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WaScubaDude
10-23-2007, 18:42
As I understand CF is the great equalizer?? Regardless of PSI etc CF is what matters. 19cf is aprox. 1/5th of 100cf?? Just want to paint a clear picture for new divers. Chime in techies.

RoadRacer1978
10-23-2007, 18:53
To save CompuDudes fingers from relentlessy typing all this info again I'll put it here for everyone.

Al.80:
2800psi=72cf
2900psi=75cf
3000psi=77.4cf
3100psi=80cf (overfill, dangerous to Al)
3200psi=83cf (overfill, dangerous to Al)
3300psi=85cf (overfill, dangerous to Al)

Al.80N
2800psi=66cf
2900psi=68cf
3000psi=70cf
3100psi=73cf
3200psi=75cf
3300psi=77.4cf
3400psi=80cf (overfill, dangerous to Al)

HP100:
2800psi=81cf
2900psi=84cf
3000psi=87cf
3100psi=90cf
3200psi=93cf
3300psi=96cf
3400psi=99cf
3442psi=100cf
3500psi=102cf (slight overfill, not dangerous to steel)
3600psi=105cf (slight overfill, not dangerous to steel)

http://forum.scubatoys.com/tanks/2335-neutrally-buoyant-aluminum-80-tank-2.html#post62528

BSea
10-23-2007, 18:55
As I understand CF is the great equalizer?? Regardless of PSI etc CF is what matters. 19cf is aprox. 1/5th of 100cf?? Just want to paint a clear picture for new divers. Chime in techies.
I had this discussion a while back at my LDS. We were discussing using an LP 85 or an HP 120 when both were filled to 3000 psi. I couldn't get him to understand that they both had almost the same psi at that pressure. He kept saying sure, but the 120 is bigger. I said true, but the they have about the same amount of air, and the lp85 is much lighter. I tried the pound of feathers vrs a pound of lead thing, but I still don't think he got it.

fire diver
10-23-2007, 21:45
As I understand CF is the great equalizer?? Regardless of PSI etc CF is what matters. 19cf is aprox. 1/5th of 100cf?? Just want to paint a clear picture for new divers. Chime in techies.

It's easier to understand the correlation if the tank volume is expressed in terms of water. Such as a 12lt tank, etc. That paints the image of a constant size, with varying pressures of gas pumped into it.

FD

WaScubaDude
11-04-2007, 23:43
As I understand CF is the great equalizer?? Regardless of PSI etc CF is what matters. 19cf is aprox. 1/5th of 100cf?? Just want to paint a clear picture for new divers. Chime in techies.
I had this discussion a while back at my LDS. We were discussing using an LP 85 or an HP 120 when both were filled to 3000 psi. I couldn't get him to understand that they both had almost the same psi at that pressure. He kept saying sure, but the 120 is bigger. I said true, but the they have about the same amount of air, and the lp85 is much lighter. I tried the pound of feathers vrs a pound of lead thing, but I still don't think he got it.

This speaks to what I am getting at. does the lp hp matter at all! or is it just the 85cf vs 120cf. As I understand it the CF would represent the size of the phone booth that would hold Xcf at the surface. So 85 HP and 85 LP are equal???? Is this not so??

Judge
11-05-2007, 08:10
I may be wrong but with the LP80 vs the HP 120: the 120 will have more air due to the increased volume. It will take more air to reach 3000psi than the 80 will. They will have same psi, but volume is what keeps you alive.

Puffer Fish
11-05-2007, 08:46
And I thought I understood this...

There are two things here

1. Tank internal volume...

2. How many cf the tank will hold.... Internal volume x number of atmospheres of pressure.

Two tanks, with the same internal volume, will hold exactly the same at the same pressure.

But, if one is rated at 3442, and the other at say 2400, then the 3442, when filled to it's spec pressure, will have more air.

Does a 100 cf Faber XF tank have a larger volume than an AL 80? Yes it does, but not by as much as the 100 versus 80 would indicate.

I like the reference to the actual tank volume, because that can be just as important as the pressure.

The effect is that the 100 cf Faber, if only filled to 3,000, is still a 87 cubic ft tank, that weighs less and takes less weight over an AL80... that sort of comparison should be part of any dicussion, if one is going to buy tanks.

