PSI to Atmospheres

What a Cylinder Gauge Reading Means in Atmospheres

Every gas cylinder on a rack carries a number stamped near the neck and another one showing on the regulator's high-pressure gauge, and in North America both are in psi. Expressing that reading in atmospheres turns it into something you can reason with: an atmosphere is one filling of a container at ambient pressure, so the atmosphere count is roughly how many container-loads of gas are packed inside. That is the arithmetic behind cylinder capacity, purge calculations and the reason a compressed-gas cylinder is treated as a hazard even when it is full of nothing more exciting than nitrogen.

The factor: 1 psi = 0.068 045 964 atm, because one atmosphere is 14.696 psi. A size K oxygen cylinder charged to 2 200 psig therefore sits at 2 200 × 0.068 045 964 = 149.70 atm above ambient, which is 150.70 atm measured absolute.

Three Kinds of Cylinder, Three Kinds of Gauge Reading

Permanent gases

Oxygen, nitrogen, argon, helium and hydrogen stay gaseous at room temperature, so the gauge falls steadily as gas is drawn off. Pressure is a genuine contents gauge for these.

Liquefied gases

Carbon dioxide, nitrous oxide and propane sit as liquid under their own vapour pressure. The gauge holds almost steady until the last liquid boils away, so it tells you temperature, not contents.

Dissolved acetylene

Acetylene is unstable when compressed on its own, so the cylinder is packed with a porous mass soaked in acetone and the gas dissolves into it. Service pressure stays near 250 psi as a result.

Residual pressure valves

Many cylinders now hold back a small positive pressure — a fraction of an atmosphere — so that air, moisture and contamination cannot creep back into an emptied shell.

From the Regulator Gauge to an Atmosphere Count

The sequence below is how a cylinder reading normally gets used: read it, convert it, work out what it is worth, and then check a supplier's metric datasheet against it.

Enter the high-pressure gauge reading

Type what the inlet gauge shows — 2200, 1450, 250, whatever it is — into the left field. The atmosphere figure resolves on the right as each digit lands, and a comma typed as the decimal mark works the same as a dot.

Turn atmospheres into a gas volume

Multiply the atmosphere count by the cylinder's water capacity to approximate the free gas inside. A 50-litre shell at 150 atm holds on the order of 7 500 litres, though real high-pressure gases deviate from that ideal figure.

Read a metric specification backwards

European datasheets often express a fill in atmospheres or bar. Tapping the swap arrow (↔) runs atm → psi so a 150 atm rating becomes 2 204.4 psi; the reverse multiplier is 14.696.

Copy the value into the cylinder log

Each field has a copy control that lifts the digits alone, with no unit and no spacing, straight into a stock sheet or a purge calculation. Ctrl + C inside the field produces the same clean string.

Cylinder pressure is a safety matter, not just arithmetic. Never fit a regulator rated below the cylinder's service pressure, never let oil or grease near an oxygen fitting, and always secure a cylinder upright before opening a valve.

Service Pressures Across the Cylinder Rack

Nominal service pressures at around 70 °F, spanning the range you meet in a typical gas store. Liquefied products are quoted at their vapour pressure at that temperature; the rest are charging pressures. Atmospheres are gauge values, matching the gauge reading in the middle column.

Cylinder class	Type	Service pressure (psi)	In atmospheres
Acetylene	Dissolved in acetone	250 psi	17.01 atm
Propane	Liquefied, vapour pressure	145 psi	9.87 atm
Nitrous oxide	Liquefied, vapour pressure	745 psi	50.69 atm
Carbon dioxide	Liquefied, vapour pressure	830 psi	56.48 atm
Oxygen, size K	Permanent gas	2 200 psi	149.70 atm
Argon or nitrogen, size K	Permanent gas	2 265 psi	154.12 atm
Helium, high fill	Permanent gas	2 640 psi	179.64 atm
Nitrogen or helium, tube	Permanent gas, ultra-high fill	6 000 psi	408.28 atm

The spread is the point. Acetylene at 17 atm and a helium tube at 408 atm are both "a full cylinder", and the same regulator will not do for both. It also explains why acetylene lines and oxygen lines behave so differently on a welding cart even though the two cylinders stand side by side.

Handy Details When You Live Among Cylinders

Either box takes the input

Start from the gauge reading or start from an atmosphere figure quoted in a specification — whichever you have — and the opposite field tracks it without a submit step.

Bar is one search away

Cylinder paperwork from outside the United States is written in bar far more often than in atmospheres. Both dropdowns cover all 26 pressure units, so a 200 bar rating can join the comparison instantly.

Reverse for a supplier datasheet

The arrow control switches the page to atm → psi, which is the direction you need when a metric fill rating has to be matched against a regulator marked in pounds per square inch.

Enough decimals for a log entry

Results carry up to eight decimal places, so a weekly cylinder audit can record 149.70 atm rather than a coarse rounding that hides a slow leak.

Cylinder Pressure Questions

Is my cylinder gauge showing psig or psia, and does the difference matter at 2 200 psi?

Practically every regulator gauge is psig: it reads zero in open air and shows how far above ambient the cylinder sits. Absolute pressure, psia, adds the 14.696 psi the atmosphere is already applying. At a full charge the distinction barely registers — 2 200 psig is 2 214.7 psia, so 149.70 atm gauge against 150.70 atm absolute, a difference under 0.7 %. It matters at the other end of the scale: a cylinder showing 30 psig holds 44.7 psia, half as much gas again as the gauge suggests, which is exactly the region where cylinder-change decisions are made.

Why is acetylene held near 250 psi when oxygen goes to 2 200 psi?

Acetylene decomposes explosively when compressed as a free gas, without needing any oxygen present. The cylinder is therefore filled with a solid porous mass saturated with acetone, and the acetylene dissolves into the solvent rather than existing as compressed gas. That chemistry, not the strength of the steel, sets the ceiling at roughly 250 psi, about 17 atm. It also explains two handling rules: keep the cylinder upright so the acetone stays in the mass, and withdraw gas slowly, because drawing too fast pulls acetone out with the acetylene.

How much gas is actually in a cylinder if I know its pressure and water capacity?

For a permanent gas, multiply the absolute pressure in atmospheres by the water capacity: 150 atm in a 50-litre shell is about 7 500 litres of gas at ambient pressure. Treat that as an upper estimate, because real gases stop obeying the ideal relation at high density — nitrogen and oxygen both become harder to compress above roughly 100 atm, so a cylinder holds somewhat less than the simple product predicts. Suppliers publish the measured contents on the label, and that figure is the one to trust for stock planning.

Why does a cylinder read higher after a day standing in the sun?

Nothing has been added — the gas is simply warmer, and in a fixed volume absolute pressure rises in proportion to absolute temperature. A cylinder charged to 2 200 psig at 70 °F reaches roughly 2 451 psig if its contents climb to 130 °F, moving from 149.70 to about 166.8 atm on the gauge. This is why fill pressures are always stated at a reference temperature, why a cylinder filled fast feels warm and reads low again once it cools, and why storage away from direct sun and heat sources is a standard rule.

Why should a cylinder never be run all the way down to zero?

Once the inside drops to ambient, any leak path lets damp air migrate inward, and moisture in an empty shell means internal corrosion, rust particles in the next fill and a contaminated product for the customer after that. Leaving a modest positive pressure keeps the flow outward. Many suppliers now fit a residual pressure valve that closes automatically while a small charge remains, and users who handle plain valves are told to shut off well before the gauge reaches the bottom of its scale and to return the cylinder marked as empty rather than genuinely empty.