Comparing tanks of different sizes and pressures

[Lorenzoid]

@wheeledgoat you're way ahead of the game, so kudos. For the first 10 years or so of my diving I never really thought about volume, never dived anything but an Al 80, and returning to the boat with at least 500 psi was all there was in my world as far as what I now know as "gas management."

Metric tanks sure would be easier, though. Yeah, a 12 liter tank is indeed the "inside volume of the tank"--how much water it would hold if you filled it with water.

 

[Kevrumbo]

. . .
Why? I just cannot seem to grasp why a rated pressure (some number an engineer decided was the max safe pressure for that vessel) has anything to do with what's in the tank now. Shouldn't it simply be a matter of volume and pressure? (...and nRT which are all constants in this case...)

That is why i prefer the metric system.

a 12l tank at 200bar is ~2400l
a 10l tank at 230bar is ~2300l
a 11l tank (AL80) at 200bar is ~2200l
a 15l tank at 200bar is ~3000l

Just take the tank volume and multiply with the pressure in bars (up to about 260bar)

With the imperial system, the tank is already rated by how much it holds. You don't do any math. 80cuft tank holds 80cuft. You only need to do math if you're doing something like filling to more or less than the rated pressure.

. . .And the formula Volume X Pressure is a very simple thing to remember. . .

@wheeledgoat and @kelemvor :

It's easier to fundamentally explain and conceptualize the rated volume and service pressure of a scuba cylinder based on the European/Asian surface atmosphere reference convention of 1 bar: The common AL80 tank has a metric cylinder rating factor of 11 liters/bar, or in other words, at the surface of 1 bar, if you pour water into the cylinder, the measured volume it can contain is 11 liters. (It's easier and more intuitive to work with Metric Cylinder Ratings like 11 liters-per-bar at a reference surface pressure of 1 bar, rather than cf/psi like 0.025 cf/psi at a normalized 14.7psi surface pressure Imperial Units reference for the AL80 tank).

So when pressurized with breathing gas to any value up to its recommended Service Rating (207 bar for the 11L per bar Alu cylinder in this example ), a cylinder carries an equivalent volume of free gas much greater than its water capacity, because the gas is compressed to several hundred times the standard surface atmospheric pressure of 1 bar (as opposed to water which is incompressible). So if you have a gas pressure reading of 200 bar in your AL80 tank, you have a total available ideal free gas volume of 200 bar multiplied-by 11 liters/bar or 2200 liters.

 

[NAM001]

A side note to al this. Say you have an al 80 and you are diving at 33 ft. (2atm) with a sac of .5. thats 1 cu ft per minute. You should have 80 min till dry. you should be using roughly 40 psi a min.,,,,,,, If you are using 100 psi you need to start suspecting a leak or something else that would explain the increased rate of consumption. because in 30 min you will be dry.

 

[Diver0001]

I initially wanted to know exactly how much of an upgrade a HP100 (@3442psi) represented over an AL80 (@3000psi), i.e. how much more air was I going to have. This has sent me down an rabbit hole from which I'd like to request some help to escape.

This post explains "Tank Factors" which at first blush seemed to give me what I needed. But as I'm trying to understand it, I can't get my head around it. Why does the rated pressure of a tank matter?

Even this Tank Air Volume calculator over at DiveBuddy uses the rated pressure as part of the calculation.

Why? I just cannot seem to grasp why a rated pressure (some number an engineer decided was the max safe pressure for that vessel) has anything to do with what's in the tank now. Shouldn't it simply be a matter of volume and pressure? (...and nRT which are all constants in this case...)

In most of the world tanks have a volume and a pressure in Bar. For every bar you get one unit of volume. So, for example, a 10 litre tank has an internal volume of 10 litres. When filled to 200 bar it contains 10*200= 2000 litres of gas. With this starting point, things like SAC calculations become very easy.

In the American system a 100cf tank will have roughly 100cf of gas in it at its working pressure. An 80cf tank will contain roughly 80cf of gas when filled. Therefore a 100cf tank contains a percentage of 100/80 (125%) as much gas as an 80cf tank. In theory you should get about 25% more time out of the 100cf tank then. For example, on a dive to 60 feet (18m) if you were diving for 40 min with an AL80 then you will roughly be able to stay an extra 10 minutes using the HP100.

I hope this gives you a way to think about it without going to a lot of calculations.

R..

 

[Magnus Lundstedt]

In the American system a 100cf tank will have roughly 100cf of gas in it at its working pressure. An 80cf tank will contain roughly 80cf of gas when filled. Therefore a 100cf tank contains a percentage of 100/80 (125%) as much gas as an 80cf tank. In theory you should get about 25% more time out of the 100cf tank then. For example, on a dive to 60 feet (18m) if you were diving for 40 min with an AL80 then you will roughly be able to stay an extra 10 minutes using the HP100.

An AL80 is rely an AL77 for some reason named AL80, so it holds roughly 77cf at stated working pressure.

 

[Kevrumbo]

(@KWS , here is a side bar to your side note above . . . and shows why it's easier to understand and figure with tanks of different sizes & pressures in Metric versus US Imperial Units).

Below are some example pressure Surface Consumption Rate (SCR) values for a variety of common cylinders with an arbitrary volume SCR (also known as volume SAC rate or RMV):

Given a arbitrary nominal volume SCR of 22 liters/min per ATA (that's 0.78 cuft/min per ATA in US Imperial Units, a reasonable & achievable volume SCR for most novice divers, and exercise level for fit advanced divers with sustained active finning):

Cylinder Size | Pressure SCR
11L/bar tank (AL80): 2bar/min per ATA;
12L/bar tank (Steel HP100): 1.8bar/min per ATA;
13L/bar tank (AL100): 1.7bar/min per ATA;
15L/bar tank (Steel HP119): 1.5bar/min per ATA;
16L/bar tank (Steel HP130): 1.4bar/min per ATA;
11L Twins (Double AL80's): 1bar/min per ATA;
12L Twins (Double HP100's): 0.9bar/min per ATA;
16L Twins (Double HP130's): 0.7bar/min per ATA.

