29 replies to this topic

[Sharktooth]

Hi Everyone,
Just don't get this. Guess I'm too hard headed
My cave diving buds and I had a discussion on LP vs. HP tanks they dive 120 LP 2460 + I dive a 100 HP 3440.
They said there tanks have more volume. I agreed. But all tanks being of equal size the LPs still have more air. One of them whipped out his calculator ran some numbers and said my tank was the equivalent to a 71.2 / 80 LP. I did not see the equation. Their analogy was thus; if the boat or shop could only fill up to 2500 psi then the 2460 would be full where as the 3440 would not The tanks both being 100s. Again I agreed but had to leave still not sure of the logic. My question is if both tanks are equal and filled to 2500 psi aren't they both holding the same volume or amount of air? And does anyone know the calculations they where using? Math uck!
Any info would help.
Thanks
Sharktooth

[Walter]

My cave diving buds and I had a discussion on LP vs. HP tanks they dive 120 LP 2460 + I dive a 100 HP 3440.

I love this stuff.

My question is if both tanks are equal and filled to 2500 psi aren't they both holding the same volume or amount of air?

No.

And does anyone know the calculations they where using?

I know how to get the correct answer. I can't verify what they were using.

First, let's define. 120 LP 2460 means the tank holds 120 cubic feet of air at 2460 PSI. 100 HP 3440 means the tank holds 100 cubic feet of air at 3440 PSI.

You want to see what they's each hold at 2500 PSI.

120/2460 = X/2500, X = the amount of air in cubic feet the 120 LP will hold at 2500 PSI.

Simply cross multiply to solve for X. 120*2500 = 300,000 = X*2460 therefore X = 300,000/2460 = 121.95

100/3440 = Y/2500, Y = the amount of air in cubic feet the 100 HP will hold at 2500 PSI.

Simply cross multiply to solve for Y. 100*2500 = 250,000 = Y*3440 therefore Y = 250,000/3440 = 72.67

The reason is your HP 100 has approximately the same internal volume as an old steel 72. The 120's have a larger internal volume. One mistake we often make is equating PSI with volume. When the DM says, "Return to the boat with 500 PSI," he's drilling a bad concept into your head, ie PSI equals volume when it doesn't. If you and your buddy both return (using the tanks described above) to the boat with 500 PSI, you'll have 14.53 cubic feet of air while your buddy will have 24.39 cubic feet. If 14.53 is good enough for you, why isn't it good enough for your buddy? If your buddy returns with 300 PSI, he'll have 14.63 cubic feet. That would be a more reasonable directive, but harder to measure.

[sapphire]

Walter,

Thank you very much for an explanation that even my math-challenged brain can understand. But now you raised another question for me. I am in the process of deciding what type of tank to purchase to replace a too-huge HP120 that I got a great deal on... it is just too big and heavy for me.

I have been advised to look at an LP 100 or 95, instead of an HP... the rationale was that I could get a "better" fill on the SoCal dive boats, who cannot fill HP tanks to full pressure.

SOOOOO.... if an HP100 (which they can only fill to 3000) and an LP100 (which they could fill to 3500) both have the same amount of air in them...

AND.... one will register 3000 on my SPG and one will register 3500...

Do I have the same amount of air or not? Will the difference in pressure in the tanks make a difference in how I use it? What would be the advantage of the LP then?



Hopefully noone will laugh at my question... it is now bugging me...

jennifer

[Diverbrian]

Personally, I use HP steels for my back gas.

I will be using a LP tank when I eventually get around to using pure oxygen as a stage bottle and shops don't like boosting the pressure of pure oxygen any higher than they need to.

I am never fortunate enough to be on dive boats that have a compressor so worrying about whether the boat can fill my tank is a non-issue. My personal experience is that the LP steels tend to be extremely heavy and bulky. I have buddies that dive LP 125's and I don't even like to think about lifting them.

As to the question, you would have far more air in the LP tank as you will have more internal space to meet your volume at a lower rated rated rated pressure.

The LP tank that you refer to:

3500 psi (actual pressure)/2460 psi (rated pressure) * 100 cu. ft (capacity when at rated pressure) = 142 cu. ft. of air

vs.

HP

3000 psi/ 3500 * 100 cu. ft= 85 cu. ft. of air

Of course, you wouldn't be pushing hydro on the HP tank if you filled it that way continously. If you filled both to rated pressure, you would obviously have the same amount of air in both tanks.

