500 psi Is Not 50 bar: Imperial and Metric Conventions in Diving

On a liveaboard in the Galapagos, a cruise director briefed the divers to be back on the boat with "500 psi or 50 bar". One of the guests, an American, later told ScubaBoard he had pointed out that the two are not the same number.^{[1 ]} He was right, and the gap is not small. A diver who follows the metric half of that briefing surfaces with 45% more pressure in the cylinder than one who follows the imperial half. Nobody on that boat was confused about physics. They were using two rules that grew up in two unit systems and sound alike when you say them quickly. Diving is full of these, and most of them only matter on the day you rent a cylinder, read a gauge or set a computer in the system you did not learn in.

The reserve that changes size at the border

Start with the arithmetic. One bar is 14.5038 psi. Fifty bar is therefore 725 psi, and 500 psi is 34.5 bar. The difference is 15.5 bar, or 225 psi, which on a single aluminium 80 is about 170 litres (6 cubic feet) of gas. Neither number is the conversion of the other. Each is simply the roundest figure in its own unit near the bottom of the gauge.

So where did the two numbers come from? A long ScubaBoard thread asked exactly that in 2007, and the answers are worth reading because none of them is "because the physics says so".^{[2 ]} One poster pointed at the gauge: a pressure gauge has to span 0 to 300 bar or more, so small differences near the bottom of the dial are hard to read, and "the red line traditionally indicated a minimum tolerance of a gauge". Another pointed at regulators: some lower-cost regulators breathe noticeably worse below about 300 psi, so a reserve keeps them in the range they were built for. Others put it down to history and to a number that is easy to remember. A later thread made the gauge point more sharply. A Bourdon-tube gauge, the mechanical kind most divers carry, reads best in the middle of its range, so the last few hundred psi are exactly where you should believe it least.^{[3 ]}

All of those are good reasons to keep something in hand. None of them produces 500 psi rather than 400 or 600, or 50 bar rather than 40. The figures are conventions. Useful ones, but conventions, which is why they do not survive the trip from one unit to the other.

A pressure is not an amount of gas

The bigger problem with any fixed reserve is that pressure only tells you how full a cylinder is, not how much gas is in it. The amount is the pressure times the cylinder's water volume. At the low pressures a reserve lives at, that simple product is close enough; real-gas effects only start to matter well above 100 bar.

The same 50 bar is 555 litres in a rental AL80 and 1200 litres on a twinset. If 50 bar is the right reserve for one of them, it is the wrong one for the other. The poster who asked about doubles back in 2011 made the same point from the other side: rather than carry a fixed figure from cylinder to cylinder, he wanted to "evaluate my gas needs and dive appropriately".^{[3 ]} That is what technical divers call minimum gas, or rock bottom: work out the gas two divers would need to sort out a problem at the deepest point and share one cylinder all the way to the surface, and make that the pressure you start your ascent with.

Here it is with round assumptions: two divers each breathing a stressed 20 L/min at the surface (40 L/min together), one minute at the bottom to sort the problem out, an ascent at 9 m (30 ft) per minute, and a three-minute stop at 5 m (15 ft), on the EN 13319 depth scale with a 1.013 bar surface.

Two things stand out. At 18 m on an AL80, rock bottom lands near the US habit of 500 psi, which may be part of why that habit stuck in warm-water recreational diving. At 30 m on the same cylinder it is 850 psi, well above both 500 psi and 50 bar, and that is the gauge reading to leave the bottom with, not to arrive at the boat with. On a twinset the picture flips, and a fixed 50 bar is more than the shared ascent needs. Your own numbers will differ with your breathing rate, your ascent and your stops. The method is the point: it turns "how much should I keep" into a figure for this dive and this cylinder, in whichever unit your gauge reads.