BSea
11-05-2007, 10:19
This speaks to what I am getting at. does the lp hp matter at all! or is it just the 85cf vs 120cf. As I understand it the CF would represent the size of the phone booth that would hold Xcf at the surface. So 85 HP and 85 LP are equal???? Is this not so??
I Made a mistake in my earlier post (I really should proof read these). I said "they both had almost the same psi at that pressure." I should have said "the both had almost the same volume at that pressure.

And you are right. an LP 85 will hold the same amount of air (volume) as an HP 85. It just that the HP will be smaller in size, and lighter.


And I thought I understood this...

Does a 100 cf Faber XF tank have a larger volume than an AL 80? Yes it does, but not by as much as the 100 versus 80 would indicate.
I agree with you on everything except the above. Why wouldn't there be 20 Cubic feet of air difference as indicated by 100-80? Maybe I'm missing something. (it sure wouldn't be the 1st time:smiley5:)

Puffer Fish
11-05-2007, 10:55
[quote=WaScubaDude;86844This speaks to what I am getting at. does the lp hp matter at all! or is it just the 85cf vs 120cf. As I understand it the CF would represent the size of the phone booth that would hold Xcf at the surface. So 85 HP and 85 LP are equal???? Is this not so??
I Made a mistake in my earlier post (I really should proof read these). I said "they both had almost the same psi at that pressure." I should have said "the both had almost the same volume at that pressure.

And you are right. an LP 85 will hold the same amount of air (volume) as an HP 85. It just that the HP will be smaller in size.


And I thought I understood this...

Does a 100 cf Faber XF tank have a larger volume than an AL 80? Yes it does, but not by as much as the 100 versus 80 would indicate.
I agree with you on everything except the above. Why wouldn't there be 20 Cubic feet of air difference as indicated by 100-80? Maybe I'm missing something. (it sure wouldn't be the 1st time:smiley5:)[/quote]


I should have been clearer.. volume in this case refers to the actual internal size of the the tank.

When you fill a tank with say 77.4 cubic feet at 3,000 psi, you are putting 204 atm more gas in it (as we are starting at 0 =14.7. That means, the volume of the tank is actually....roughly .38 cubic feet inside (Watch me do the math wrong). With an identical volume tank, at 3442 psi in it, it would be 234 (approx) atm's...and around 88.8 cubic feet. The internal size of the tanks are just .427 versus .38 or around 12% larger.. the rest is due to the pressure differences.

Note: every number here is actually off, as gas laws are not such easy math, but they should be close.

If every tank you have can be filled to its rated pressure, you are luck.. as many places cannot do the 3,442 thing.

So which high pressure tank has the closest internal volume and should you buy it, if you cannot get high pressures fills? Well a LP 85 worthington has an internal volume of .473 and a wothington X7 - 120 has an internal volume of .513.. so at the same pressure the 120 has more air (92 cubic ft). There is a weight difference of 4 lbs... A little more air... a little more weight.. would seem to be a draw..ok, there is a price difference.. a size difference, but just going on the two tanks, I would rather have the HP one... as some places i could get a much bigger fill... hope I made at least some sense there.

BSea
11-05-2007, 11:04
I should have been clearer.. volume in this case refers to the actual internal size of the the tank.

When you fill a tank with say 77.4 cubic feet at 3,000 psi, you are putting 204 atm more gas in it (as we are starting at 0 =14.7. That means, the volume of the tank is actually....roughly .38 cubic feet inside (Watch me do the math wrong). With an identical volume tank, at 3442 psi in it, it would be 234 (approx) atm's...and around 88.8 cubic feet. The internal size of the tanks are just .427 versus .38 or around 12% larger.. the rest is due to the pressure differences.

Note: every number here is actually off, as gas laws are not such easy math, but they should be close.

If every tank you have can be filled to its rated pressure, you are luck.. as many places cannot do the 3,442 thing.

So which high pressure tank has the closest internal volume and should you buy it, if you cannot get high pressures fills? Well a LP 85 worthington has an internal volume of .473 and a wothington X7 - 120 has an internal volume of .513.. so at the same pressure the 120 has more air (92 cubic ft). There is a weight difference of 4 lbs... A little more air... a little more weight.. would seem to be a draw..ok, there is a price difference.. a size difference, but just going on the two tanks, I would rather have the HP one... as some places i could get a much bigger fill... hope I made at least some sense there.
Ok, that makes sense.

