Calculating SCUBA Cylinder Capacities

[DiveGearExpress]

See our new articles on Calculating SCUBA Cylinder Capacities and Z Factors for SCUBA

Technical divers need to know exactly how much breathing gas they have as they enter the water, because if they didn't plan correctly then bad stuff can happen. You can just accept that the names used to describe some SCUBA cylinders have a weak relationship to their actual free air capacity, because otherwise this STEM heavy article is going to take you on a journey through some gas thermodynamics for divers. If you don't find that interesting, skip to the Table of True Capacities for Common SCUBA Cylinders.

 

[tbone1004]

if you're going to do that, can you include at 3600psi/248bar since that is what most tech fills are?

 

[DiveGearExpress]

if you're going to do that, can you include at 3600psi/248bar since that is what most tech fills are?

Done.

 

[Jack Hammer]

Interesting article, Thanks

 

[Compressor]

Very good article; can we get the tables as a PDF file? Worth saving for sure.
Thank you.

 

[DiveGearExpress]

Very good article; can we get the tables as a PDF file? Worth saving for sure.
Thank you.

Thanks!

As for a PDF, I'd rather not engrave them in stone formally yet... these Z Factors can and sometimes do change over time... and I'll keep them updated. Plus they haven't had enough peer review, too new. Luxfer used an old table from a 1970's handbook that set Z at 1.0325 and they rounded to 1.033; but some thermodynamic constants have changed in the 2000's, which then ripples through those tables and is part of the explanation for their 77.4 vs my 77.6 on an AL80 (Z=1.032). No reason you can't do it yourself, but I would prefer to let that table get a few more eyeballs first. Google already has it... just google search "compressibility factor for scuba"

 

[JohnnyC]

I hate math. Math sucks. To avoid doing math, I just use metric measurement for volume and pressure. I ended up making myself a little table of common US and metric tank sizes and their volumes at varying common pressures in both psi and bar.

 

[DiveGearExpress]

I hate math. Math sucks. To avoid doing math, I just use metric measurement for volume and pressure. I ended up making myself a little table of common US and metric tank sizes and their volumes at varying common pressures in both psi and bar.

"... conversions between metric and imperial cylinder free capacities are more complex than simple units conversions [...] The reason is small differences in temperature and pressure between the EN/ISO and DOT standards. This is why simple units conversion of free capacity between imperial and metric cylinders is improper."

 

[JohnnyC]

"... conversions between metric and imperial cylinder free capacities are more complex than simple units conversions [...] The reason is small differences in temperature and pressure between the EN/ISO and DOT standards. This is why simple units conversion of free capacity between imperial and metric cylinders is improper."

It's simply a table of correlations. Think of it like the ratio deco of gas planning. They're more than close enough for quick calculations, and it's much easier to do math than multiplying by tank factors. Add in carrying a camera so you need more gas in your wing, or cooler water, or feeling chilly so you gas up your drysuit a little more, and your ultra-precise imperial gas calculations don't matter any more. So small differences in standards is a specious argument. The liquid volume of a cylinder is constant. The idea that you should be calculating your available volume of gas based on a non-constant variable is pretty silly.

@Kevrumbo had a really good post somewhere on the superiority of metric units regarding scuba diving. Fact is, the imperial way of measuring tank capacities is pretty silly when it comes to ease of use and functionality in situ.

 

[АлександрД]

all based on Van der Waals equation - Wikipedia
Just comparation of ideal and real gases had variations....

BTW - what is your best SPG, used for gas mixing? I think it is something like KELLER digital Manometer ECO 1 (Ei) or DG25 Digital Pressure Gauge ?
Accuracy - 0.5%
Why needs four digits after point?