[Sharktooth]

Thanks Walter,
Now I understand after seeing the equation. I believe that is the same calculation they were using. My buddy's tanks are double LP 2460 + 120s which he wants to sell me. He wants double 130 LPs. I do not dive with him he dives with two others one is full cave cert. The other 2 are trying to get there. I'm not ready for that need rescue an O2 provider 1st (working on that). My steel HP 3442s do just fine for me now. I am hungry to learn and mathematically challenged.
Again thank you very much Walter.
Hi Sapphire,
There are several web sites that you can go to compare tank stats. I do not know if I can post them here so I did not.
What I do know & that is not much. Is a shop should only fill a tank to the working pressure that is stamped on the tank.
So lets say you have a 120 LP tank with a WP of 2460 + (+ means they can fill the tank 10% over the stamped pressure for the first five years ). This tank should only be filled to 2706 ( 2700) psi. If the shop is filling it to 3500 not only is it illegal but they are putting their & your life at risk. How ever unlikely, the tank could explode. The pressure ratings are put there for our safety. Walter please correct me if I'm wrong you've more experience than I. I'm still learning how to correctly fill and service tanks. My steel tanks are close to the same weight as an 80 alum. & are negative buoyant at 500 psi. I love my tank for the diving I do.
No way would I laugh at any questions asked about our hobby. Sorry so wordy.

Eric

[Walter]

There are a couple of issues with what is apparently happening on some dive boats in California. Jennifer and I talked about it on the phone last night to help me better understand those issues.

First, Brian's math is correct (why wouldn't it be, he's a bright guy). The only problem is we have to made assumptions on the working pressure of the two different tanks. Until we know the actual working pressure, the volumes are guesses. Jennifer's going to get the rated pressure of the tanks at which she's looking, then she can make an informed decision.

Next, Eric is correct about overfilling tanks. While it is unlikely the tank will explode, it is a possibility. It is more likely the burst disk will fail (which is its purpose). Overfilling tanks will also reduce their life.

Apparently, the staff on dive boats are telling Jennifer they can fill low pressure tanks to 3500 PSI (a serious no no), but can only fill high pressure tanks to 3000 PSI. This is something I don't understand. If the boat can fill DIN tanks at all and their compressors will fill one tank to 3500 PSI, there's no reason they can't fill the DIN tank to 3500 PSI as well. They tell Jennifer they don't have the necessary adaptor. This makes no sense to me at all. Can anyone shed some light on this practice?

Eric, you bring up an excellent point on steel vs AL with regard to weight and buoyancy. Many people assume steel tanks are heavier because the sink. That's usually not the case for tanks with similar volumes.

[canuckdiver]

Walter,

Thank you very much for an explanation that even my math-challenged brain can understand. But now you raised another question for me. I am in the process of deciding what type of tank to purchase to replace a too-huge HP120 that I got a great deal on... it is just too big and heavy for me.

I have been advised to look at an LP 100 or 95, instead of an HP... the rationale was that I could get a "better" fill on the SoCal dive boats, who cannot fill HP tanks to full pressure.

SOOOOO.... if an HP100 (which they can only fill to 3000) and an LP100 (which they could fill to 3500) both have the same amount of air in them...

AND.... one will register 3000 on my SPG and one will register 3500...

Do I have the same amount of air or not? Will the difference in pressure in the tanks make a difference in how I use it? What would be the advantage of the LP then?



Hopefully noone will laugh at my question... it is now bugging me...

jennifer

Jen, try a set of E7-100's.

These are about the same size as my 72's, and hold more than enough air for any dive you are going to do.

They are also small, and relatively light, so they are easy on the back.

Most shops can fill aluminum tanks, which means it's usually pretty easy to get a 3000psi fill, wich gives you at least the same volume as an AL80, with the better buoyancy characteristics.

MHO

[AndrewJD]

A simple method of working with disimilar tanks.

Basically we want to come up with a factor that gives us the volume per unit pressure. We call this the "Tank Factor" or TF.

TF = Total Volume [cuft]/Working Pressure [psi]*100

Example

An aluminum 80 has a working pressure of 3000psi and holds 77 cuft of gas.

TF = 77 cuft/3000psi*100 = 2.567 cuft/100psi. For simplicity we'll call it 2.5 cuft/100psi.

A LP95 has a working pressure of 2640psi and holds 95cuft of gas.

TF = 95 cuft/2640psi*100 =3.598 cuft/100psi. Again for simpicity we'll call it 3.5 cuft/100psi.

So how to use this.

If I'm diving my LP95 and my buddy is diving an Al80 we need to account for the disimilar tanks when doing gas management. From the tank factors we know that I have more gas per 100 psi then my buddy. Say for a 100' dive we determine that we each need 24 cuft of gas to get both of us to the surface with a normal ascent. I'll call this Rock Bottom and this will change based off of depth.

Now to calculate the turn pressure (TP) for each tank.

TP = RB/TF

LP95 TP = (24cuft)/(3.5cuft/100psi) = 685psi ----> 700psi

Al80 TP = (24cuft)/(2.5cuft/100psi) = 960psi -----> 1000psi

So when I reach 700 psi with my LP95 or my buddy reaches 1000psi with his Al80, whichever comes first, its time to call the dive and ascend.

Jonathan

[oatmeal769]

Confusing indeed.

First off, physical size of the tank matters not to gas volume, or pressure.

Second, As far as I know, 'HP' and 'LP' are a bit of a misnomer, unless more definition is applied. The word 'Pressure' as it applies to compressed gas in metal cylinders means different things to different people, depending on the industry, the cylinder, the application, etc. In most American recreational scuba circles, HP mostly means a cylinder whose working pressure is above 3000 PSI. 'LP' More or less means anything under 3,000 PSI working pressure. I have heard the ubiquitous aluminum 80 termed both as an 'HP' and an 'LP' tank. I'm sure someone else can comment more on where the LP and HP designations came from, I just know they both hurt when dropped on my foot.

I think the issue of confusion besides labeling is tank volume, and tank pressure. Pressure (PSI) is not the same as volume (cubic feet). It's easy to tell how much gas (volume) you have when a cylinder is pressurized to its working, or 'rated' pressure. A scuba cylinder is most often referred to (in America) by the cubic feet of air it holds at it's rated pressure. I.E. a '95, a 120, an '80' a '72' etc.

If you stick 'LP' or 'HP' in front of those numbers, it more or less describes the general range of the maximum pressure the tank was designed (or deemed safe by one agency or another) to be filled. So, an 'HP'120 filled to it's rated pressure of 3,500 PSI holds about 120 cu, ft. of gas. an 'LP'95 holds about 95 cu. ft. of gas at it's rated pressure of 2,400 PSI. My father is known to hold close to 40 cu. ft. of gas after eating too much broccoli.

It can get tricky to guess how much gas is in a cylinder when it is only partially filled, unless you know the magic happy gas formula. Just like Walter & Andrew described, There's also a slightly more concise way to figure it. The goal is to be able to convert a known pressure (PSI) to a volume (cu. ft.) Also, to liberate some Mexican zebras from their oppressive regime, but that's another topic... Then you can easily figure how much gas you have.

As far as dive boats in California, no offense to any crew reading, but always talk to the captain to ascertain the capabilities of his boat. Walter's right, there is a lot of bad information out here, and I'd say the volume / pressure issue and HP / LP thing confuses many crew, and the people you get on the phone. They want to be helpful, so they give out wrong info instead of finding out for sure. ...Duuuude, like, MASSIVE pressure dude! Totally MASSIVE ! !

I use a so called HP80 for most of my recreational dives, and have been on (I think) every single dive boat from Ventura to San Diego in the last 5 years. Some with 'vintage' (read older) compressor systems are only able to fill to 2,500, but they are few and far between. A few only fill to 3K, but they are decreasing in numbers too. Most well run charters here are capable of handling 3500 PSI fills all day long. The HP thing has been around a while now, and most operators know they need to have the capability to serve this growing customer base. When in doubt, call and if you get anything other than a sure answer, ask for the owner dude, or captain dude.

There are a couple boats that do indeed pull the 'We don't have an adaptor for DIN valves' story. That's when I like to whip out my trusty adaptor (which any shop can get you for about $15-$20) and say, 'We don' need no stinking adaptor! I have one of my own!'. If you decide on a HP cylinder, an adaptor is a good extra to have in your save a dive kit. I also think that if a boat still doesn't handle what has become commonplace among divers, They don't need to have my business. There are LOTS of boats out here.

[sapphire]

Confusing indeed.

Gosh, I feel I have been hanging out in the garage too long...

First off, physical size of the tank matters not to gas volume, or pressure.

Duly noted. I think I get this, actually.

  I'm sure someone else can comment more on where the LP and HP designations came from, I just know they both hurt when dropped on my foot.

I have to say I agree here !!

  My father is known to hold close to 40 cu. ft. of gas after eating too much broccoli.

Also, to liberate some Mexican zebras from their oppressive regime, but that's another topic... 