Puffer Fish
11-05-2007, 11:14
I should have been clearer.. volume in this case refers to the actual internal size of the the tank.

When you fill a tank with say 77.4 cubic feet at 3,000 psi, you are putting 204 atm more gas in it (as we are starting at 0 =14.7. That means, the volume of the tank is actually....roughly .38 cubic feet inside (Watch me do the math wrong). With an identical volume tank, at 3442 psi in it, it would be 234 (approx) atm's...and around 88.8 cubic feet. The internal size of the tanks are just .427 versus .38 or around 12% larger.. the rest is due to the pressure differences.

Note: every number here is actually off, as gas laws are not such easy math, but they should be close.

If every tank you have can be filled to its rated pressure, you are luck.. as many places cannot do the 3,442 thing.

So which high pressure tank has the closest internal volume and should you buy it, if you cannot get high pressures fills? Well a LP 85 worthington has an internal volume of .473 and a wothington X7 - 120 has an internal volume of .513.. so at the same pressure the 120 has more air (92 cubic ft). There is a weight difference of 4 lbs... A little more air... a little more weight.. would seem to be a draw..ok, there is a price difference.. a size difference, but just going on the two tanks, I would rather have the HP one... as some places i could get a much bigger fill... hope I made at least some sense there.
Ok, that makes sense.


Thanks, I was at the end of my math skills there..

It is really interesting... if the numbers are somewhat different... say a LP85 worthington filled to a cave fill (3,000) versus a HP worthington 100 that you can get filled to 3,300. Most would pick the LP.. but the HP is a lighter and has more air (and smaller) tank.

in_cavediver
11-05-2007, 12:29
I should have been clearer.. volume in this case refers to the actual internal size of the the tank.

When you fill a tank with say 77.4 cubic feet at 3,000 psi, you are putting 204 atm more gas in it (as we are starting at 0 =14.7. That means, the volume of the tank is actually....roughly .38 cubic feet inside (Watch me do the math wrong). With an identical volume tank, at 3442 psi in it, it would be 234 (approx) atm's...and around 88.8 cubic feet. The internal size of the tanks are just .427 versus .38 or around 12% larger.. the rest is due to the pressure differences.

Note: every number here is actually off, as gas laws are not such easy math, but they should be close.

If every tank you have can be filled to its rated pressure, you are luck.. as many places cannot do the 3,442 thing.

So which high pressure tank has the closest internal volume and should you buy it, if you cannot get high pressures fills? Well a LP 85 worthington has an internal volume of .473 and a wothington X7 - 120 has an internal volume of .513.. so at the same pressure the 120 has more air (92 cubic ft). There is a weight difference of 4 lbs... A little more air... a little more weight.. would seem to be a draw..ok, there is a price difference.. a size difference, but just going on the two tanks, I would rather have the HP one... as some places i could get a much bigger fill... hope I made at least some sense there.
Ok, that makes sense.


Thanks, I was at the end of my math skills there..

It is really interesting... if the numbers are somewhat different... say a LP85 worthington filled to a cave fill (3,000) versus a HP worthington 100 that you can get filled to 3,300. Most would pick the LP.. but the HP is a lighter and has more air (and smaller) tank.

Figure Cave fill in an LP85 is really 3600-3800, not 3000. At 3800, its a 122cf, at 3600 its 116cf, at 3300 its 106cf.

In the end, capcitity, size, and characteristics determine the suitability of a tank. The LP85 and HP100 are really good balanced tanks. The 104-130's are as well for the 'need more air' crowd. Some get a bit bad on the out of water weight, some are too short and some to tall for many divers.

Its all nothing more than a balancing act.

CompuDude
11-05-2007, 14:31
It is really interesting... if the numbers are somewhat different... say a LP85 worthington filled to a cave fill (3,000) versus a HP worthington 100 that you can get filled to 3,300. Most would pick the LP.. but the HP is a lighter and has more air (and smaller) tank.

It all depends on the availability of cave fills. Not many local shops here will overfill an LP tank more than a couple hundred psi. So you'll always have less than an actual cave fill, and less than an HP100 would have, even underfilled.

Filled to the exact same pressure, an LP85 will hold a tinge more than the HP100, but not enough to really be significant. And around here, even the worst shops will fill to 3300 psi in an HP tank, but when they only fill your LP tank (2650 fill pressure) to 2800, you're going to have a lot more gas with 3300 in your HP100 than with 2800 in your LP85.