What planners and computers do with it

Software has to pick a number, and the choices show how unsettled the convention is. Subsurface, the open-source log and planner, ships its planner with a reserve preference of 40 bar: in recreational mode it checks that the gas above that 40 bar covers the ascent.^{[4 ]} Its manual says a value of 50 bar is often used and lists 40 bar, 50 bar, 25% and a third as the figures agencies assume.^{[5 ]} The same project added a separate minimum-gas calculation in 2017, which prints rock bottom for the bottom gas from a problem-solving time and a stress factor on the SAC.^{[4 ]} Shearwater's Perdix 2 works the other way round: it computes Gas Time Remaining as the time you can stay before a direct ascent would leave you at your reserve pressure, a figure you set yourself.^{[6 ]}

Dive Kit offers five reserve rules, in the Gas Usage tool and in Settings › Diving: Thirds, Fourths, Fifths, Sixths and Fixed. Thirds is the default. On a 200 bar (2900 psi) fill it keeps 67 bar (970 psi) and turns a there-and-back dive at about 131 bar (1900 psi).^{[10 ]} The Fixed rule starts at 50 bar, and with the pressure unit set to psi it reads 725 psi, not 700 or 750. The app keeps the reserve in bar and converts it for the screen, so it shows you the setting it will actually use instead of a rounder number that is not the same reserve.

If you want the US habit, set it. In psi the stepper moves on a 50 psi grid: one press from 725 lands on 700 (48.3 bar) or 750 (51.7 bar), and four more presses down reach 500 psi (34.5 bar). In bar it moves in 5 bar. Whatever rule you pick, Gas Usage flags any plan that ends under 50 bar (725 psi) as critical.^{[10 ]} Dive Kit does not compute rock bottom for you. If you work it out by hand as above, Fixed is the rule to type it into.

Cylinder size: cubic feet at pressure, or litres of water

The reserve problem has a twin in how cylinders are named. In the US a cylinder is named for the gas it delivers at its working pressure; elsewhere it is named for the water it holds. An AL80 is 77.4 cubic feet at 3000 psi, and the same cylinder is an "11.1 litre" at 207 bar.^{[7 ]} Neither name converts into the other without the pressure. A diver who asks a US shop for "a twelve" and a European diver who asks for "an eighty" are both missing half the description. The full story, including why an AL80 holds about 77 rather than 80 cubic feet, is in What "AL80" actually means, and the cylinder sizes table lists both names side by side.

Breathing rate: why psi per minute does not travel

Breathing rate has the same trap in a different place. A surface breathing rate of 20 L/min is 0.71 cubic feet a minute, and that volume is yours: it moves with you from cylinder to cylinder. The pressure rate does not. On an AL80, 20 L/min drains about 26.5 psi (1.8 bar) a minute at the surface. On twin 12s it is 12.2 psi (0.83 bar) a minute. A diver who quotes their SAC as "about 25 psi" is telling you the cylinder they learned on as much as their lungs. Convert a pressure rate to a volume with the cylinder it was measured on, then carry the volume. The SAC/RMV calculator does that conversion, and SAC, RMV and SRMV sorts out the names.

Depth: round pairs that are not conversions

Training materials pair depths that are close but not equal: 18 m and 60 ft, 30 m and 100 ft, 40 m and 130 ft. In fact 60 ft is 18.3 m, 100 ft is 30.5 m and 40 m is 131 ft. On a dive the difference is a few tenths of a metre. In a calculation that ends in a limit, it decides which way you round.

The maximum operating depth (MOD) is the clearest case. EANx32 at a PPO₂ of 1.4 bar has a MOD of 33.6 m on the EN 13319 scale, which is 110.2 ft. Round that down in feet and you get 110 ft. Round it down to 33 m first and then convert, and you get 108 ft, a figure 2 ft shallower than the gas allows. Rounding down is the right direction for a limit; doing it twice, once in each unit, throws depth away for nothing. Dive Kit's gas tools compute the MOD in metres and convert the exact figure, so Gas Specs shows EANx32 at 1.4 bar as 33.6 m in metric and 110.2 ft in imperial: 33 m or 110 ft once you round down for a plan.^{[9 ]} The textbook versions differ again: (1.4 ÷ 0.32 − 1) × 10 gives 33.75 m, and the same formula with 33 ft per atmosphere gives 111.4 ft. The MOD calculator shows the computed figure in either unit.

