surface pressure drops. Dive Kit uses the standard atmosphere for the altitude you enter:

Altitude	Surface pressure
Sea level	1.013 bar
500 m	0.955 bar
1000 m	0.899 bar
2000 m	0.795 bar
3000 m	0.701 bar

The deco engine receives the same figure (1013.25 mbar at sea level), so the schedule, the gas tools and the PPO₂ column all start from one surface.

Water density and metres per bar

A metre of sea water weighs more than a metre of lake water, so a bar of pressure takes fewer metres. Dive Kit keeps one table of water densities and derives metres per bar from it: metres per bar = 100 ÷ (density × 9.80665).

Water Density setting	Density	Metres per bar
Fresh	1.000 kg/L	10.20
EN 13319 (default)	1.020 kg/L	10.00
Salt	1.025 kg/L	9.95
Red Sea	1.028 kg/L	9.92

EN 13319 is the default in every tool, the deco planner and the buoyancy calculator included. It is the European standard for dive instruments, and many dive computers use it by default: their depth reads 10.00 m for every bar of water pressure. Planning in EN 13319 means your plan and such a computer read the same depth for the same pressure, and it makes “10 m per bar” exact. Pick Salt, Fresh or Red Sea when you want the true depth in that water.

Gas switches: a 0.03 bar PPO₂ tolerance

A gas counts as breathable at a depth while its PPO₂ there is no more than 0.03 bar above its cap. The MOD Dive Kit reports stays exact (the depth where PPO₂ equals the cap). The switch rule and every PPO₂ warning allow the 0.03 bar alike, so a gas the planner switches onto is never flagged as over its cap.

The reason is the 6 m stop. With the real surface pressure, pure oxygen at 6 m in EN 13319 water sits at 1.613 bar, a hair over a 1.6 cap, and its MOD is 5.8 m. Read strictly, no planner would put you on oxygen at the stop it exists for. The tolerance keeps that switch in every water type (the Red Sea is the worst case, 1.618 bar at 6 m) without letting a gas creep meaningfully deeper: 0.03 bar is about 0.3 m of depth for oxygen and 0.6 m for EANx50. A 7 m oxygen stop (1.71 bar) is still refused.

Air is 21% oxygen and 79% nitrogen

Dive Kit treats air as 21/79, the convention every training agency teaches, in blending, EAD, END and gas density alike. Real dry air is 20.95% oxygen with almost 1% argon; treating it as 21/79 changes a gas density figure by about 0.35%, below what any screen shows.

Real gas, not ideal gas

At fill pressures, compressed gas does not follow “volume × pressure”: an AL80 at 3000 psi holds about 4% less air than that product says. Dive Kit uses compressibility (Z) tables for oxygen, nitrogen and helium mixtures, evaluated along a 15 °C isotherm, for cylinder contents, gas usage, remaining pressure, the SAC/SRMV calculator’s measured litres and every blending calculation. A measured SRMV therefore plans the same dive back to the same end pressure; the ideal figure read a few percent high, because near 200 bar a bar of air holds about 12% fewer litres than volume × pressure says. SAC in bar per minute is the gauge drop over the load and does not go through litres, so it is not the ideal conversion of the SRMV. The blender assumes 20 °C ambient and a fill at ambient temperature unless you enter another fill temperature; a hot fill shows its settle step as it cools.

Gauge and absolute pressure

Cylinder pressures on screen are gauge pressures, what a gauge reads (zero at the surface). The physics (Dalton’s and Boyle’s laws, real-gas tables) needs absolute pressure, gauge plus surface pressure. Dive Kit converts at that boundary, so every mixing and top-up calculation runs on absolute pressure. That is why a top-up result differs slightly from the gauge-only sum taught in blending courses.

Defaults

• Cylinder: one aluminium 80, 11.1 L of water volume filled to 206.84 bar (3000 psi), whenever you have not picked a cylinder.
• Reserve: the rule of thirds; the fixed-reserve option starts at 50 bar. A cylinder below 50 bar is flagged Critical in the planner.
• Gas density: computed at 15 °C (59 °F) water unless you set a temperature. Limits are 5.2 g/L (caution) and 6.2 g/L (critical).
• END warning: 30 m (98 ft).
• PPO₂ caps: 1.4 bar for lean mixes and air, 1.5 for mid mixes, 1.6 for rich mixes and oxygen.

Rounding

• Best mix rounds oxygen down to a whole percent (never richer than the depth allows) and helium up (never more narcotic than you asked for).
• MOD rounds down: to 0.1 m where it is a fact about a gas, to a whole metre where it is a recommendation (Best Mix).
• Limits compare after rounding to what the screen shows, and trip only when strictly past: density at one decimal, END and OTU at whole units, the hypoxic PPO₂ floor at two decimals. A density that reads 5.2 g/L is not over a 5.2 limit. PPO₂ against a cap is the exception: it uses the 0.03 bar tolerance above, not rounding.

Worked comparisons

Salt water unless stated. “Rule of thumb” uses 10 m per bar and a 1 bar surface.

Question	Rule of thumb	Dive Kit	What moves it
MOD of EANx32 at 1.4 bar	33.75 m	33.4 m (33.6 m in EN 13319)	real surface, 9.95 m per bar
MOD of EANx32 at 1.6 bar	40.0 m	39.6 m	same
Best nitrox for 30 m at 1.4 bar	35% (EANx35)	34.7%, so EANx34	oxygen rounds down
EAD of EANx32 at 30 m	24.4 m	24.4 m	the offsets cancel
Oxygen at the 6 m stop, EN 13319	1.60 bar	1.613 bar; MOD 5.8 m; still switches at 6 m	0.03 bar tolerance
Air in an AL80 filled to 3000 psi	2,296 L	2,214 L	real gas, absolute pressure
Topping 100 bar of EANx32 with air to 200 bar	26.5%	26.7%	absolute pressure, real gas

Compared with other tools

Many dive computers use the EN 13319 scale by default, so in the default water type your plan and such a computer agree on depth. Planners that use the 1 bar = 10 m convention work on the same depth scale; the remaining difference is the surface, 1.013 bar in Dive Kit where the convention uses 1 bar, which moves a MOD or a switch depth by about a tenth of a metre. How Dive Kit’s schedules compare in detail is on Compared to MultiDeco.

When to still use the rules

Use the rules in the water, on the boat, and in your agency’s exams. They land within about half a metre and one percent of oxygen of Dive Kit’s numbers. The textbook MOD is slightly deeper than the physical one, so keep the margin you were taught. Use Dive Kit’s numbers for the plan itself.