I just love finding these gems buried in tech descriptions!

As far as dive boats in California, no offense to any crew reading, but always talk to the captain to ascertain the capabilities of his boat.  Walter's right, there is a lot of bad information out here, and I'd say the volume / pressure issue and HP / LP thing confuses many crew, and the people you get on the phone.  They want to be helpful, so they give out wrong info instead of finding out for sure. ...Duuuude, like, MASSIVE pressure dude! Totally MASSIVE ! !

No wonder I am confused...

There are a couple boats that do indeed pull the 'We don't have an adaptor for DIN valves' story.  That's when I like to whip out my trusty adaptor (which any shop can get you for about $15-$20) and say, 'We don' need no stinking adaptor!  I have one of my own!'.  If you decide on a HP cylinder, an adaptor is a good extra to have in your save a dive kit.

This boat I was on had the adapter, but I still only got fills to 3000 in the borrowed HP100...

I still don't know what tank to get, but I know a heckuvalot more than I did yesterday about the issues. Thanks, guys, for sharing your knowledge I appreciate it!

jennifer

[oatmeal769]

Here's some trivia on filling cylinders...

Any dive shop or boat worth it's salt isn't going to fill your cylinder beyond what the DOT rating is for the tank. With aluminum cylinders, and HP steel cylinders, this is the safe route, and is right on the money. You may hear though that some LP cylinders can be 'overfilled' by hundreds of PSI. While any shop in it's right mind would never fill beyond the DOT or TC (canada) rating and rightfully so, it is nonetheless true.

For example, some (but not all) Faber steel '95's' are designated by Faber for 5,000 fill cycles at 207 BAR. 207 BAR = 3001.5 PSI. So even though the DOT 'rated' pressure is 2400, or 2640, it's completely fine to fill the cylinder to 3K as long as you have a valve and burst disk that are made for that pressure. The trouble is finding a shop to do it for you.

The hydro test pressure on these cylinders is 4K, and the minimum cylinder burst pressure is 6400K. The deep dark secret of many 'tec' divers is that they routinely fill these cylinders to 3K - 3.5K and stay completely within 'safe' parameters.

Edited by oatmeal769, 10 June 2004 - 11:42 AM.

[DandyDon]

You guys lost me back there somewhere. Here's my understanding on...

My cave diving buds and I had a discussion on LP vs. HP tanks they dive 120 LP 2460 + I dive a 100 HP 3440.

A 120 LP 2460, taken to a compressor that cannot fill beyond 3000#, can be filled to capacity = 120 cf.

However, a 100 HP 3440 taken to the same compressor and filled to only 3000# will have:

3000 x 100 = 87.2 cf, right...?
3440

Did I agree with anything already said...?

[Walter]

No, Don, you'll have approximately 72 cu ft, not 87. See my first post above.

[zendiver]

Now this is what I am talking about....some good ole equations relating to diving...don't get much better...thanks to all that has responded. VERY good and useful information.

[oatmeal769]

Not to ruffle any feathers, but I am of the opinion that Don's answer is correct. This is the calculation of which I spoke earlier...

It can get tricky to guess how much gas is in a cylinder when it is only partially filled, unless you know the magic happy gas formula. Just like Walter & Andrew described, There's also a slightly more concise way to figure it. The goal is to be able to convert a known pressure (PSI) to a volume (cu. ft.) Also, to liberate some Mexican zebras from their oppressive regime, but that's another topic... Then you can easily figure how much gas you have.

The way I do this is with a cylinder 'baseline.' a Baseline is how much volume one PSI takes up (in cubic feet) in a given cylinder at it's rated pressure. once I know that, I can just multiply that 'baseline' number by the number of PSI a tank has in it to find the volume of air in it given as cubic feet. It's easy to find, except for me until I finally learned it, as I am retarded.

Divide the cylinder's rated volume by it's rated pressure.

Once you have this baseline number, you just multiply it by the pressure in PSI you know you have in the tank to find the volume of gas, (cubic feet) in the tank.

Example: an Aluminum 80 (with a rated fill pressure of 3,000 PSI) has 2350 PSI of pressure in it, how much air in cubic feet does it have?

80 / 3000 = 0.267

0.267 times 2350 =62.7 cu. ft.

Another example:

A HP80 with a rated fill pressure of 3,440, filled to 3,000

80 / 3440 = .02907

.02907 times 3,000 = 87.2 cu. ft.


That's all there is to it.

Huh Huh... That was cool...
Yeah! Math Sucks! Heh Heh!