steel tanks?

[Indian Valley Scuba]

You (or the filler) might get seriously hurt filling a low pressure tank to 3,500 psi.

I am sure the poster was just using the physical size as an example, not the thickness of the material, pressure rating, burst disc rating, etc.

I would suggest you find a friendly local dive shop that will talk to you and show you how the tank neck markings are, so you can get a better handle on what you are looking at.

 

[awills101]

You (or the filler) might get seriously hurt filling a low pressure tank to 3,500 psi.

I am sure the poster was just using the physical size as an example, not the thickness of the material, pressure rating, burst disc rating, etc.

I would suggest you find a friendly local dive shop that will talk to you and show you how the tank neck markings are, so you can get a better handle on what you are looking at.

i would never fill a tank past it's pressure rating, and im sure my dive shop wouldnt either it was just as example for my question about hp an lp tanks of the same cf holding the same amount of air

 

[Indian Valley Scuba]

Without getting to much into math, there are two finite differences in the discussion here.

One, two cylinders that are physically the same size, meaning you could in fact pour the same volume of liquid into them, have the same starting capacity.

However, when you are talking about a compressable medium, such as air, that changes things, and tank volumes here in the states are quoted in terms of cubic feet of capacity of air, based on standard temperature and pressure. So the ending pressure of the vessel is what the ultimate capacity is all about. If a cylinder is designed and rated for "x' psi as a maximum, then that is what determines it's designated capacity, such as a LP104. Even though it might have the same physical space inside as that HP130 used in the earlier example, you could not put the same amount (# of cubic feet) of air in there since you would have to compress the air that much more, resulting in a higher pressure.

So, to summarize, physical tank volume and theoretical tank volume are two very different animals.

Go visit that local friendly dive shop, and if you're anywhere near us, come stop in. We'll teach you all you want to know about tanks and then some!

 

[awills101]

i understand what your saying about physical size of the tanks, but i know what i needed to know and that is that and lp and hp 100 will hold the same amount of air the lp will just be bigger than the hp becuase it holds the air at a lower pressure. right?

 

[Indian Valley Scuba]

If they are rated to the same capacity yes, a LP100 and an HP100 will both hodl 100 CF of gas at their design pressure rating. So obviously the LP will have to be smaller cause the bubble of air can't be squeezed as tightly with the lower pressure rating.

Sounds like you got it! Good luck in buying your tanks.

If you want a handy spec sheet on tanks, go to ScubaGearPlus and click on "Tanks" and then "Cylinder Specifications". All the dimensions, capacities and everything else is there for you.

 

[awills101]

thanks for your help!

 

[DA Aquamaster]

You (or the filler) might get seriously hurt filling a low pressure tank to 3,500 psi.

I am sure the poster was just using the physical size as an example, not the thickness of the material, pressure rating, burst disc rating, etc.

I would suggest you find a friendly local dive shop that will talk to you and show you how the tank neck markings are, so you can get a better handle on what you are looking at.

I won't argue with that directly or dispute the DOT regs covering tank fills, burst discs, etc. But it is common practice in North Florida to fill 2400 psi tanks to 3600 psi.

In that case a Worthington LP 95 that weighs 42 lbs is 8.0" in dia and 23.7" tall will hold 93.3 cu ft at 2640 psi and 126.8 cu ft at 3600 psi. A Faber 95 has the same basic dimensions and capacity but weighs only 37 lbs.

In comparision a Worthington X8-130 weighing 43 lbs, 8.0 in dia and 25.5" tall will hold 131.4 cu ft at 3442 psi and 100.8 cu ft at 2640 psi.

So in many cases N Fl cave divers will opt for a Faber 95 and save 12 pounds per set of doubles compared to an HP130 and get close to the same volume of gas. With larger LP tanks like the LP 108, the divers in question will get more gas 148 cu ft per tank at 3600 psi than they could otherwise get with available HP tanks.

Long term, this practice of overfilling to what amounts to the test pressure of the tank has not resulted in catastrophic failure but has I think resulted in much shorter life spans for some of the LP tanks involved with the result that they will fail a hydro test that they would otherwise not fail in the forseeable future.

