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Vlane
10-28-2009, 23:08
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 (http://www.scubatoys.com/store/detail.asp?product_id=X7100) 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
10-28-2009, 23:25
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
10-29-2009, 07:55
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 (http://www.scubatoys.com/store/detail.asp?product_id=X7100) 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
10-29-2009, 08:02
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
10-29-2009, 13:28
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
10-29-2009, 15:16
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
10-29-2009, 16:53
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
10-30-2009, 16:06
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
10-31-2009, 01:14
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
11-01-2009, 07:36
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.

Vlane
11-02-2009, 19:54
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.
I've been looking, out of curiosity, and LP tend to only be an inch or two taller than HP tanks. I can see how that might slightly affect trim but not by much.

Can someone elaborate on the + that some LP tanks have? The LP tanks that ST carry have the +, but is it uncommon to not have the +? Also, can a tank lose the + rating after years of having the 10% overfill or cave fill?

in_cavediver
11-02-2009, 20:46
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.
I've been looking, out of curiosity, and LP tend to only be an inch or two taller than HP tanks. I can see how that might slightly affect trim but not by much.

Can someone elaborate on the + that some LP tanks have? The LP tanks that ST carry have the +, but is it uncommon to not have the +? Also, can a tank lose the + rating after years of having the 10% overfill or cave fill?

The '+' rating allows for a 10% overfill of the tank. IE, most LP tanks are rated at 2400psi fill pressure but with the '+', they get filled to 2640. It should also be noted that the rated capacity of most LP tanks is at 2640, not 2400 psi. This is also limited to the 3AA steel tanks - commonly referred to as LP steels in the scuba arena.

As for which is elgible - any 3AA steel tank is eligible to qualify for the '+' rating. All it has to do is pass hydro at the '+' standard. This standard is valid only for the hydro period in question. Its possible for a tank to have the '+' rating for 5 years, not have it for 5 years and then again have it for 5 years. It all depends on the hydro test performed.

The last bit is cave fills. These simply are gross overfills in LP steel tanks and are not legal, allowed by the DOT or recommended by any tank manufacturer. They are also fairly common in North Florida. For an example - take my PST LP104's they are 106cft@2640. A cave fill puts them between 3600-4000psi. At 4000psi, they have around 160cft of air in them. (145cft at 3600psi). Cave fills are all about getting more air. A set of LP121's pumped to 4000 is almost 400cft of air. More cft means more time underwater on thirds. These do cause damage to the tanks and will shorten the life of the tank. That said - I have seen quite a few 15-20 year old steel LP tanks that have known a life of cave fills which keep passing hydro.

navyhmc
11-02-2009, 20:49
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.

Oops, you're right...sorry. Note to self, don't post when you're THAT tired.

CompuDude
11-02-2009, 22:45
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.
I've been looking, out of curiosity, and LP tend to only be an inch or two taller than HP tanks. I can see how that might slightly affect trim but not by much.

Can someone elaborate on the + that some LP tanks have? The LP tanks that ST carry have the +, but is it uncommon to not have the +? Also, can a tank lose the + rating after years of having the 10% overfill or cave fill?

In_Cave gave the detailed response. I'll summarize... sort of.

Most new LP tanks (if not all) are sold with the + rating stamp, which allows a 10% overfill. The tanks are all rated to 2400, so adding 10% gets you 2640.

LP tanks are sold assuming the plus rating. So an LP 95 only carries 95 cf of gas if you fill it to 2640 psi (ie, 10% over the stamped rating of 2400 psi)

After 5 years pass, and it's time for hydro, you have to take the LP tank to a hydro facility that is willing and able to give it the extra test needed to extend the + rating. A tank can pass the hydro but not the + test, at which point you can only (legally) fill it to 2400 psi, and it no longer holds 95cf of gas with a legal fill, it now holds 86cf. (Many simply continue to overfill and ignore this, but ...)

As I alluded earlier, there are a number of hydro stations that cannot (or at least that's what they claim) or will not perform the extra steps needed to restore the + rating to the tank... sometimes they simply don't know, other times they just can't be bothered, who knows. It's not supposed to be that way, but such is life. Most places around Los Angeles fall into this category, unfortunately. So unless you're an overfiller in cave country, if you choose LP tanks you'll want to be sure you have access to a hydro facility that can and will do the + rating steps.

That's pretty much the basics.

Vlane
11-03-2009, 01:16
Thanks for clearing the + up. It seems odd that a tank is advertised as 95 cf tank, but if it doesn't pass a certain aspect of hydro it is no longer a 95 cf tank.

WD8CDH
11-03-2009, 09:25
I have only seen two 3AA tanks manufactured after 1960 that did not have the + from the factory. I was able to get them (yes, they were mine:smiley5:) tested and stamped for the + on subsequent hydros.

Some, but unfortunately not all 3AA tanks have their REE value stamped on the tanks. That value is what the hydro shop needs to determine if the tanks can be stamped with the +.

in_cavediver
11-04-2009, 05:27
I have only seen two 3AA tanks manufactured after 1960 that did not have the + from the factory. I was able to get them (yes, they were mine:smiley5:) tested and stamped for the + on subsequent hydros.

Some, but unfortunately not all 3AA tanks have their REE value stamped on the tanks. That value is what the hydro shop needs to determine if the tanks can be stamped with the +.

