LP TANKS

[Christopher McCaskie]

I have en diving for over 30 yes and never got into the LP tank thing- till now, only because I got two tanks for $300 so had to jump. So questions regarding blow pressure tanks… and if I seem like I’m being a it leads, I’m truly not, I’m just trying to get it

1. First of all, I get the cave fill aspect, but if your not doing a cave fill, what is the advantage of a LP Tank?
1-b Also, if your gonna take a tank rated for 2800 PSI AND BLOW IT UP TO 3700 your increasing the capacity of- say a lp108 from 108 cf to 142 cf which is about 130% higher than its rated, so why not take an hp 100 and fill it up 130cf or 4550- both are 30 higher than rated. Don’t get me wrong, I would never fill a tank to 4500 but I don’t get what makes the lo safe to go that far over its rating.

2. Assuming you don’t cave fill… If a 100cf tank holds 100cf at 3500psi and you surface at 500psi then you have 14cf left.
If you take an LP 108 cf tank AT 2800 psi and bring it down to 500 psi, you have 19.2 cf of air left, so should you bring it down to 365 psi so you have the same amount by volume of air or are you just shorting yourself?

 

[tmassey]

1. First of all, I get the cave fill aspect, but if your not doing a cave fill, what is the advantage of a LP Tank?

Practically, IMHO, none. I’d never own LP tanks without being able to get at least a *moderate* overfill. Otherwise I’d buy the HP (rough) equivalent and get the capacity.

However, there are tanks that don’t have a direct HP equivalent: LP50, LP72, and Faber LP85 (with the narrower diameter). So, if you want those tanks for whatever reason, you’ll have to go with the LP tanks, overfill or not.

1-b Also, if your gonna take a tank rated for 2800 PSI AND BLOW IT UP TO 3700 your increasing the capacity of- say a lp108 from 108 cf to 142 cf which is about 130% higher than its rated, so why not take an hp 100 and fill it up 130cf or 4550- both are 30 higher than rated. Don’t get me wrong, I would never fill a tank to 4500 but I don’t get what makes the lo safe to go that far over its rating.

It’s a thickness and metallurgy thing. The LP (3AA) standard was made like most of a century ago and is over-engineered for the lack of fine control over steel attributes. Overfilling greatly lessens the margins built into 3AA tanks, but their margins were high to begin with. Modern steels are much better categorized, and that’s why HP tanks can be roughly the same weight and dimensions as a 3AA tank but be rated so much higher. They assume much more consistent steel and can be pushed closer to their limits.

2. Assuming you don’t cave fill… If a 100cf tank holds 100cf at 3500psi and you surface at 500psi then you have 14cf left.
If you take an LP 108 cf tank AT 2800 psi and bring it down to 500 psi, you have 19.2 cf of air left, so should you bring it down to 365 psi so you have the same amount by volume of air or are you just shorting yourself?

How about a completely different way of answering that question: ‘500 PSI on the surface’ is completely stupid.

500 psi on the surface basically assumes an aluminum 80. If you’re not using an aluminum 80, that entire thing is meaningless. What if you were using a 30 ft.³ tank? Trying to surface with only 500 psi would be ridiculous. What if you had a 300 ft.³ tank? Trying to surface with 500 psi would be vast over kill. Those are extreme versions of your example, but the same thinking.

The other reason I think that any psi at the surface measurement is stupid is: the time you need to be worried is not at the surface, it’s at the bottom before you start to come up. So how exactly do you know when you should leave the bottom? 500 psi at the surface only tells you whether you did it right or not once it’s too late to do anything about it. In my opinion, it’s completely stupid in every way. Not everybody agrees, and everyone can figure out what works for them, but to me it makes very little sense, even when you *are* using an aluminum 80.

This probably isn’t the right context for a detailed answer, but Google “rock bottom gas management“. That will give you all kinds of things to think about and how to figure out exactly how much gas you need, regardless of tank type, starting pressure or dive depth.

 

[Bowers]

I second tmassey ^ and will add that increased gas volume is only part of what makes them great for certain dives.

Buoyancy is a major factor for many of us. If im diving a dry suit and can have a tank thats more negative, then it will help me achieve a proper ballast of weight that is better distributed and needs less lead.
For cold dives with thick thermal protection, hp steels will achieve this even better. But for lighter layered drysuit dives lp tanks are closer to the amount of weight needed.

(Tank life expectancy is a tiny benefit as well)

 

[ps444mike]

depends on dive boats.. some only fill to 3000 hot.. so my lp 50s (2640) get filled plus a little extra..

 

[inquis]

should you bring it down to 365 psi so you have the same amount by volume of air

The 500 psi "rule" is based on the ubiquitous AL80 and provides about a pound of air or 13 cuft. Yes, for a larger tank like an LP85 or HP100 (these are darn near the same internal volume), then 425 psi gives the same amount of gas.

Definitely research "Rock Bottom" or "Min Gas" because that will tell you when to start your ascent. The admonition to "finish with X psi" is not terribly helpful on its own.

 

[inquis]

However, there are tanks that don’t have a direct HP equivalent: LP50, LP72, and Faber LP85 (with the narrower diameter)

The HP100 is effectively equivalent to the LP85 (1% less internal volume). While the HP100 is 7.25" diameter vs 7.0" LP85, they both fit in the boat rack expecting a 7.25" AL80.

OP, steel tanks (both LP and HP) are desirable because you'll use less lead than an aluminum tank. Between those two steels, however, the HP is more popular because most people rely on a shop for fills. If you fill yourself (or shop will overfill), the LP is nice because it's a few pounds lighter climbing the ladder.

 

[LI-er]

if I seem like I’m being a it leads, I’m truly not, I’m just trying to get it

I never even considered the possiblity that you're being a it leads.

In fact I don't have a clue what being a it leads even is.

 

[newman_diver]

LP steel tanks are also good if HP fills aren’t available. I imagine that is less of an issue now but back in the days getting anything over 3000 psi was a luxury.

I don’t think you will find too many cases where you will find a shop willing and/or able to give you a “cave fill” on an HP tank. Typically the valves on the cylinders aren’t rated for that pressure either.

LP tanks are also easier to partial pressure blend Nitrox into without needing a booster pump.

I personally have both and look at the cylinder characteristics above anything else.

 

[rjack321]

I don’t think you will find too many cases where you will find a shop willing and/or able to give you a “cave fill” on an HP tank. Typically the valves on the cylinders aren’t rated for that pressure either.

In part because only a couple select HP tanks are "overbuilt" to the 3AA standard. The HPs are exemption tanks. Completely different wall thickness and metallurgy

 

[Capt Jim Wyatt]

Many/Some dive shops don't have the capacity to fill pressures beyond 3000-3200 PSI. So if you have a HP tank rated at 3442 psi you cannot get the tank filled to rated capacity.

LP tanks, rated at 2640 can get filled to, and beyond capacity at these shops.