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Z Factors for SCUBA

The easiest method to determine true gas capacity of SCUBA cylinders is to use the Compressibility Factor (Z). To calculate true capacity, divide ideal capacity by the Z Factor obtained from the table below. See our article Calculating SCUBA Cylinder Capacity for more information.

Gas Mixture
Common Name †
2015 psig
70°F
2640 psig
70°F
3000 psig
70°F
3442 psig
70°F
3600 psig
70°F
207 bar_g
15°C
232 bar_g
15°C
300 bar_g
15°C
Nitrox 50 (deco) ≈ 0.9773 ≈ 0.9897 ≈ 1.0007 ≈ 1.0178 ≈ 1.0248 ≈ 0.9938 ≈ 1.0077 ≈ 1.0575
Nitrox 32 ≈ 0.9912 ≈ 1.0080 ≈ 1.0214 ≈ 1.0412 ≈ 1.0491 ≈ 1.0153 ≈ 1.0316 ≈ 1.0868
air (dry) ≈ 0.9984 ≈ 1.0173 ≈ 1.0320 ≈ 1.0532 ≈ 1.0615 ≈ 1.0263 ≈ 1.0438 ≈ 1.1020
Trimix 21/35 (normoxic) ≈ 1.0636 ≈ 1.0901 ≈ 1.1066 ≈ 1.1280 ≈ 1.1359 ≈ 1.1065 ≈ 1.1243 ≈ 1.1764
Trimix 18/45 ≈ 1.0736 ≈ 1.1010 ≈ 1.1176 ≈ 1.1389 ≈ 1.1467 ≈ 1.1185 ≈ 1.1362 ≈ 1.1869
Trimix 10/50 (aka Heliair) ≈ 1.0784 ≈ 1.1064 ≈ 1.1233 ≈ 1.1446 ≈ 1.1524 ≈ 1.1246 ≈ 1.1423 ≈ 1.1927
Trimix 15/55 ≈ 1.0798 ≈ 1.1075 ≈ 1.1241 ≈ 1.1449 ≈ 1.1524 ≈ 1.1256 ≈ 1.1429 ≈ 1.1918
Trimix 10/70 (hypoxic) ≈ 1.0833 ≈ 1.1104 ≈ 1.1262 ≈ 1.1458 ≈ 1.1529 ≈ 1.1284 ≈ 1.1448 ≈ 1.1902

oxygen (pure) ≈ 0.9387 ≈ 0.9383 ≈ 0.9424 ≈ 0.9515 ≈ 0.9558 ≈ 0.9328 ≈ 0.9400 ≈ 0.9738
helium (pure) ≈ 1.0668 ≈ 1.0869 ≈ 1.0985 ≈ 1.1125 ≈ 1.1175 ≈ 1.1008 ≈ 1.1126 ≈ 1.1445

† This table is arranged so that interpolation is possible for most pressures and mixtures. As the Z Factor value increases, the free gas capacity decreases. At pressures 2015 psig and lower, the Z Factors for Nitrox and air are very close to, or a little better than, unity with ideal capacity. At the highest pressures, as the mixture gets very rich in helium the Z Factors are less predictable because helium is weird stuff. Just because this table lists Z Factors at higher pressures doesn't mean you can or should fill tanks to that pressure, especially with pure oxygen.

☆ Heliair 10/50 is not a particularly good breathing gas for open-circuit divers. Heliair always is less than optimal for either PO2 or narcosis, depending on your TOD. However, Heliair is easy to make so it is widely used and the reason it appears in this table.

This Z Factors table was prepared using the NIST REFPROP database and software, version 10 (beta) copyright 2018. The software, developed by the National Institute of Standards and Technology (NIST), calculates thermodynamic properties of fluids and their mixtures. REFPROP is based on the most accurate pure fluid and mixture models currently available. For calculation purposes, the gases were defined to be pseudo-pure air (dry), Nitrox mixtures (O2, balance N2) and Trimix (O2, Helium, balance N2) mixtures composed of the stated percentages on a molar basis and all without the trace gases or contaminates typically found in the breathing gases used for SCUBA. That means these perfect gas Z Factors would be a little different for the stuff in your tank, but not enough to be significant.



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