26 replies to this topic

[Dive_Girl]

Remember your Instructor's admonition? "These are FINS, not flippers. This is your MASK, not your goggles."

What was this Instructor's problem? Who cares? So what?

Well, the fact is terminology is important. It may be fun, it may cost you a beer when you're at the lake on your checkout dives, but what happens later?

It is important to continuously learn in diving, and one of the ways is to demonstrate your attention to the details. Your peers and the professionals around you will be assessing you based on many factors, but if you're asking your buddy to turn on your "oxygen", the chances of being brought into a discussion about planned run time, bottom time and gas management for the dive may be nil.

Here's one that I'll be the first to admit confused me when I was starting out - maybe it's because I am not terribly mechanically inclined or that I'm a girl but working pressures had me confused to begin with. For example, I thought those short 80s that had 3400psi had MORE air in them than the tall 80s with 3000psi. 3400 is more than 3000, right?? Anyone see where I was mistaken?

[Walter]

I thought those short 80s that had 3400psi had MORE air in them than the tall 80s with 3000psi. 3400 is more than 3000, right?? Anyone see where I was mistaken?

That's a common misconception for beginners. You have lots of company.

[Dive_Girl]

I thought those short 80s that had 3400psi had MORE air in them than the tall 80s with 3000psi. 3400 is more than 3000, right?? Anyone see where I was mistaken?

That's a common misconception for beginners. You have lots of company.

Any takers on explaining working pressure?

[Walter]

I thought those short 80s that had 3400psi had MORE air in them than the tall 80s with 3000psi. 3400 is more than 3000, right?? Anyone see where I was mistaken?

That's a common misconception for beginners. You have lots of company.

Any takers on explaining working pressure?

Working pressure is the pressure stamped on the tank, 3000 PSI for an AL 80, 2250 for most ST 72s.

[Dive_Girl]

that tells me where to look to find the working pressure of a tank, but I am looking for an explaination of what working pressure is; one that would make it clear to a diver that doesn't understand it's meaning.

[Walter]

Working pressure is the maximum fill pressure for a tank, unless that tank is a steel tank allowed a 10% overfill.

[peterbj7]

It's worth explaining the difference between the systems used by the US and most of the rest of the world. This has nothing to do with the material a tank is made from.

In the US system the volume of a tank is denoted by the amount of air (gas!) at normal surface conditions that can be compressed into it at its working pressure. Knowing the physical size of the tank isn't enough, nor is knowing what the pressure actually is nor indeed what the maximum permitted pressure is. What you need is the "rated pressure". As Walter said, most aluminum tanks have the same pressure for "rated" as "maximum", but that isn't so for steel.

In the other system the actual volume of the tank is measured, ie. the volume of water that it can contain. Now all you need to know is that measure and the actual pressure of the tank and you know how much air/gas you have inside. Simpler and with less room for error.

Many recreational divers won't see the significance of this, because many of them will not have had any training in gas management. But it does matter, and particularly so to that strange breed of human we've been looking for an alternative name for, "technical divers". These people need to know how much gas they're carrying because they've already worked out roughly how much gas they need, so this then tells them whether they'll have enough.

Knowing the pressure of a tank obviously isn't enough to decide whether you're going to live or not (!). A small tank will run down quicker than a large one starting from the same pressure simple because there isn't as much gas in it.

The measures of gas that are used are cubic feet or litres, with around 28 of the latter to one of the former. "Surface Air Consumption" or SAC is given based on either of these units, so you can see that SAC doesn't mean a lot unless you know how much gas you're carrying.

Sorry to insult some peoples' intelligence, but I don't think all our readers know all of this, and they need to both to understand the thread and to become better divers. This is after all a forum for newer divers.

[annasea]

No insult taken, Peter. Thank you for such a detailed response!

[Dive_Girl]

Sorry to insult some peoples' intelligence, but I don't think all our readers know all of this, and they need to both to understand the thread and to become better divers. This is after all a forum for newer divers.

This is a forum to assist newer divers, so thank you for your sharing of detailed info!!

I thought those short 80s that had 3400psi had MORE air in them than the tall 80s with 3000psi. 3400 is more than 3000, right?? Anyone see where I was mistaken?

