I completely agree that the metric to imperial tank measurement conversions
suck, but I think there may be more to it than just the compressibility and
temperature aspects. The Luxfer cylinder site seems to have been down about two
weeks, but the third-party references I can find in the online tank catalogs in
the US, etc. certainly reference that Luxfer AL80 as 77.4cuft, not 80cuft. So,
those instructors are just looking at manufacturer spec sheets, like I did when
I got my Faber 120 steel.
It doesn't make sense to me that a manufacturer like XS scuba (the link I
provided earlier with the Faber Steel tanks) would change measurement metrics
between tanks on the same table, so if the nominal cuft values for something
like a current Faber 120 or 119 are even used to initially convert to a metric
equivalent, it seems that something will be off for one tank or the other. From
memory (flying atm), I think the 119 tank in that table held something like
123cuft as opposed to the 120cuft listed for an HP120. If it's not the 119
specifically, it's one of the other tanks on that table.
As an imperial tank user, it's just really confusing to have Subsurface
reporting different cuft values than the tanks I'm using, even if the
underlying conversion calculations are correct. And, I'm still not convinced
those calculations are correct across our spectrum of standard tanks. I'll look
into it further, when I get back to my computer. Do metric tank volumes already
account for the valve displacement?
On February 23, 2016 6:46:44 PM EST, Linus Torvalds
<torva...@linux-foundation.org> wrote:
>On Tue, Feb 23, 2016 at 2:18 PM, Richard Houser
><r...@divinesymphony.net> wrote:
>> In my classes back in Michigan, and during checkout dives I witnessed
>as a
>> bystander in Ohio and Florida, instructors with at least 5 different
>shops
>> (SSI/PADI/NAUI) have explicitly reminded students that when doing
>their
>> logs, air capacity in an AL80 is 77cuft, NOT 80.
>
>That's complete bullshit. Sorry. That's an oversimplification of the
>problem, and it's an over-simplification to the point of just being
>wrong.
>
>The fact is, it depends on the exact manufacturer, but also people are
>entirely confused about what "true capacity" is.
>
>For example, a Luxfer AL80 (which is just about the most common one
>out there) is a 11.1L bottle ("wet volume", aka "metric size", aka
>"not completely buggered and screwed up idiotic imperial units").
>
>That's the only _real_ "true capacity". The wet volume of the cylinder
>is the *actual* volume of the cylinder, with no idiotic "air at
>rpessure and temperature" effects.
>
>And guess what? When you look at what we translate "AL80" into, that's
>*exactly* what we give:
>
> cylinder vol=11.097l workpressure=206.843bar
>
>so our cylinder calculations are 100% correct. We never actually use
>"80" in any calculations, except to initialize that metric volume that
>we got right.
>
>So we actually use a 11.1 liter volume (and the "workpressure" is
>actually immaterial for everything, the _only_ use for that is for the
>crazy imperial conversion).
>
>Now, where does that "true capacity" come from? It's some fairly BS
>measure that comes from how idiotic the imperial cylinder measurements
>are to begin with, and involves a test of how much air you can
>actually get out of the cylinder. The difference to actual 80 cuft
>seems to come from a few places:
>
> (a) when you empty a cylinder, it ends up not empty (as in a vacuum),
>but full of air at 1 atm.
>
> So even if there was 80 cuft of air in the cylinder, you won't
>get all out of *out* of the cylinder, there will be that 11.1L
>(roughly 0.4 cuft) remaining in the bottle.
>
> It's somewhat unclear whether people use absolute pressure or
>gauge pressure for the working pressure, so that 0.4 cuft is actually
>debatable, but the "naive" math model (which is what I'd expect the
>cylinder sizing to be, considering all the other things going on)
>would give you a 0.4 cuft deficit.
>
>So that accounts for part of it, but only 0.4 cuft. Where did the rest
>go?
>
> (b) air is not actually entirely compressible.
>
> This is a fairly small factor at 3000psi, but it's a factor.
>HOWEVER. The rule for cylinder sizing is that the stated cylinder size
>is basically the "theoretical" size, not the real size.
