Wow! It blows my mind that you couldn’t find manufacturer official specs on the PST LP 95 or PST LP 104, which are probably their best selling steel tanks.
Quick question: I see that you were able to find specs on the PST LP 80. When PST is calculating their capacity are they using the plus hydro?
In other words, I’m trying to determine (authoritatively) whether the tank factor for a PST LP 80 is 3.0530 or 3.3583? And same for a 104.
To answer your first question first and briefly (the only time I'll be brief in this whole post!
): Manufacturers use the maximum pressure available to that tank at the time they sell it to you. After all, who would sell a tank showing less than its maximum legal volume? And this is shown in the spreadsheet: all service pressures used are the plus-rated pressures where such a thing exists.
Now for your second question not briefly (in fact, *FAR* from briefly: it's a simple question but with a complex answer):
First, tank factors should *NOT* be based on service pressure rating, working pressure, fill pressure or any other such pressure. For speed and convenience people often do, but when you're calculating in advance there is no good reason to do so. Instead, they should be based on the tank's *WATER* capacity. The beauty of water capacity and tank factors is that they work no matter what you call the tank (e.g. LP95 or HP119), what pressure the manufacturer thinks you should fill the tank to, how much they lie about (I mean, embellish for marketing purposes) the gas capacity of the tank, or even what pressure *you* fill the tank to. They stay the same no matter what. (Except when they don't: that's covered later.)
And once you know the water capacity, you no longer have to care about how much gas *they* say the tank will hold. If you really care about the answer, you just figure it out for yourself, so you don't have to guess how they did it. (Of course, you *do* have to depend on the accuracy of the water measurement........)
So, for your example: Let's figure out the tank factor for the PST LP80. We only need a single number: the water capacity of a PST LP80. And that is 12.86 Litres. Where did that come from? That's the beauty of the spreadsheet: it has a link to show where all the info came from -- in this case, a PDF from PST that lists that information. (By the way, that's why I don't have the LP95 or LP104 or LP85 or whatever listed: the PDF doesn't list them, and I don't have any other authoritative references. Find me a company-provided resource that provides water volume (and weight and dimensions) and I'll gratefully add them!
)
Now for the math. Tank factor is cubic feet per 100 PSI. We need to know how many atmospheres 100 PSI is: 100 PSI tank / 14.7 PSI Atmospheric = 6.80. Now, take the water volume and multiply it by this factor: 12.86 L * 6.80 = 87.5 L. Now, convert that to Cubic Feet: 87.5L * 0.0353147 ft3/L (I just Google for that conversion) = 3.09. Your tank factor is 3.09 ft3 / 100 PSI.
Now, an observant reader might notice that this tank factor does not match either of the tank factors in the spreadsheet! Why not? Two different reasons. First, the air tank factor in the spreadsheet does *not* use the water volume, but rather the manufacturer-stated air capacity (again, always at the plus pressure). For the EAN32 tank factor, that *is* based off the water volume, but it *also* takes gas compressibility (z factor) into account, which throws off the answer by a few percent (4.91%, to be exact). This example used water volume, but no z factor, so it's 4.91% higher.
So this brings me to the final point: *NONE* of this stuff is particularly exact. First off, we are using one or more manufacturer-supplied numbers (at least either water capacity or gas capacity). It's from an authoritative source, but that does not necessarily mean that it's actually *accurate*. Second, there's a wide variety of ways of using those numbers to get to a tank factor. As you've seen, with the information we have for this single tank, we can calculate *three* different tank factors: one based on gas capacity without z factor, one based on water capacity without z factor, and one based on water capacity *with* z factor (for EAN32). And these varied from 2.94 to 3.09 for the exact same tank, using what is *supposed* to be consistent information provided from a single manufacturer-supplied source!
And remember, tank factors are rounded down to the nearest half-digit (unless it's *real* close to the higher value). So getting the number particularly precise doesn't really overly matter. Even in your example: 3.05 or 3.35 get rounded to the same value: 3.0.
(Oh, and giving 104/108's a tank factor of 4.0 is really not right. They are 3.8's, not 4.0 If you're doing something with 104's where the tank factor really matters, make sure you know what you're doing -- or maybe just create a plan where you're not depending on fractions of a tank factor...)
In the end, this is why people end up using rules of thumbs to determine tank factors. KISS: AL80's are 2.5, LP85's are 3.0, LP95/HP120's are 3.5 and 104/130's are 4.0 (even though they're *not*). It's precise enough. If you're using tank factors it's because you're probably at a dive site (or even worse, actually gearing up or in the water!) and trying to do this quickly in your head, and simple means less chance for mistakes. If you're *not* yet at the dive site, then using tank factors isn't really necessary: get out a calculator and calculate the actual gas volumes involved.
Anyway, rant over. I hope this answers your question, though I'm certain it's more than you care about!
I'm happy to answer questions you might have, but I would suggest if you have further questions, it might be better to break this off into a new thread: it really has nothing to do with the spreadsheet itself, and I'd rather not use this thread as a catch-all for unrelated discussions. (Yeah, I know it's ScubaBoard, but a man can dream...)
