LP vs HP steel tanks

[Superlyte27]

Is 3600 psi a misprint? The huron scuba chart shows a LP 95 as 2640 psi when +'d. Is it regular to fill more than 900psi over recommended max? What does this practice do to the life expectancy of a tank?
Thanks OND

So the next time you see a set of used tanks for sale, remember that the scuba police isn't around to restrict the ability to overfill tanks. I have wondered if this type of diver uses these tanks and then sells them to unsuspecting divers claiming they will pass hydro. They probably will pass hydro, however, how much of their usefull like has been reduced. I will continue to buy my tanks new or from trusted dive buddies.

Dennis[/QUOTE]

LOL, here we go again. 10 years ago OMS was marketing Faber's. In their warranty literature it said right on the front, guaranteed 10,000 fills at 4000 psi. Of course the DOT made them remove the literature but... I've recently got rid of a set of doubles that were 20+ years old with at least 1000 fills at 3600+, and they pass hydro every time. I went to bigger tanks. There is no issue with a properly maintained LP Steel tank failing hydro and fill pressure under 4000psi is not an issue of maintenance.

 

[Superlyte27]

The more interesting question to ask is how do you legally defend this practice in the event of an injury or death at the filling station or anywhere else as a result of any failure of tank, valve, etc.

Don't know, there's never been one.

 

[Superlyte27]

Comparing LP95 to HP100 is not a proper comparison.

XS Scuba Worthington Steel Cylinder Specifications

LP95 is comparable to HP119.

So when you do that advantages (other than price) of LP95 are gone.

Negative Ghost rider, your math is really bad.

95/2640*3600=130 on an LP95

119/3442*3600 = 124 on an HP119

Multiply that times 2 tanks for Doubles config and that's 12 more cu' of air in a cheaper tank with better buoyancy characteristics, and a work horse record of decades without issue.

But hey, what do I know, you're the same guy that insisted on 36% Nitrox at a cost of 12 bucks per tank for a 20' reef dive. :0

 

[Troutmaster]

Negative Ghost rider, your math is really bad.

95/2640*3600=130 on an LP95

119/3442*3600 = 124 on an HP119

Multiply that times 2 tanks for Doubles config and that's 12 more cu' of air in a cheaper tank with better buoyancy characteristics, and a work horse record of decades without issue.

But hey, what do I know, you're the same guy that insisted on 36% Nitrox at a cost of 12 bucks per tank for a 20' reef dive. :0

I am guessing he is refering to physical size and weight of the tanks, not the volume of gas at a given service pressure.

The machines that make tanks are expensive. So for almost every LP tank there is an HP tank that has very similar size, shape and weight, but is made from steel with different tensil strenght. The steel changes the service pressure.

You are not going to blow either tank up filling it over capacity (within reason). If you are given a choice of two tanks, both the same size and weight and one with a higher service pressure, take the HP tank if you think you ever going to use it somewhere that will only fill it to rated capacity. Like any dive shop not in North Florida. I fill my own tanks on a 6000psi bank system so any tank I buy is HP when I am done (no, I do not fill to 6000psi, that is just what the bank has on tap)

If you get a good deal on an LP tank, snatch it up, you can not have too many tanks and switching to steel over aluminum will get some of the lead off your belt.

 

[DA Aquamaster]

And I thought that the only difference between LP and HP was the pressure stamped on the tank due to to silly DOT regulations. When in Florida I fill both LP and HP to 4,000 psi unless Wayne is behind in charging up his bank. I may be a little off on some of the numbers, but an LP 85 is basically an HP 100, while an LP 95 is an HP 119. It would sure be easier to just adopt the European standard of actual volume in liters so that the answer to this question would be obvious.

You are correct in terms of a worthington HP119 having very similar dimensions to a Worhington LP 95, but they are not the same tank with nust a different service pressure.

The Worthington LP 95 is basically a 14.5 liter tank (2642 liters at 182 bar) and the HP 119 is a 14.6 liter tank (3482 liters at 237 bar) - but they are not the same tank as the external dimensions are slightly different, and the engineering standards, materials and wall thicknesses are different.

I agree with you that having a tank's internal volume identified makes sense as you can multiply the volume time the atmospheres to get the volume. But its not a cure all.

In fact a bar is 14.5 psi while an atmosphere is 14.7, so bars don't exactly equal atmospheres.

Seems like a small point but its really not. What makes it work in Europe is that bars are "close" to atmopsheres and that SPG's are calibated in bars so each bar will have an amount of gas close to the internal volume of the tank marked in liters.

But in the same sense I know that a faber LP 95 (roughly a 15 liter tank) holds 95.1 cu ft at 2640 psi or .036 cu ft per psi. (95.1 psi/2640=.036). So it's exactly the same concept as using bars and liters, you just use 100 psi increments (3.6 cu ft per 100 psi) for convenience. It's called a "tank factor" in most techncial dive training texts in the US.

