Bottom time on LP vs HP

[steveann]

Since you ctarted it cerich....

I personally don't know why you'd want to overfill an HP tank. The stress on the equipment is not really worth it. Why not just get an LP tank and overfill that and still stay under the HP pressure ranges?

 

[Mark Vlahos]

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?

Your bottom time will be based on depth, Nitrogen loading time, and available breathing gas.

Since your question here is really about only breathing gas, for this discussion lets dismiss the other considerations entirely.

A low pressure cylinder filled to its rated pressure will contain its rated capacity of gas, if that cylinder is overfilled it will contain more gas. A high pressure cylinder filled to its rated pressure will contain its rated capacity of gas, if that cylinder is undrefilled it will contain less gas. I wrote a chart and posted it to this thread, if you look at the chart you will see how much gas is available to you at various pressures. If you look at two different cylinders...

A Low Pressure 85 filled to 3100 PSI contains approximately 100 cubic feet of gas.

An E7-100 filled to 3442 PSI contains approximately 100 cubic feet of gas.

Since your bottom time (for the purposes of this discussion) is limited by gas supply, each cylinder will give you the same length dive.

Determine how much gas you will need for a dive, and then look at the chart to find the right cylinder and pressure for that cylinder to give you the required gas supply.

Mark Vlahos

 

[S. starfish]

Chances are you will get a longer bottom time on the LP simply because it has a much lower working pressure and so is much easier to over fill. But if you overfill both an HP and an LP to hold the same amount of air it's not going to make a bit of difference to your bottom time.

 

[cerich]

Since you ctarted it cerich....

I personally don't know why you'd want to overfill an HP tank. The stress on the equipment is not really worth it. Why not just get an LP tank and overfill that and still stay under the HP pressure ranges?

I wouldn't. I do what you just said.

But he didn't want whats better he wanted an answer to a question that wasn't too clear so I did my best to answer.

The whole tank/volume/pressure thing is hard for lots of divers unless they get into tech where for many it's the first time it is ever explained. It should be taught at Open Water IMHO

 

[Mark Vlahos]

I wouldn't. I do what you just said.

But he didn't want whats better he wanted an answer to a question that wasn't too clear so I did my best to answer.

The whole tank/volume/pressure thing is hard for lots of divers unless they get into tech where for many it's the first time it is ever explained. It should be taught at Open Water IMHO

My deco class was the first place it was really explained to me. The chart I wrote is a pretty clear illustration of the relationhips, at least for those who have trouble with the math as I did. When I did the math it only provided me with isolated snapshots, the chart lays out the relationship over lots of cylinder types and pressures.

Mark Vlahos

 

[The Kraken]

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?

OK, both tanks 100 cf
One H/P
One L/P

Same pressure: Let us assume a pressure of 2000 psi

All other things being equal:

L/P nominal pressure for 100 cf = 2000 psi
H/P nominal pressure for 100 cf = 3000 psi

You will get a longer bottom time with the L/P because you have 100 cf.
The H/P tank will contain only 66.6... cf because it is on 2/3's full.

But now you have to throw the big kahuna into the deal - the SIZE of THE TANK.

the K

 

[Courtneyf82]

Well, I'm totally confused. I have used steel lp 80's for my first 25 dives and just got my HP 80 2 dives ago, and now when I dive with the HP I get at least 20 more min of bottom time, my buddy who uses very little air, and always finishes with >= 1000psi when I am empty (same tank steel lp 80), now has to be the one to surface first. Why is that?

 

[steveann]

Courtney, did changing tanks change your weighting (add or drop lead?) I'll bet that is a big reason. More mass to move around in the water means more work means more air used. Also, the HP 80 is a true 80, where an LP 80 may not be.

 

[limeyx]

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

I also find there is something of a psychological factor too.
I dive an LP85 and LP80. For some reason, I burn through air more quickly with the 85 compared to the 80 at the same PSI. My buddy noticed the same thing.

 

[CD_in_Chitown]

I also find there is something of a psychological factor too.

If you can measure it in empirical numbers its physiological too and should be calculated into your planning RMV. I use multiple RMVs depending on the planned activity, the equation doesn't change but the variables do for work/hunting, dive/normal swim, and deco RMV rates.

Your experience may be related to drag or other stress from the tank, knowing your estimated RMV in each set-up makes it a non-issue. Plan your dive, dive your plan. Tank enough gas to do so and return with 1/3rd for reserve.:14: