Tank Sizing Questions

[Vlane]

Out of curiosity, and attempting to expand my scuba knowledge, I started looking at tanks and sizes. Quickly after that things went WAY over my head.

1. Why is it that HP tanks, while having around the same cubic feet of a LP tank, are able to have a much higher service pressure? Also, looking at the chart at the bottom, why do some HP tanks have more cubic feet but the same service pressure?

2. What is the difference between a LP fill and a HP fill? I know that some places don't offer HP fills, but if there aren't HP fills available can you still get the typical 3000 PSI fill?

3. Why are LP tanks the preferred tanks in cave country. LP tanks have a lower service pressure, and I know that it isn't uncommon to get overfills to 3600, but why not just get a HP tank to begin with (something to do with HP fills availability)?

Please go as in depth as you need to on these, just please try and explain it in a way that is easy to understand haha. Also, feel free to give any other little tidbits of information as well.

[comet24]

1. LP and HP tanks are made with different materials thus different certified service pressures. Look at the tank dimensions they are different. X7 means a 7.25 inch diameter and X8 means 8 inch diameter.

2. You can always put less air in a tank. So yes you can fill a HP tank with 3000psi. You can do some simple algebra and calculate how much gas is in a tank at a certain pressure.

3. LP and HP tanks trim out differently in the water. LP tanks are lighter then HP tanks.

[scubadiver888]

Originally Posted by Vlane

Out of curiosity, and attempting to expand my scuba knowledge, I started looking at tanks and sizes. Quickly after that things went WAY over my head.

1. Why is it that HP tanks, while having around the same cubic feet of a LP tank, are able to have a much higher service pressure? Also, looking at the chart at the bottom, why do some HP tanks have more cubic feet but the same service pressure?

The cubic feet is the relative to the dimension of the cylinder. If you look at the diameter times the height you will notice that the 80, 100 and 120 all have the same diameter but the 100 is taller than the 80 and the 120 is taller than the 100. Thus, bigger cylinder means it will hold more air at the same pressure.

You can also look at the 100 and 119. They are the same height but the 119 is a larger diameter. That 3/4 of an inch larger diameter equals 19 cubic feet more air.

2. What is the difference between a LP fill and a HP fill? I know that some places don't offer HP fills, but if there aren't HP fills available can you still get the typical 3000 PSI fill?

A LP fill should be 2400 PSI. If the cylinder has a + symbol you can get an additional 10% or 2640 PSI. HP fill is typically 3442 PSI. There are some cylinders which will take 3500 PSI and I know of one which will go as high as 4400 PSI.

If you don't get the cylinder filled to the recommended pressure, the volume will be lower. For example, if you have a HP100 cylinder. It will have 100 cubic feet at 3442 PSI. If you fill this cylinder to 3000 PSI it will only have 87.2 cubic feet of air (3000 / 3442 * 100). You can even fill it to low pressure or 2400 PSI (2400 / 3442 * 100) and the cylinder will have 69.7 cubic feet.

My shop has two banks of air. One bank is at 3000 PSI and the other bank is at 3500 PSI. They will fill all the cylinder from the 3000 PSI bank. If you have a high pressure cylinder they will top it up with the 3500 PSI bank.

I plan on getting an HP100. A typical AL80 is something like 78 cubic feet when filled to 3000 PSI. The HP100 is 87.2 cubic feet when filled to 3000 PSI. So even when short filled, the HP100 will hold more air than an AL80.

3. Why are LP tanks the preferred tanks in cave country. LP tanks have a lower service pressure, and I know that it isn't uncommon to get overfills to 3600, but why not just get a HP tank to begin with (something to do with HP fills availability)?

In my area, none of the shops will give you a 'cave' fill. If you bring in an LP cylinder they will fill it to 2640 PSI. Some people like this because the higher pressure is harder on the regulator, you have no concerns about getting a short fill (most the time I get an AL80 with only 2800 PSI). They are also lighter and have better buoyancy characteristics.

For the areas that do have 'cave' fills, they get all the benefits of the LP cylinder plus they overfill them and therefore get more air supply then you would get from a HP cylinder. For example, if I take an LP95 and fill it to 3442 I get something like 123 cubic feet. HP119 or HP120 is only going to give me 119 or 120, respectively. Some cave divers have told me they fill their LP95 to 4000 PSI. That would be close to 144 cubic feet.

Personally, as someone who fills his own cylinders, I would be a little concerned about filling a cylinder rated for 2640 PSI to 3442 PSI. That is filling the cylinder to beyond 30% its recommended service pressure. People who do this make statements like, "They pressurize the cylinder to 5280 PSI when they do hydrostatic testing." This is done once every five years. You fill a cylinder from 30% to 50% above its recommended rating, daily, and you are going to cause metal fatigue. I would not want to be there when that cylinder fails.

One last tidbit of information. If the cylinder is an HPxxx then at the recommended pressure (3442 PSI) it will hold xxx cubic feet. However an LPxxx at the recommended pressure (2400 PSI) will only hold ~90% of xxx cubic feet. For example, an LP95 filled to 2400 PSI will be 86.4 cubic feet (2400 / 2640 * 95 and 2640 = 2400 + 10%). Only if the LP cylinder has the + stamp can you over fill it by 10% to make it 95 cubic feet.

[Scuba Pete]

lp tanks are also very popular in cave country because they are a proven tank. People have dove the lp104 for 20-30 years down there. The newer HPversion the hp130 is only a few years old and does not have the longevity worked out yet. take a 104 the service pressure is 2640, pump that to 3600 yet got 142cf of gas. take the hp 130 service pressure is 3442 pumped to 3600 you got 135cf of gas. So you get more gas from the lp tank than you do a hp tank that is supposed to be comparable to the lp tank. There are people down there that do that on a daily basis and their tanks last for 10+ years. There just isnt enough time that has passed for the hp tanks to "prove" themselves.

[Vlane]

I think the hardest part of this is trying to grasp that even though there might be the same amount of PSI in the tank, there may be more/less cubic feet of air. To me it seems like 2 tanks that both have 3442 PSI in them should have the same amount of air, but that's not the case. I realize PSI is a measure of pounds per square inch, but I guess I've never thought about it in terms of cubic feet of air compared with the size of the tank. Now it makes sense why cave country the preferred tank is LP.

[CompuDude]

Originally Posted by comet24

3. LP and HP tanks trim out differently in the water. LP tanks are lighter then HP tanks.

Not much difference in trim.

Only some LP tanks are lighter than HP tanks... they're generally pretty close.

[in_cavediver]

Everything is pretty well covered but I'll add the last bit on LP tanks - cost. In the past, they have been quite a bit cheaper than the HP equivalent. For instance, I got (4) lp 85'S for around $225 a tank. At that time, the HP 100's were all over $300. Tank prices fluctuate so I can't tell you now how much of factor this is.

If you go some places, you will see a LOT of LP tanks simply because they last forever. I have some LP72's older than I am. They are the same 3AA guidelines used all the way back to WWII. The were old before the HP tanks even existed. Being all that was available, thats what was used. A wealth of knowledge was built up for them and the cave fill grew from that.

I lied - one last bit. The 'HP' tanks are know as exemption tanks because they are produced on an exemption certificate issued by the DOT. This certificate defines hydro procedures, test procedures etc and must be renewed on a timeline. If the exemption is not renewed, the tanks are condemned when they come due for the next hydro test. This may or may be an issue to you but if you own the older PST HP tanks (3500psi), you might be concerned since PST isn't really around much. There are others from the Alluminum tanks that were produced under exemptions rather than 3AL standard and some of those are condemned based on expired exemption. The 3AA is a standard maintained by the DOT (same as 3AL for AL tanks). Its good for as long as the DOT recognizes it (used on LP tanks).

[WD8CDH]

Originally Posted by comet24

3. LP and HP tanks trim out differently in the water. LP tanks are lighter then HP tanks.

That depends on whether you are comparing same physical size or same rated capacity. If you are comparing same rated capacity, a HP tank is almost always quite a bit lighter than a LP tank (primarily because it is physically smaller).

If you are comparing same physical size HP and LP tanks, the LP tank will be about the same weight or just a few pounds lighter than the HP tank.

[navyhmc]

Not to mention LP's are a lot bigger LP 121 is 38", a HP 120-7 is 28" and HP1119-8 is 24 a 130 is 25. the LP121 is huge!

[in_cavediver]

Originally Posted by navyhmc

Not to mention LP's are a lot bigger LP 121 is 38", a HP 120-7 is 28" and HP1119-8 is 24 a 130 is 25. the LP121 is huge!

Navy, your numbers are off. The OMS LP121 is 29" and the OMS LP 131 is 31". The PST LP 121 is about 29.4" .This is compared to the PST E7-120 which is 28.5"

IN general - they are pretty close in length with a slight nudge longer to the LP tanks.