>
> As seen before, we actually use the correct _real_ size for a
>Luxfer AL80 bottle. Don't believe me? Go look at the Luxfer data
>sheets, it really is 11.1 liter wet size. And just turn Subsurface
>into metric mode, and you really will see 11.1 liter, not 80 cuft.
>
>So that's another small part of it, but at 3000 psi it's probably not
>noticeable. We're (again) talking about fractions of cuft. It gets
>much more noticeable with HP cylinders.
>
> (c) I suspect that the defining *test* for "true capacity" is
>somewhat misleading in itself.
>
> I don't have any hard data for this, but I think that what the
>true capacity test does is let the air out and measure it. Which,
>thanks to bernoulli's law will actually measure colder air than is in
>the cylinder (air cools down when it expands), which in turn by boyle
>will shrink the air (colder air is denser).
>
>So when you measure the amount of air coming out of a cylinder, you
>are inherently also measuring the effects of a temperature
>differential.
>
>In other words, there are several causes of confusion here.
>
>But what you should take away from this is:
>
> - there's a hell of a lot of confusion about imperial cylinder sizes
>
> - the people who wrote the code in subsurface do actually know what
>they are talking about, to the point where I can pretty much guarantee
>that we know _better_ than your poor scuba instructors who just "know"
>that the "actual size" of a cylinder isn't the one stamped on it, but
>have little idea why.
>
> Don't get me wrong: I'm not trying to call out the scuba
>instructors. They are right, its' just that things are more
>complicated, and imperial sizes are laughably crap.
>
> Dirk actually has a math major, I have just a strong minor in math,
>and both of us kind of know the physics. We've actually *cared* about
>this issue, because it keeps coming up.
>
> - subsurface actually does get the calculations right, in that we
>only ever use the metric wet volume (11.1l) for real calculations, and
>we actually do take both the "cylinder still contains air" _and_ the
>"air isn't actually perfectly compressible" into account when we do
>our SAC rate calculations.
>
> - however, subsurface cannot take the temperature differential into
>account, so I'm not claiming our math is perfect either. Our math is
>_good_, but in the end, you will have to understand that the
>temperature of the cylinder, and the temperature of the air coming out
>of it does actually matter. It's a real issue. Some of the pressure
>drop when you dive literally comes from the cylinder cooling down
>under water. We've all seen pressures drop from 3100 psi ("nice
>overfill") on the boat in the tropics to 2800psi within minutes of
>jumping in the water. That's about 10% right there. There are smaller
>effects from the smaller temperature changes while under water, and
>there is also the bernoulli effect when just letting the air out (in
>order to breathe it).
>
>In short: we pretty much account for everything _but_ the temperature
>drop. The temperature drop you will generally see as a much higher SAC
>rate at the start of the dive: subsurface will blame _you_, not the
>temperature.
>
>Sadly, the tempoerature drop is not only likely the biggest effect,
>it's also the one that we simply cannot account for. We could *try* to
>estimate how quickly a cylinder cools down as you jump in the water,
>but it would depend on things like "did you go down immediately, or
>were you lollygagging on the surface waiting for your buddies?" So we
>don't really even try.
>
>Anyway, the primary take-away from all this really should be:
>
>IMPERIAL CYLINDER SIZES ARE CRAZY.
>
>Just don't use them. Use the metric ones. That's what subsurface uses
>for any calculation anyway.
>
>But if you want to take something else from this thing, please let it
>be that subsurface actually does quite a competent job. When we
>compute the metric size, we do the trivial ideal gas with no
>corrections thing, but that's because that is how imperial cylinder
>sizes are defined. But when we compute actual air use, we take the
>physical compressivity at a particular pressure into account, and we
>obviously also take the "remaining air" into account, so things like
>the "you can't actually get all the air out" is not an issue, because
>we've done the SAC rate calculations using the air that remains in the
>cylinder.
>
>We don't compensate for temperature, and you really can't. You should
>just generally be aware of it. Colder water will make your SAC-rate
>higher: mostly because you just actually tend to use more air (your
>body will work at keeping your temperature up), but partially because
>the air you breathe will be denser, so you'll have a higher effective
>SAC-rate _purely_ from temperature effects too.
>
>Sadly, the temperature effects often dominate.
>
> Linus
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