So while I agee the diver or filler "might" get seriously hurt by this practice, it has not yet happended to date and I am pretty sure the poster was not just using it as an example.

I also think that if Worthington, PST or Faber came out with an LP 108 sized HP steel tank with a capacity of 150 cu ft, cave divers would buy it. Until then, LP 108's are going to get fillled to 3600 psi.

 

[halocline]

If they are rated to the same capacity yes, a LP100 and an HP100 will both hodl 100 CF of gas at their design pressure rating. So obviously the LP will have to be smaller cause the bubble of air can't be squeezed as tightly with the lower pressure rating.

I'm sure you meant to say that the LP tank must be bigger, not smaller.

 

[halocline]

I would suggest you find a friendly local dive shop that will talk to you and show you how the tank neck markings are, so you can get a better handle on what you are looking at.

Unfortunately, some dive shop employees have a poor understanding of tank specs and capacities. The few that post regularly on SB are exceptions to the rule, I'm afraid. One good source of basic info about tank markings is in the Vance Harlow book on regulator repair. I'm sure there are websites as well that clearly explain basic info about scuba cylinders.

 

[SparticleBrane]

After looking at Indian Scuba's website and some of the information on there about cyilnders, I feel the need to speak about some of the things they have posted there...

Here is the total quote:

Myth #4 You can never get a full fill if your shop doesn't fill tanks in a water bath

All cylinders should be filled slowly so heat should never pose a problem. Slow filling also extends the life of the cylinder by "working" the cylinder walls less from fill to fill. For an optimal fill, they can be left connected to a regulated fill system and allowed to top themselves off as any gas expansion caused by a warm fill dissipates. If for some reason a cylinder is filled quickly, or other cylinders are in the queue, then simply letting a filled cylinder sit and cool and then topping it off later will yield a more accurate fill. The whole water bath thing is another leftover from the "that's the way we've always done it" mentality - from an engineering point of view there is no significant circulation of gas within the cylinder during the filling process, and air itself has a very poor thermal conductivity coefficient, so the thermal transfer of heat from the gas to the cylinder wall and then to the surrounding room air is very slow and inefficient. Water baths at room temperature, which many shops swear by, do not have any magical abilities to reach into your cylinder and draw the heat out of the gas within, making water baths simply an unnecessary mess at any fill station. In fact, pre-chilling cylinders as some do in a water bath actually can actually cause condensation to form on the interior cylinder walls during the filling process, which obviously is something we all wish to avoid in our tanks. An additional drawback to water baths for tank fills is the number of times a fill whip gets accidently dropped into the water, then placed onto a tank valve for filling blowing the water that is on the fitting end right into the tank!

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All cylinders should be filled slowly so heat should never pose a problem. Slow filling also extends the life of the cylinder by "working" the cylinder walls less from fill to fill.

The reality is that not all cylinders will be filled slowly. Sometimes you need to fill a bunch of cylinders quickly, and it isn't economical or worth anyone's time spending hours filling cylinders that you need right now.

Water baths at room temperature, which many shops swear by, do not have any magical abilities to reach into your cylinder and draw the heat out of the gas within, making water baths simply an unnecessary mess at any fill station.

One thing I have noticed is that usually, the temps at water baths tend to be lower than the ambient air temperature, mainly due to small amounts of evaporative cooling. Either way, water can hold tens of times the amount of heat that air can, and can transfer it much quicker. Heating the cylinders through filling and having them sitting in a water bath works wonders to keep them cooler through the filling process.

In fact, pre-chilling cylinders as some do in a water bath actually can actually cause condensation to form on the interior cylinder walls during the filling process, which obviously is something we all wish to avoid in our tanks.

The maximum allowable dew point for CGA Grade E air is -65F, and is lower for oxygen-clean air. I can't say I know of anyone chilling cylinders to that temperature.

An additional drawback to water baths for tank fills is the number of times a fill whip gets accidently dropped into the water, then placed onto a tank valve for filling blowing the water that is on the fitting end right into the tank!

If it gets dropped in the water (which I have never seen happen, but I'm sure it occasionally does), just open the fill whip for a few seconds and blow any water out; it will quickly dry the fill whip.