It gets to be a tougher decision when you factor in the lower price of the LP85 compared to the HP100... but I'd rather spend a little more and be relatively assured of how much gas I'll end up with, due to the local fill policies. So I always buy HP100s.

In Florida, yes, I'd probably pick up the LP85s every time, since cave fills are so commonly available.

Puffer Fish
11-05-2007, 15:20
It is really interesting... if the numbers are somewhat different... say a LP85 worthington filled to a cave fill (3,000) versus a HP worthington 100 that you can get filled to 3,300. Most would pick the LP.. but the HP is a lighter and has more air (and smaller) tank.

It all depends on the availability of cave fills. Not many local shops here will overfill an LP tank more than a couple hundred psi. So you'll always have less than an actual cave fill, and less than an HP100 would have, even underfilled.

Filled to the exact same pressure, an LP85 will hold a tinge more than the HP100, but not enough to really be significant. And around here, even the worst shops will fill to 3300 psi in an HP tank, but when they only fill your LP tank (2650 fill pressure) to 2800, you're going to have a lot more gas with 3300 in your HP100 than with 2800 in your LP85.

It gets to be a tougher decision when you factor in the lower price of the LP85 compared to the HP100... but I'd rather spend a little more and be relatively assured of how much gas I'll end up with, due to the local fill policies. So I always buy HP100s.

In Florida, yes, I'd probably pick up the LP85s every time, since cave fills are so commonly available.

It is surprizing how much where you get your tanks filled matters. Up here.. a cave fill is under 3,000.. usually around 2,800. But I never get a 3,440 fill.. it is always around 3,300. At Fill express, I usually get a hot 3,600.. cooling to around 3,400, but if the tank is cool.. it ends up being a 3,600, but a cave fill is usually around 3,200. Looking at it another way.. that makes for different sized tanks...

CompuDude
11-05-2007, 16:00
It is really interesting... if the numbers are somewhat different... say a LP85 worthington filled to a cave fill (3,000) versus a HP worthington 100 that you can get filled to 3,300. Most would pick the LP.. but the HP is a lighter and has more air (and smaller) tank.

It all depends on the availability of cave fills. Not many local shops here will overfill an LP tank more than a couple hundred psi. So you'll always have less than an actual cave fill, and less than an HP100 would have, even underfilled.

Filled to the exact same pressure, an LP85 will hold a tinge more than the HP100, but not enough to really be significant. And around here, even the worst shops will fill to 3300 psi in an HP tank, but when they only fill your LP tank (2650 fill pressure) to 2800, you're going to have a lot more gas with 3300 in your HP100 than with 2800 in your LP85.

It gets to be a tougher decision when you factor in the lower price of the LP85 compared to the HP100... but I'd rather spend a little more and be relatively assured of how much gas I'll end up with, due to the local fill policies. So I always buy HP100s.

In Florida, yes, I'd probably pick up the LP85s every time, since cave fills are so commonly available.

It is surprizing how much where you get your tanks filled matters. Up here.. a cave fill is under 3,000.. usually around 2,800. But I never get a 3,440 fill.. it is always around 3,300. At Fill express, I usually get a hot 3,600.. cooling to around 3,400, but if the tank is cool.. it ends up being a 3,600, but a cave fill is usually around 3,200. Looking at it another way.. that makes for different sized tanks...

Most shops routinely ignore the fact that the regs allow for a 5/4 rated capacity overfill on ALL tanks (LP and HP) at 131 degrees to account for cooling after a hot fill is done. That's 25%!!! So technically, a 3442psi-rated tank could be hot-filled to about 4300psi without breaking any rules! Of course, that's harder on a shop's compressor, so it's a rare shop that will do that. Most of them hot fill to ~3500 and only if you're lucky (and have time, of course) will they top it off to bring it back up ~3500 once it cools.

Ditto for LP tanks, of course... 2650 rating means you can legitimately [hot] fill it to just over 3300 psi and allow it to cool without it being considered an overfill.

It only has to be AT rated pressure at 70 degrees F.

This requires knowing how to interpret 49 C.F.R. 173.301, of course. Lack of education and financial considerations tend to overrule common sense and actual rules, sadly.

marchand
11-05-2007, 16:16
So, if my LP120 gets a cave fill to 3600psi I think it should have about 180cf of gas. did I do my math right? If I didn't could you tell me the correct cubic footage at that pressure.