Given a arbitrary nominal volume SCR of 15 liters/min per ATA (0.53 cuft/min per ATA in US Imperial Units, relaxed with minimal finning for advanced divers):

11L/bar tank (AL80): 1.4bar/min per ATA;
12L/bar tank (Steel HP100): 1.3bar/min per ATA;
13L/bar tank (AL100): 1.2bar/min per ATA;
15L/bar tank (Steel HP119): 1bar/min per ATA;
16L/bar tank (Steel HP130): 0.9bar/min per ATA;
11L Twins (Double AL80's): 0.7bar/min per ATA;
12L Twins (Double HP100's): 0.6bar/min per ATA;
16L Twins (Double HP130's): 0.5bar/min per ATA.

Given a arbitrary nominal volume SCR of 11 liters/min per ATA (0.39 cuft/min per ATA in US Imperial Units, drift diving floating neutrally buoyant & going with the current):

11L/bar tank (AL80): 1bar/min per ATA;
12L/bar tank (Steel HP100): 0.9bar/min per ATA;
13L/bar tank (AL100): 0.8bar/min per ATA;
15L/bar tank (Steel HP119): 0.73bar/min per ATA;
16L/bar tank (Steel HP130): 0.68bar/min per ATA;
11L Twins (Double AL80's): 0.5bar/min per ATA;
12L Twins (Double HP100's): 0.45bar/min per ATA;
16L Twins (Double HP130's): 0.3bar/min per ATA.

The point is that pressure SAC rate for most nominal activity on single tank is going to fall in between 1 bar/min per ATA and 2 bar/min per ATA. And how easy is it to figure factors of numbers like 1 and 2? --All you need now is a convenient time interval like 10 minutes and your metric depth converted to ATA as multiplying factors (simply divide-by-ten and add one, so for example 20 meters depth is 3 ATA), and you will then know what your Depth Consumption in bar will be over that time interval, with a particular tank, and level of physical activity. . .

So for an example, choosing an AL80 single tank for a drift dive in Palau:

How much easier & intuitive is it to work with "1 bar/min" Metric vs "14.5 psi/min equivalent" in US Imperial Units???

My Surface Consumption Rate (SCR) after drift diving so many years on holiday in Palau's 28deg C tropical water temp is a personal best 11 litres/min per ATA.

Using this SCR value with a 11 litres/bar tank (i.e. an AL80 Cylinder):

Divide 11 litres/min per ATA by 11 litres/bar equals 1 bar/min per ATA .

So again --How much easier, intuitive & advantageous is it to work with "1 bar/min" and the metric system in general for Scuba? Well, the arithmetic can all be figured easily & quickly in your head and on-the-fly:

All my dives were on Nitrox32, averaging 20 meters depth always going with the drift current; 20 meters is 3 ATA (divide 20 by 10 and add 1 gives a depth in atmospheres absolute of 3 ATA).

Therefore at 20 meters, my 1bar/min per ATA gas pressure consumption rate will increase threefold --that is 1bar/min per ATA multiplied by 3 ATA equals a depth consumption rate of 3 bar/min at 20 meters. Hence checking my elapsed bottom time every 10 minutes, I expect to consume 30 bar (3 bar/min multiplied by 10min equals 30 bar), and accordingly I already know my SPG will read 30 bar less in that 10 minute time frame. (If however the actual SPG reading indicates 30% or more consumption than expected, then there is a leak problem or I am physically exerting/breathing harder than normal and probably would consider aborting the dive).

So by the first 10 minutes delta time at 20 meters, I expect to be down 30 bar from a full AL80 tank at 200bar, or 170bar remaining actual SPG reading (3bar/min multiplied by 10min is 30bar consumed; and 30bar consumed from 200bar total full tank is the SPG showing 170bar remaining pressure). At the end of another 10 more minutes delta time drifting along at 20 meters, I've consumed 30bar from 170bar, or 140 bar remaining in tank. And finally after another 10 minute period at the elapsed dive time mark of 30 minutes total, I've consumed 30bar delta from 140bar, or 110bar remaining and nearing half tank.

At 40 minutes elapsed time, I'm ascending off the deep wall into the shallow coral plateau around 9 meters (down 30bar from 110bar, or 80 bar remaining in tank). And finally at the 45 to 50 minute mark, I'm at 6m and my 3-5min safety stop with 60 to 70 bar left. I surface and I know even before looking at my SPG that I have around 50 bar remaining in my tank.

This is how you should actively use your SCR with your particular tank, knowing how much breathing gas you have left not only on pre-planning, but also during the actual dive at depth, real-time-on-the-fly --all with easier to use metric units . . .additionally, you have a SPG that reads in units of pressure: why not convert your SCR to a Depth Consumption Rate (DCR) in pressure units to make use of it???

In summary & recap: divide your volume SCR (or SAC/RMV rate) by your particular tank's cylinder rating factor to get a figure in pressure units per minute since your SPG reads in pressure units -not volume units. Multiply this SCR in pressure units by your planned depth in ATA, and you'll know what your Depth Consumption Rate (DCR) per minute in pressure units at that depth will be. And the Metric System for Scuba diving makes the arithmetic much easier especially if your pressure Surface Consumption Rate (SCR) turns out to be roundable up to convenient integer like 1 or 2bar/min per ATA.