Bottom time on LP vs HP

[Fire_medic_RescueDiver]

OK so I have read so any threads on steel LP vs HP, and a few people have asked the same question but no one seems to answer it, they just seem to get into the discusion about over filling. Ok lets try and get pass this and get to the basic question will you increase your bottom when diving on a LP overfilled vs HP overfilled? or will the bottom time be the same?

 

[cerich]

totally dependant on the size of the cylinder.

Take the tank size and rated pressure, divide rated pressure by the cubic feet. The number you have is how many psi per cubic foot in that cylinder. From there you can figure out how many cubic feet you have at the filled pressure(normal, under or OVER).

The more cubic feet you have the longer you can dive.

Don't overfill alum. cylinders ever!

 

[Fire_medic_RescueDiver]

Ok, if both tanks are 100cf and filled to the same pressure, and the same diver is using it on two dives the in a pool at 12 feet just laying on the bottom of the pool, which one would you get a longer bottom time?

 

[CD_in_Chitown]

How high a PSI can you get them filled to? What are they rated at?

Ok, if both tanks are 100cf and filled to the same pressure, and the same diver is using it on two dives the in a pool at 12 feet just laying on the bottom of the pool, which one would you get a longer bottom time?

IE fill two tanks to 3450 psi, one is rated at 2650 and two is rated at 3500, the diver on the bottom is breathing at .75 cuft/min, but...overfilling implies going beyond the rating, normally I get my HPs filled to 3500-3700 without issue. This isn't overfilling to me but it would be if my tanks were LP.

3450/(2650/100) = 130 Cuft Tank 1
3450/(3500/100) = 98 Cuft Tank 2

Pick a pressure, and pick an RMV in the .6 - 1.2 Cuft per min range, I'll admit to adding to the confusion in another discussion, I'll be glad to work it out how I learned and let someone punch holes in my understanding for your entertainment and our education. I am but a grasshopper, Pick some variables.

 

[cerich]

the one with the most cubic feet. You didn't give enough info to answer.

Is one a LP 100 at a rated pressure of 2250 psi? The other a HP 100 at a rated pressure of 3500 psi? If both had 3500 psi in them then the LP100 will have considerably more gas and thus last longer.

The LP at 3500 psi would have 155.55 cubic feet of gas, the HP only 100, provided all else is constant the LP has over 50% more gas thus 50% more bottom time.

You need to get with someone that can teach you this stuff !

 

[Soggy]

Cylinders in the US are rated to carry a certain amount of gas in cft at a certain psi (working cft and working psi). 100 cft of gas is 100 cft of gas, just like a pound is a pound regardless of composition.

So, if you have a LP80 rated at 2640 psi filled to 2640 psi or a HP80 rated at 3500 psi filled to 3500 psi, or an Aluminum 80 rated at 3000 psi* filled to 3000 psi, you will get the same bottom time because they all contain 80cft of gas. Pressure is irrelevent. Gas volume is what determines how much time you'll get.

However, if you have a LP80 filled to 3500 psi, you'll have ~106 cft of gas, which will last you longer than an HP80 filled to the same pressure.

Make sense?

*an aluminum 80 actually has 77.4 cft of gas at 3000 psi, but close enough

 

[steveann]

Longer bottom times really aren't a function of the overfill or tank type, but rather tank capacity and breathing rate.

A HP 80 and an aluminum 80 hold the same amount of air (okay, yeah, one holds 80 and the other holds 77.2 or something like that but for arguments sake let's say 80 for both.) If you breathe each tank the same, they'll last the same amount of time. Since the HP 80 is at 3500-ish and the aluminum 80 is at 3000, the HP 80 will lose pressure faster (HP 80 is 43.75 psi per cuft while the alum is 37.5). Still, though, you get the same amount of gas from each, 80 cuft. The benefits derived from the HP 80 are the buoyancy and the size. The HP tank is much smaller than the alum, and some people like that. But, in the end, there is no breathing air difference between the two, hence no change in bottom time (based purely on air consumption.)

In overfilled LP and HP tanks, you need to look at the capacities of those tanks. An LP 85 overfilled to 3000 psi gives you a not-so-LP 96. An HP 80 overfilled to 4000 psi gives you a bit-more-than-HP 92. So, for these two tanks and pressures, the overfilled LP 85 would give you more bottom time (all based on ideal gas laws. I'm not up for the real gas law math today, and it doesn't change it by that much.)

Conversely, an LP 66 pressed up to 2800 psi gives you an MP (if you will) 70, whereas an E7-65 pressed up to 3800 gives you an WHP (that'd be Way High Pressure) 75. In this case, the HP gives you more bottom time.

These calcs are based on 3442 HP pressure, 2640 LP pressure, nominal rated capacities, and ideal isothermic gas laws. In reality, many things will affect your bottom time, and the benefits different tanks give with regards to buoyancy and size (read drag and trim) can immensely affect your bottom time. In other words, if you want to improve your bottom time, work on your comfort in the water and trim more than your overfilling techniques.

As an example, if you take your SAC from a 1.0 to a .7 (not extrememly hard to do), you increase your bottom time by 142%. This is the equivalent of turning your HP 80 into a HP 114 with regards to bottom time (in other words you'd need 114 cuft of gas to stay down as long at SAC=1.0 as you could on 80 cuft with SAC=0.7)
To get the 114 cuft in an HP 80 would require an overfill to 4900 psi.

Hope this helps.

 

[cool_hardware52]

OK so I have read so any threads on steel LP vs HP, and a few people have asked the same question but no one seems to answer it, they just seem to get into the discusion about over filling. Ok lets try and get pass this and get to the basic question will you increase your bottom when diving on a LP overfilled vs HP overfilled? or will the bottom time be the same?

One minor Point.

500 PSI is widely considered to be a minimum. Back on the boat with 500 PSI, or better, "Rock Bottom, never less than 500 psi"

Why 500 PSI? SPG accuaracy and resolution, and necessary minimum inlet pressure for a first stage.

500 PSI in a 2250 is ~22%, 500 PSI in a 3442 is ~14%. If you had two tanks of equal capacity (at their respective working pressures) you can in theory use ~8% more of the HP tank without violating 500 PSI.

Personally I prefer LP tanks. Easier to get full fills, PP fills require lower O2 pressures.
If you are bumping up against the 500 PSI rule, get a bigger cylinder.


Tobin

 

[CD_in_Chitown]

RMV * ATA * Gas Supply = Time (?)

I usually solve this equation for Gas Supply so please correct me if I'm mistaken

.75 * 1.3 * 100 = 97.5 minutes

Tank 1 .75 * 1.3 * 130 = 126.75 minutes
Tank 2 .75 * 1.3 * 98 = 95.5 minutes

 

[cerich]

and now to get off topic...