CompuDude
11-05-2007, 16:19
So, if my LP120 gets a cave fill to 3600psi I think it should have about 180cf of gas. did I do my math right? If I didn't could you tell me the correct cubic footage at that pressure.

120cf / 2650psi = ~0.0452 cf/psi.

* 3600psi = 163 cf.

texdiveguy
11-05-2007, 16:21
Whats with this cra_ _y term...'cave fill'....its a tech fill or wreck fill!!

Puffer Fish
11-05-2007, 16:29
Whats with this cra_ _y term...'cave fill'....its a tech fill or wreck fill!!
Nope, sorry... it is a cave fill... it allows you to get deeper into the cave before you run out of air... very helpful in setting records for the farthest body retrevel.

Tech fill is with some exotic gas no one has ever heard of.. neptunium for example.

Wreck fill is a rebreather...

texdiveguy
11-05-2007, 16:30
120cf / 2650psi = ~0.0452 cf/psi.

* 3600psi = 163 cf.

I am 'nit picking'..... should read 2640psi ........ :-)

I am the WORLDS WORST typer and speller.....I don't see how the rest of you even read my posts--lol!!

texdiveguy
11-05-2007, 16:31
Whats with this cra_ _y term...'cave fill'....its a tech fill or wreck fill!!
Nope, sorry... it is a cave fill... it allows you to get deeper into the cave before you run out of air... very helpful in setting records for the farthest body retrevel.

Tech fill is with some exotic gas no one has ever heard of.. neptunium for example.

Wreck fill is a rebreather...

....... love it.......LOL!

Puffer Fish
11-05-2007, 16:38
120cf / 2650psi = ~0.0452 cf/psi.

* 3600psi = 163 cf.

I am 'nit picking'..... should read 2640psi ........ :-)

I am the WORLDS WORST typer and speller.....I don't see how the rest of you even read my posts--lol!!
Sadly, he will still not get even one more cubic feet of gas, but at least you tried.

texdiveguy
11-05-2007, 16:44
120cf / 2650psi = ~0.0452 cf/psi.

* 3600psi = 163 cf.

I am 'nit picking'..... should read 2640psi ........ :-)

I am the WORLDS WORST typer and speller.....I don't see how the rest of you even read my posts--lol!!
Sadly, he will still not get even one more cubic feet of gas, but at least you tried.

LOL....yea but its worth a try...lol!

CompuDude
11-05-2007, 16:45
Oops, good catch. Shows you how often I dive LP tanks!

RoadRacer1978
11-05-2007, 16:49
So, if my LP120 gets a cave fill to 3600psi I think it should have about 180cf of gas. did I do my math right? If I didn't could you tell me the correct cubic footage at that pressure.

120cf / 2650psi = ~0.0452 cf/psi.

* 3600psi = 163 cf.

I know I'm a newb, but never actually knew how to figure this. Thanks for the formula.

Puffer Fish
11-05-2007, 17:03
So, if my LP120 gets a cave fill to 3600psi I think it should have about 180cf of gas. did I do my math right? If I didn't could you tell me the correct cubic footage at that pressure.

120cf / 2650psi = ~0.0452 cf/psi.

* 3600psi = 163 cf.

I know I'm a newb, but never actually knew how to figure this. Thanks for the formula.
it's a ratio... 3,600/2640 x 120 also produces the same number.

Now for the bad news... gas is not quite linear in compression.. so the number is a bit off. There would really be a bit less in the tank at that pressure.

I would be happy to provide the tedious and boring reasons... someone just give me a push...

Anne Eastwell
11-05-2007, 17:04
Geez, you guys are giving me a headache! I'm glad you know what you're talking about, I still don't get it but that's not important right now...

Puffer Fish
11-05-2007, 17:04
Oh, and I just love to ramble on about random particle mathematics... really, I do...

Puffer Fish
11-05-2007, 17:05
Geez, you guys are giving me a headache! I'm glad you know what you're talking about, I still don't get it but that's not important right now...
It comes down to getting the right tank that does what you want.. taking into consideration what your dive shop wants to give you... simple really.