The sad part is the REE values are published in the books/documentation the Hydro shops are supposed to have. As Compudude has stated, the quality of hydro shops seems to vary geographically.

Good for me - I have a top notch hydro facility who are willing to answer all of my questions and know that some tanks have recommended procedures (PST HPs for instance)

redsoxphan
11-05-2009, 11:35
Thanks everyone!! This is all great info and something that I've not given much thought until reading these posts.

So, an air hog like me needs more CF (bigger tank), not necessarily more working pressure.

Flatliner
11-05-2009, 14:47
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!

I LOVE my LP121!!!!

navyhmc
11-05-2009, 17:48
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!

I LOVE my LP121!!!!

I like mine as well, but you're juuust a few inches taller than me! :smiley36:

clayton
11-06-2009, 14:28
But why use lp tanks,my 3000psi 80's weigh 44 lbs filled,my new 100cf hi press are 4 lbs lighter filled and the same physical size,as long as they are filled properly i get 20 extra cf and slight less weight,but i go offset the weight with lead so not really aconcern.

navyhmc
11-06-2009, 14:40
I have a LP because that are some areas that you flat out can't get a full HP fill...

In "Cave Country" a lot of cave divers will overfill the LP's to get more cu ft of gas. And when you're 1,000' back in a cave, any extra gas you have means life! Or deeper penetration, your choice.

in_cavediver
11-07-2009, 07:33
But why use lp tanks,my 3000psi 80's weigh 44 lbs filled,my new 100cf hi press are 4 lbs lighter filled and the same physical size,as long as they are filled properly i get 20 extra cf and slight less weight,but i go offset the weight with lead so not really aconcern.

I have LP tanks because I got really good deals on some and because the HP tanks didn't exist when I got others.

Case in point - I got an LP85 for just under $200 (OK 4 of them). The HP equivalent is over $300 for the HP100. The characteristics of the 85 and 100 are darn near the same - its just less gas (when filled to rated pressure). If you get short HP fills, you are even closer.

There are all kinds of academic reasons to say to go for one or the other the reality is for most people it doesn't matter in practice.

Splitlip
11-07-2009, 07:54
I sold my LP's because it was becoming difficult to get the over fills. My main shop at the time changed their policy, and limited the fills to the rated pressures.

A cave diving buddy was happy to take them off my hands. LOL. Up in cave country he is still regularly getting fills to 4000 on the tanks "rated" for 2400.

Here is an interesting link. Mind you, you cannot even get these aluminum tanks filled today.

How much energy is there in a filled scuba tank (http://biobug.org/scuba/scubatank/)

navyhmc
11-07-2009, 08:31
I've been looking for that link! Thanks Split!

sanjose80
11-07-2009, 19:31
I think the steel tanks are thin walled too so for the same outside dimensions have more capacity inside.

in_cavediver
11-07-2009, 20:10
I think the steel tanks are thin walled too so for the same outside dimensions have more capacity inside.

Yes but the bottoms of steel tanks are rounded where Al tanks are flat. Any minor increase in volume from wall thickness is grossly overshadowed by the loss of volume with the rounded bottoms.

navyhmc
11-07-2009, 21:33
I think the steel tanks are thin walled too so for the same outside dimensions have more capacity inside.

Wall thickness has very little impact on total gas. The pressures do.

When you calculate psi vs. physical tank volume, the 1.5" of height difference between a HP119 and HP130 is pretty close to the compressible area needed at 3442psi to make up 11 cu ft of gas. The same goes for the height of a HP100 vs HP120 vs HP80

If you compare the LP vs. the HP tanks, a HP119 is 5.2" shorter than a
LP121 (120 cu ft actual) which is about right for the cubic inch difference in volume for the lower psi.

WD8CDH
11-09-2009, 15:19
I think the steel tanks are thin walled too so for the same outside dimensions have more capacity inside.

Yes but the bottoms of steel tanks are rounded where Al tanks are flat. Any minor increase in volume from wall thickness is grossly overshadowed by the loss of volume with the rounded bottoms.

AL tank bottoms are rounded on the inside so you loose about the same. The volume differance (in cubic inch water capacity) is primarily from wall thickness.

CompuDude
11-09-2009, 16:55
I think the steel tanks are thin walled too so for the same outside dimensions have more capacity inside.

Yes but the bottoms of steel tanks are rounded where Al tanks are flat. Any minor increase in volume from wall thickness is grossly overshadowed by the loss of volume with the rounded bottoms.

AL tank bottoms are rounded on the inside so you loose about the same. The volume differance (in cubic inch water capacity) is primarily from wall thickness.

Well, the corners are rounded off a bit, but I seem to recall seeing a cut away aluminum tank, and I could swear I remember the inside bottom being pretty flat.

In fact, here's a random image from the net that shows this:

http://www.wetwebmedia.com/SystemPIX/TankShots/tank.jpg

Splitlip
11-09-2009, 18:44
The bottoms of aluminum tanks are pretty flat but do roll up into the walls. And the bottoms I believe are thicker and a beefier where the vertical walls meet the horizontal bottom. For all intents I would call it rounded too. I am trying to find a section.

Edit: Duh! about half way down in the link I posted above, you can see what I mean.