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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. This table is arranged so that interpolation is possible for most pressures and mixtures. 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.

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 Z Factors table was prepared using the NIST REFPROP database and software, version 10. 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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