CompuDude
11-05-2007, 17:24
So, if my LP120 gets a cave fill to 3600psi I think it should have about 180cf of gas. did I do my math right? If I didn't could you tell me the correct cubic footage at that pressure.

120cf / 2650psi = ~0.0452 cf/psi.

* 3600psi = 163 cf.

I know I'm a newb, but never actually knew how to figure this. Thanks for the formula.
it's a ratio... 3,600/2640 x 120 also produces the same number.

Now for the bad news... gas is not quite linear in compression.. so the number is a bit off. There would really be a bit less in the tank at that pressure.

I would be happy to provide the tedious and boring reasons... someone just give me a push...

I don't think we really need to go into a discussion of ideal gas laws, do we? ;)

Btw, do they even make LP120s? LOL

moosicman
11-05-2007, 17:31
what is the difference in al80 and al80n?

BSea
11-05-2007, 18:07
I would be happy to provide the tedious and boring reasons... someone just give me a push...
I'm not going to push.


Btw, do they even make LP120s? LOL
I've never heard of 1, but there are so many. But it must be big.

texdiveguy
11-05-2007, 18:25
what is the difference in al80 and al80n?

Al80N= Neutrally Buoyant Aluminum 80 Tank, Scuba Tanks, Luxfer, Neutrally Buoyant Aluminum 80 Tank (http://www.scubatoys.com/store/detail.asp?product_id=AL80N)

Steve Scuba
11-05-2007, 18:26
I don't think we really need to go into a discussion of ideal gas laws, do we? ;)

Btw, do they even make LP120s? LOL

Wow, the moment I read that, PV=nRT popped into my head. That's some old info filtering its way to the surface after years of disuse. My Chemistry prof would be proud. I guess he did his job.

in_cavediver
11-05-2007, 18:40
Btw, do they even make LP120s? LOL

Yep - They make them but were more common as LP 121's. They are 8" diameter and around 29" tall. Someone is making a new HP tank called the HP149. They actually aren't bad 'big' tanks for tall divers. Think the guys who like the HP 120's over the HP100's.

I know several people who use them as sidemount tanks. I have yet to meet someone who uses it as a standard single though.

Now, you want a water heater, Heiser makes/made an 190 (4400psi). It was something like 62lbs negative full, 46 negative empty and 87lbs out of the water. http://www.huronscuba.com/equipment/scubaCylinderSpecification.html

Puffer Fish
11-05-2007, 18:41
So, if my LP120 gets a cave fill to 3600psi I think it should have about 180cf of gas. did I do my math right? If I didn't could you tell me the correct cubic footage at that pressure.

120cf / 2650psi = ~0.0452 cf/psi.

* 3600psi = 163 cf.

I know I'm a newb, but never actually knew how to figure this. Thanks for the formula.
it's a ratio... 3,600/2640 x 120 also produces the same number.

Now for the bad news... gas is not quite linear in compression.. so the number is a bit off. There would really be a bit less in the tank at that pressure.

I would be happy to provide the tedious and boring reasons... someone just give me a push...

I don't think we really need to go into a discussion of ideal gas laws, do we? ;)

Btw, do they even make LP120s? LOL
Actually I was hoping for a good quantum mechanics discussion at the best, or a comparison of ideal gas laws and the problem with it's high pressure mathematics at the worst.

Yes, Virginia, they do.. and they have... there are actually a PST, Worthington, Faber and OMS tanks (not all being made today, obviously).. with my favorite being the PST.. a 51.3 lb monster that, when full is 10 lbs negative...Ah, a pair of doubles is truly a manly set of tanks.. what with valves and regs and boots and bands would weigh in at, oh just over 120 lbs, and be around 30 lbs negative...truly an air filled rock.

Puffer Fish
11-05-2007, 18:46
what is the difference in al80 and al80n?

Al80N= Neutrally Buoyant Aluminum 80 Tank, Scuba Tanks, Luxfer, Neutrally Buoyant Aluminum 80 Tank (http://www.scubatoys.com/store/detail.asp?product_id=AL80N)


One of the truly stupid concepts in diving... lets make the tank heavier so it will not float..you could just glue some weight to a standard one and do the same thing (it is not as buoyant, because it weighs more...doh)

Anne Eastwell
11-05-2007, 18:53
Geez, you guys are giving me a headache! I'm glad you know what you're talking about, I still don't get it but that's not important right now...
It comes down to getting the right tank that does what you want.. taking into consideration what your dive shop wants to give you... simple really.