So if I have 2000 psi remaining in my Faber LP 95, I know I have 72 cu ft of gas remaining (20x3.6=72).

I also know that a tank with a larger internal volume also has a larger tank factor. For example an X7-100 is basically a 12 liter tank and has a tank factor of 2.8. When you compare that to a 15 liter LP 95 it's obvious the "smaller" X7-100 has a smaller tank factor but holds similar volume at the service pressure because the service pressure is higher (2640/182 bar for the LP 95 versus 3442/237bar for the X7-100).

In short, you don't really gain anything just by noting the internal volunme of a tank in liters - you also have to consider the service pressure and whether you use bars and liters or psi and cu ft, it all works the same. The only difference is that Fabers and soem other tanks come stamped with their internal volume in liters while you;d have to use simple math to determine internal volume in cubic feet - or reference a chart that lists tank factors.

Either way, it's still obvious.

 

[DA Aquamaster]

Is 3600 psi a misprint? The huron scuba chart shows a LP 95 as 2640 psi when +'d. Is it regular to fill more than 900psi over recommended max? What does this practice do to the life expectancy of a tank?
Thanks OND

As noted above it is not a misprint, it's a common N FL cave fill. And as noted above, operational history suggests the practice is not dangerous - certainly safer than the actual drive to the dive shop.

The other side of the coin (suggested by indian valley scuba above in regards to liability with an overfill) is that we have too many effing lawyers and an over litigious society.

----

Superlyte's math is fine but his starting assumptions are not 100% correct.

An X8-119 is really not a 119 at all but rather a 123 cu ft at 3442 psi, so at 3600 psi the 42 pound X8-119 holds 128 cu ft compared to a 38 pound Faber LP 95's 129 cu ft. In comparison the 42 pound Worthington LP 95 holds 93.3 cu ft at 2640 psi and 127 cu ft at 3600 psi.

At 3600 psi all three tanks are ballpark identical (127 to129 cu ft) and the two worthington tanls weigh essentially the same, while the Faber is 4 pounds lighter.

So given access to 3600 psi fills for low pressure tanks, I'll pick the Faber 95 every time. If I can't get overfills, I'll pick the X8-119 as it ensures I can get 123 cu ft of gas almost anywhere.

I own both, a steel HP & a steel LP. I use the LP only if I'm going to be on a boat that can't fill over 3000psi. No one likes getting a short fill, especially on a boat....

The "LP is better on boats due to low fill pressure" argument is not accurate when comparing similar dimensioned tanks. Looking at the numbers above it is obvious that an LP 95 or an X8-119 are going to hold the same gas at the same pressure.

So if the boat only fills to the DOT approved service pressure, at best I am only going to get 2640 psi and 93.3 cu ft in a Worthington LP 95. In contrast even if I only get 3000 psi in my 3442 psi service pressured X8-119, I am still going to get 107 cu ft of gas in my "underfilled" tank. Same size tank, same weight, same boat, but 14 cu ft more gas.

If I can get the same 3000 psi on my LP 95 then it's still only a draw and the LP 95 offers no advantage.

It's obvious over the long term in a larger number of scenarios that you are better off with the 3442 psi tank even if you are getting short fills.

Outside of N FL cave fills and slightly lower purchase price, low pressure tanks really don't offer any advantage compared to a similar dimensioned 3442 psi tank.

 

[Superlyte27]

Unless you fill your own tanks or 95 percent of your diving is in north florida. I meet both of those qualifications.

 

[CaboJim]

"The "LP is better on boats due to low fill pressure" argument is not accurate when comparing similar dimensioned tanks. Looking at the numbers above it is obvious that an LP 95 or an X8-119 are going to hold the same gas at the same pressure. :

Your point is only valid if your LP tank's capacity is smaller than your HP tank's capacity. The LP tank I have is an 85 cu ft vs my HP tank is a 72 cu ft......

 

[iztok]

Negative Ghost rider, your math is really bad.

95/2640*3600=130 on an LP95

119/3442*3600 = 124 on an HP119

Multiply that times 2 tanks for Doubles config and that's 12 more cu' of air in a cheaper tank with better buoyancy characteristics, and a work horse record of decades without issue.

But hey, what do I know, you're the same guy that insisted on 36% Nitrox at a cost of 12 bucks per tank for a 20' reef dive. :0

Now lets do the real math based on facts from: XS Scuba Worthington Steel Cylinder Specifications

LP95:

93.3/2640*3600 = 127.2

HP119:

123.0/3442*3600 = 128.7

For a total of 3 cuft advantage for HP119 doubles vs LP95.

As far as buoyancy characteristics are concerned, difference in weight of tanks is 0.1lbs and difference in length is 0.3". Which would indicate perhaps less than 2% change in buoyancy characteristics.

BTW: Where/when did I insist on 36% fill for 20ft dive?

 

[Superlyte27]

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