Tank Pressure questions

[dsevern]

I've been tank shopping of late and a little confused.

I've always used the standard 3000 psi Al80 (77 cu. ft)

Now I see Luxfer has a HP 80, 3500 PSI and 83 cu.ft and then I saw another post asking about steel vs al and they mentioned high pressure and low pressure steel 72's.

So what's the deal with pressure? I understand I could stay down at a given depth for a longer time with a higher volume (cubic feet) tank, but how does the tank pressure come into play? or WHERE does it come into play?

Thanks in advance!

 

[mike_s]

The higher pressure tanks are more compact, giving you more cubic feet in a smaller tank size.

A tank will deliver it's rated cubic feet (at rated pressure) regardless of it's a regular tank, a low pressure (LP) tank or a High Pressure (HP) tank.


why do some people prefer LP tanks? because they typically either like to overfill them or want to reduce the work load on either their first stage or their compressor system.

Why do some people prefer HP tanks? more air at the rated pressure for the size of the tank.



NOTE: most HP tanks rated at 3500psi or above have a DIN valve on them, which is one thing you'll see different about HP tanks. But just because it's got DIN doesn't mean it's HP. many people prefer the DIN on their tanks. (Especially LP tanks they will overfill).

 

[scubawally]

think of tank pressure like density and then t becomes a little bit easier. When they list the "size" of a tank they mean the volume of air it would hold if that volume of air was at atmospheric pressure and the tank was at its rated (working) pressure. for example an AL80 holds 77.7 Cuft of air when it is pressurized to 3000 psi. The same tank also holds only 38.8 cuft at 1500 psi. The length of your dive is dependant on the amount of air you bring down and not so much related to pressure of your tank.

for example if you have a steel 130 at 1500 psi (half empty) you still have more air than you would have if you were diving with an Al63 pressurized to 3000psi. therefore you will be able to conduct the same dive profile. The only place where this gets a little sticky is when you are diving really deep in order to breath you need to have more pressure in your tank than the surrounding water pressure (you have to dive very deep for this to stat to be a problem)

As for the Al 80 is does have 77ish cuft but for naming purposes and ease of terminology it is rounded up to 80. the steel 72 is an older tank and contains 72 cuft but it is a low pressure tank (2600 psi)

 

[Otter]

...snip...

So what's the deal with pressure? I understand I could stay down at a given depth for a longer time with a higher volume (cubic feet) tank, but how does the tank pressure come into play? or WHERE does it come into play?

Thanks in advance!

I think there may be a more fundamental question being asked (or I may be wrong).

A scuba tank is fixed in size, therefore to get more gas into the tank, you must compress the gas (i.e. raise the pressure). We often used 'pressure' as a proxy for volume (e.g. how full is the tank). Take your 'Aluminum 80' which as you correctly point out, holds 77 cu ft of air at its service pressure of 3000 psi. At 1500 PSI it would hold 1500/3000 or 1/2 of the 77 cu ft volume or 38.5 cu ft of air.

So when you see different size tanks and different working pressures, you are seeing the different tradeoffs between size and pressure are being made.

I would add another advantage to LP tanks that folks is SoCal sometimes consider is that not all dive boats can/will deliver 3500 psi. So if you want a 'full fill', you go with LP but lug around a physically larger tank.

edit: Looks like during my distraction someone else helped out...sorry for the redundancy.

 

[Igodeep]

I don't mean to complicate the discussion, but you may also want to look at the bouyancy characteristics of the tanks. Some tanks will stay negative when empty, some will get positive, and some can get very positive. You need to understand this and account for changes in bouyancy. For example, you could start a dive with an AL80 and be weighted perfectly, but at the end of the dive when the tank is 3-4 lbs positive, you may not be able to hold your safety stop.

I also agree with Otter, sometimes it can be very difficult to get a full fill on a 3500 psi tank. I personally prefer LP Steel tanks.

 

[SteveC]

This subject comes up from time to time so let me mention that all 80 cf tanks get about 5lbs lighter at the end of a dive. Not just aluminum.