Thank you for not just ignoring me...:smiley9:

Puffer Fish
11-05-2007, 18:53
Btw, do they even make LP120s? LOL

Yep - They make them but were more common as LP 121's. They are 8" diameter and around 29" tall. Someone is making a new HP tank called the HP149. They actually aren't bad 'big' tanks for tall divers. Think the guys who like the HP 120's over the HP100's.

I know several people who use them as sidemount tanks. I have yet to meet someone who uses it as a standard single though.

Now, you want a water heater, Heiser makes/made an 190 (4400psi). It was something like 62lbs negative full, 46 negative empty and 87lbs out of the water. http://www.huronscuba.com/equipment/scubaCylinderSpecification.html
That would be Faber.. I have their 133.. nice tank and only 42 lbs, and the same height as an AL 80... put into a complete package, including reduced lead.. and there is around 8 lbs difference between it and the AL80, with a lot more air...

Puffer Fish
11-05-2007, 18:55
Geez, you guys are giving me a headache! I'm glad you know what you're talking about, I still don't get it but that's not important right now...
It comes down to getting the right tank that does what you want.. taking into consideration what your dive shop wants to give you... simple really.


Thank you for not just ignoring me...:smiley9:
Would never do that... never. It was timing.. we were right in the middle of math calculation.. and tunnel vision had set in...:smilie40:

marchand
11-05-2007, 19:49
I'm pretty sure the service pressure is 2400psi on my LP120. 2640psi is with the 10% overfill. so, is it 120cf at the service pressure or the overfill pressure?

Puffer Fish
11-05-2007, 20:02
I'm pretty sure the service pressure is 2400psi on my LP120. 2640psi is with the 10% overfill. so, is it 120cf at the service pressure or the overfill pressure?
Good news and bad..the good news is that most of the LP120/121 tanks were bigger than 120 at the 10% overfill level

Bad news is that none were 120 at the 2400 psi level.

A pst lp120 was 122.5 at 2640 and a faber LP120 was 125 at 2640.. need the tank maker to get the specifics.

in_cavediver
11-05-2007, 21:26
I'm pretty sure the service pressure is 2400psi on my LP120. 2640psi is with the 10% overfill. so, is it 120cf at the service pressure or the overfill pressure?
Good news and bad..the good news is that most of the LP120/121 tanks were bigger than 120 at the 10% overfill level

Bad news is that none were 120 at the 2400 psi level.

A pst lp120 was 122.5 at 2640 and a faber LP120 was 125 at 2640.. need the tank maker to get the specifics.

The good new is in North Florida, the Faber LP120 is really a faber 180......

marchand
11-06-2007, 00:27
mine is a pst which puts it's cave fill to about 170cf.

WaScubaDude
11-06-2007, 00:41
Whats with this cra_ _y term...'cave fill'....its a tech fill or wreck fill!!
Nope, sorry... it is a cave fill... it allows you to get deeper into the cave before you run out of air... very helpful in setting records for the farthest body retrevel.

Tech fill is with some exotic gas no one has ever heard of.. neptunium for example.

Wreck fill is a rebreather...

This is so damb funny, and I think it was lost to all the math geeks.

WaScubaDude
11-06-2007, 00:50
Soooo all you math wizzes. Is 85cf LP and 85cf HP with no over fill or other funny business pretty much equal?? =??

CompuDude
11-06-2007, 02:49
Soooo all you math wizzes. Is 85cf LP and 85cf HP with no over fill or other funny business pretty much equal?? =??

Cuuuuute..... :smiley32:

Puffer Fish
11-06-2007, 05:40
Soooo all you math wizzes. Is 85cf LP and 85cf HP with no over fill or other funny business pretty much equal?? =??
Actually....no... as there is a major weight and size and buoyancy difference... amount of air may be the same.. but everything else is different.

marchand
11-06-2007, 14:16
Soooo all you math wizzes. Is 85cf LP and 85cf HP with no over fill or other funny business pretty much equal?? =??
Actually....no... as there is a major weight and size and buoyancy difference... amount of air may be the same.. but everything else is different.

and I would rather have the LP85. it is really easy to get cave fills around here...