Water: 10 metres per bar, 33 feet per atmosphere

Those two textbook formulas hide the next convention. The metric rule of thumb is that 10 m of sea water adds 1 bar; the unit behind it, the metre of sea water (msw), is defined as a tenth of a bar. The imperial rule is that 33 ft of sea water adds one atmosphere, from a sea-water density of 64 lb/ft³.^{[8 ]} They are not the same rule in two units. 33 ft is 10.06 m, and one atmosphere is 1.013 bar, so each rule is consistent with itself and they disagree with each other by just under one percent. Mix them, 10 m per bar on one line and 33 ft per atmosphere on the next, and the error compounds.

Real water varies more than either rule. Dive computers measure pressure and turn it into depth with an assumed density, and many use the EN 13319 instrument standard, 1.020 kg/L, which makes 10 m per bar exact. In fresh water the same computer reads about 2% shallow: 29.4 m when you are at 30.0 m.

Dive Kit uses EN 13319 by default in every tool, so a plan and a computer on that scale read the same depth for the same pressure, and offers Fresh, Salt and Red Sea for the true depth in that water. The conventions page lists every value and how far it moves a result.^{[9 ]}

Bar, psi, gauge, absolute and ATA

A cylinder gauge reads zero at the surface. It shows gauge pressure, the pressure above the air around it. Gas physics needs absolute pressure, gauge plus the atmosphere: 1.013 bar, or 14.7 psi, at sea level. The US writes the difference into the unit (psig for gauge, psia for absolute); metric divers usually say "bar" for both and let the context decide. The diving word for absolute pressure in atmospheres is ATA, so 99 ft of sea water is 4 ATA by the 33 ft rule.

At 30 m on the EN 13319 scale the water adds 3 bar and the absolute pressure is 4.01 bar (58 psia). Your breathing rate, your PPO₂ and your gas density all scale with the 4.01, not the 3. Cylinder pressures stay gauge pressures everywhere on a dive. Dive Kit shows gauge pressure on screen and converts to absolute inside every mixing and top-up calculation, which is why a blending result can differ slightly from the gauge-only sum taught on courses.^{[9 ]}

Temperature and lead

Temperature is the easy one, once you know a few anchors: 28 °C is 82 °F, 20 °C is 68 °F, 10 °C is 50 °F and 4 °C is 39 °F. The formula is °F = °C × 1.8 + 32. It matters for exposure protection, and it matters for gas: a cylinder filled hot loses pressure as it cools in the water.

Lead is where a unit mistake shows up on the first descent. A diver who wears 6 kg at home and asks a US shop for "six" gets 6 lb, which is 2.7 kg, and is 3.3 kg light. Going the other way, 6 kg is 13.2 lb, so the same diver needs about 13 lb. A 2 lb block is 0.91 kg, so a belt built from US blocks rarely lands on a round kilogram figure. The cylinder adds its own swing: an AL80 goes from about 0.8 kg negative full to about 2 kg positive empty, a 2.8 kg (6 lb) change over the dive. The weight calculator works in either unit, and a check dive with a near-empty cylinder settles the rest.

When you dive in the other system

• Reserve: ask whether the briefing means a turn pressure or a surfacing pressure, and convert it. 50 bar is 725 psi; 500 psi is 34.5 bar.
• Cylinder: get the water volume and working pressure, not only the nickname. An AL80 is 11.1 L at 207 bar.
• Breathing rate: carry it as a volume (L/min or cu ft/min), never as bar or psi per minute.
• Depth limits: convert the exact figure and round once, down. EANx32 at 1.4 bar is 33 m or 110 ft, not 108.
• Water: know which density your computer assumes, and expect a fresh-water depth to read about 2% shallow on an EN 13319 setting.
• Lead: convert before you ask. 1 kg is 2.2 lb.

The unit converter covers pressure, volume, depth, temperature, weight and breathing rate, and shows the factor it used. The gas consumption calculator takes either unit for the reserve rules above. None of this replaces a briefing, your training or your own gauge. It just makes sure the number you heard and the number you dive to are the same number.