Since no detailed explanation has been offered, I am going to offer an explanation. I realize this subject can spring off and get very detailed, but I just want to focus on a less complex concept regarding working pressures to begin with.

Some divers who have been exposed to both aluminum and steel tanks may have noticed that an aluminum 80 cubic foot tank (referred to as AL 80) and a high pressure (referred to as HP) steel 80 cubic foot tank are different physical sizes. The AL80 is taller (at about 26 inches) than the steel HP 80 cubic foot tank (at about 20 inches).

1. First, aren't all 80 cubic foot tanks the same "size" (size)?

No. Steel is stronger than aluminum, so steel tank walls can be thinner than aluminum tank walls. As steel is stronger, traditionally steel is also chosen for use when creating higher pressure tanks. So an HP steel tank can hold the same "amount" of air/gas as an aluminum tank but in a smaller container.

2. Second, ok so there are different tank sizes, so don't they all hold different amounts of air/gas (volume)?

An AL 80, a steel HP 80, and a steel LP 80 will all have [relatively] the same volume, 80 cubic feet of air [77.4-80 cuft actual capacity - please see chart below], when each are filled to their listed working pressures.

In the US system the volume of a tank is denoted by the amount of air (gas!) at normal surface conditions that can be compressed into it at its working pressure.

3. Third, so what is working pressure?

Working pressure is the term used to note what pressure (psi) a tank should be filled to, to yield the cubic feet of air/gas the tank is listed at. For example, in order for an AL 80 tank with a working pressure of 3000psi to hold 80 cubic feet of air/gas [77.4 cuft actual capacity - please see chart below], it must be filled to 3000psi. In order for a steel HP 80 tank with a working pressure of 3500psi to hold 80 cubic feet of air/gas, it must be filled to 3500psi. In order for a steel LP 80 tank with a working pressure of 2640psi to hold 80 cubic feet of air/gas [78 cuft actual capacity - please see chart below], it must be filled to 2640psi. Therefore if all are filled to their working pressures, an HP 80 does not have [relatively] more air/gas (or some would think of it as 500psi more) than an AL 80, and an AL 80 does not have more air/gas (or some would think of it as 360 psi more) than an LP 80. They all have [relatively] 80 cubic feet of air at their working pressures.

But if an AL 80 and an HP 80 are both filled to 3000 psi, the

small tank will run down quicker than a large one starting from the same pressure simple because there isn't as much gas in it.

An AL 80 with a working pressure of 3000psi filled to 3000psi should yield 80 cuft of gas [77.4 cuft actual capacity - please see chart below]. An HP 80 with a working pressure of 3500psi filled to 3000psi should yield 68.57 cuft of gas (80/3500 * 3000), thus "there isn't as much gas in it."

[PerroneFord]

Except when 80cuft is actually 77.4

[Dive_Girl]

Except when 80cuft is actually 77.4

yep tanks argh! I focused on "volume" and not capacity...and a steel LP 80 holds 78 and a steel HP 80 actually holds 80, so they are somewhat close. I was trying to avoid the nitty nitty and focus on the general concept. BUt thank you for pointing that out and I've modified my post above to note the actuals using the chart below.

[Walter]

3. Third, so what is working pressure?

Working pressure is the term used to note what pressure (psi) a tank should be filled to, to yield the cubic feet of air/gas the tank is listed at.

Close. That's true for aluminum bottles, but not for steel. Steel cylinders only yield the listed volume when filled to 10% over their working pressure.

[Dive_Girl]

3. Third, so what is working pressure?

Working pressure is the term used to note what pressure (psi) a tank should be filled to, to yield the cubic feet of air/gas the tank is listed at.

Close. That's true for aluminum bottles, but not for steel. Steel cylinders only yield the listed volume when filled to 10% over their working pressure.

I didn't know that. So in order for a shop to fill a steel to 10% more than the working pressure as stamped on your tank, you'd need the "+" symbol stamped behind it, correct? And so I should have my HP 80s filled to 3850 to yield 80 cuft?

[Walter]

Exactly. I know you're aware, but to make it clear for those who aren't. The + has to be stamped at the time of the latest hydro to be valid.

[peterbj7]

Another aside. The standard OC tank in Europe is the 12 litre steel. The standard in the US, the 80cu.ft. aluminum, is around 11 litres. So the European tank contains about 9% more than the US one.