The perils of owning LP tanks in the Midwest

[formernuke]

They might also want to avoid overfilling to avoid potential liability to someone injured as a result (and a lawyer could attempt to use a violation of the CFRs as evidence of best practice). But "it's illegal"--as in, government authorities care about this--seems a stretch.

Exactly, if injury or property damage occurs overfilling in violation of the CFR which is on the books against the law is prima facia evidence of negligence. Good luck defending that law suit.

 

[James79]

Not just a question for John, but everybody: Educate me here as to how tanks are rated. If a tank is LP vs. HP, is there not some inherent difference in the tank? Thicker walls, different threads, something like that? Why would you buy an LP tank and then outfit it with burst discs well beyond what it is supposed to use? Is it just to cheat the amount of gas that you are able to put in it, even though that might not be the safest idea?

Kmarks,

I am not an expert, and all of this is just my unofficial opinion, but my personal decision point on is based on a few factors:

Number one is metallurgy.... I will not overfill aluminum tanks (and even understand some shops having a limit on how old of aluminum tanks they will fill regardless of alloy) due to the fact that AL fatigues when stressed (no matter how little the stress, there is some fatigue. More stress, more fatigue). So overfilling Al tanks WILL shorten their safe service life. Steel, on the other hand, has a "Fatigue Strength." Stress below that limit has no permanent effect on it (hence industrial tanks from the 1900's still being in service and passing hydro). And Hydro test pressure should (as I understand it) be at the bottom end of the allowable fatigue strength. So, in theory overfills below Hydro pressure "Should" be harmless.

Second is that Hydro tests should never burst a tank. The safety margin to bursting is a lot higher... the hydro test measures tank "Stretch" at high pressure, and fails it if it stretches too much (over simplified).

Third - HP vs LP; HP tanks (that I know of) are produced on an exception permit... they hydro at 3/2 service pressure rather than 3AA's standard of 5/3 service pressure. So the margin is smaller. But HP tanks are still heavier (thicker walled than LP tanks).

Fourth - burst discs; a lot of people who overfill run higher rated burst discs to reduce risk of it blowing at their preferred fill pressure. The "correct" burst disk is rated to blow at hydro pressure (+/- 100psi I believe).

So what I prefer to do is use old school 72's (2250 psi steels that are + rated for 71.2 cf at 2475 psi) with correct burst discs (pop at 3750 psi) and fill to 3000 psi. This buys me 86 CF of air in a tank that slightly smaller than an AL80, weighs less, but is negative buoyant (let's me take 5 lb off my belt) and and still has a 750 psi margin on the burst disc (which should still do its job in the event of a fire, etc.) HP steels are more negative than I need (or want) and heavier on land, so I prefer not to use them. And I hate AL80s with a passion... they are the worst of all worlds for a personally owned tank (in my opinion).

Your risk decisions must be your own, but this was my reasoning and where I ended up.

Respectfully,

James

 

[rx7diver]

One shop I’d emailed told me ... they weren’t risking their shop and employees for a $15 fill.

[HIJACK]
$15 for 85 ft^3 (one cylinder) of banked EAN32? Is this typical? (Seems high to me.)
[/HIJACK]

rx7diver

 

[formernuke]

[HIJACK]
$15 for 85 ft^3 (one cylinder) of banked EAN32? Is this typical? (Seems high to me.)
[/HIJACK]

rx7diver

Nitrox 22-40 here is 16 per tank.

 

[tmassey]

$10 Air and $15 Nitrox 32 (banked or otherwise) is typical for the Detroit area. Sometimes it’s lower.

SCUBA Services - Fluid Tech Services

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[HIJACK]
$15 for 85 ft^3 (one cylinder) of banked EAN32? Is this typical? (Seems high to me.)
[/HIJACK]

rx7diver

 

[Marie13]

[HIJACK]
$15 for 85 ft^3 (one cylinder) of banked EAN32? Is this typical? (Seems high to me.)
[/HIJACK]

rx7diver

$12-$15 here in the Chicago area, depending on shop. Air is around $10.

 

[tbone1004]

@James79 burst discs are 90% of test pressure
AL tanks are still required to pass 10,000 cycles to test pressure or 100,000 cycles to service pressure. Filling to 3300psi cold won't bother them.
HP steel tanks being heavier is not just wall thickness, they use more dense alloys as well.

 

[boulderjohn]

Some places rob you blind for nitrox. One of the people with whom I dive regularly said the local shops where he used to live charged $25 for an AL 80.

In contrast....

1. There is no local deeper diving in Colorado, so most shops, including the one I use most, do not offer nitrox. That shop does, however travel to areas where they do a lot instruction, and they like to have their instructors on nitrox for those trips because of the number of dives they do. As part of my friendly relations with the shop, I make their nitrox for them by putting the O2 in the tanks that they then top off themselves. I don't charge anything. I figure that if I did charge the per cubic foot price that I charge other divers when I make their deco mixes (etc.), i would be charging them about $3 for the O2 in each tank. If I charged wholesale, it would be less.

2. Fill Express, the now gone fill shop I used to use in Florida, had 32%, 36%, and 40% banked. If you rented a tank from them, you could get air, 32%, 36%, or 40%, with no difference in price.

 

[Marie13]

I got a good deal on Nitrox - just picked up my tanks. $10/each for the LP85s for banked 32%. I think I’ll be getting fills throughout the winter for my mine diving there. I much prefer diving Nitrox. It will just mean two round trips of 40 min/30 min each way (tanks get dropped off and I get a proper fill) but that’s much closer than where I used to go.

 

[OrcasC205]

Our local shop has an annual "All-you-can-breath" fill plan for banked 32%. At $450 for an individual or $650 for a Family plan, it works out well if you do a lot of local diving. I think between my wife and I we got about 100 fills this year, so it works out to $6.50/fill (a little less actually since the shop extended the contract by two months to make up for the Covid shutdown this spring.) Per-tank charge is $15/fill, so my breakeven point is 43 fills/year.