Puffer Fish
11-06-2007, 14:56
Soooo all you math wizzes. Is 85cf LP and 85cf HP with no over fill or other funny business pretty much equal?? =??
Actually....no... as there is a major weight and size and buoyancy difference... amount of air may be the same.. but everything else is different.

and I would rather have the LP85. it is really easy to get cave fills around here...


And that is why they make so many different sizes.. me, I have HP100's, HP117's and HP133's, as I can get 3,400 psi nitrox fills where I dive.

BSea
11-06-2007, 15:31
Soooo all you math wizzes. Is 85cf LP and 85cf HP with no over fill or other funny business pretty much equal?? =??
Actually....no... as there is a major weight and size and buoyancy difference... amount of air may be the same.. but everything else is different.

and I would rather have the LP85. it is really easy to get cave fills around here...


And that is why they make so many different sizes.. me, I have HP100's, HP117's and HP133's, as I can get 3,400 psi nitrox fills where I dive.

I asked an engineer, a Salesman, and an accountant this question:
Is 85cf LP and 85cf HP filled to standard pressure equal?

and here's their answers.

Engineer: Hmmm 85.00000 cubic feet compared to 85.00000 curic feet. Yes they are equal.

Salesman: Why heck no buddy. Look at that HP tank right there. It's $75 more for a reason, and that reason is added value that you can take home with you today. Will that be cash or credit card?

Accountant. (He looks over his shoulder to see if anyone is listening then says) Do you want it to be?

WaScubaDude
11-06-2007, 16:01
Soooo all you math wizzes. Is 85cf LP and 85cf HP with no over fill or other funny business pretty much equal?? =??
Actually....no... as there is a major weight and size and buoyancy difference... amount of air may be the same.. but everything else is different.

and I would rather have the LP85. it is really easy to get cave fills around here...


And that is why they make so many different sizes.. me, I have HP100's, HP117's and HP133's, as I can get 3,400 psi nitrox fills where I dive.

I asked an engineer, a Salesman, and an accountant this question:
Is 85cf LP and 85cf HP filled to standard pressure equal?

and here's their answers.

Engineer: Hmmm 85.00000 cubic feet compared to 85.00000 curic feet. Yes they are equal.

Salesman: Why heck no buddy. Look at that HP tank right there. It's $75 more for a reason, and that reason is added value that you can take home with you today. Will that be cash or credit card?

Accountant. (He looks over his shoulder to see if anyone is listening then says) Do you want it to be?

Thank You. I was looking for all the really smart math guys to weigh in with yes 85cf = 85cf. More for the new divers who may be confused about what CF means. I explain it in terms of how big is the "phone booth" the air fits into at the surface. I think this is a good analogy. Others?

How about one more tank challenge...It doesn't matter that an Aluminum 80 ends up bouyant at the end of the dive, it's the Delta, or difference between weight in water full and weight in water empty that matters. Then it's just a question of how many lbs. on the "belt' vs. lbs on the back.
Your thoughts??

CompuDude
11-06-2007, 16:20
Thank You. I was looking for all the really smart math guys to weigh in with yes 85cf = 85cf. More for the new divers who may be confused about what CF means. I explain it in terms of how big is the "phone booth" the air fits into at the surface. I think this is a good analogy. Others?

How about one more tank challenge...It doesn't matter that an Aluminum 80 ends up bouyant at the end of the dive, it's the Delta, or difference between weight in water full and weight in water empty that matters. Then it's just a question of how many lbs. on the "belt' vs. lbs on the back.
Your thoughts??

Not true. You have a significant net reduction of weight carried with the steel tank.

Steel HP80 = 28.0 lbs
Steel HP100 = 33.0 lbs
Alum. 80 = 31.6 lbs

The tanks weigh about the same on land (actually, a little more with the HP100 but still under 2# difference, and 3.5 LESS with the HP80) and yet you have to carry more lead on your belt with the Al.80 (approx 6-7 lbs) due to the buoyancy characteristics. (-3 for HP80, -2.5 for HP100, +4 for Al.80)

So going from the Al.80 to an HP80, you drop 7 pounds due to the different buoyancy characteristics, and then the tank itself weights 3.6 pounds less, for a net weight reduction of 10.6 pounds... that's weight on your feet on dry land. For the HP100, that's 6.5 lbs for the buoyancy, but add ~1.5 for the tank weight, so a net reduction of 5 lbs.

The buoyancy swing during the dive would be roughly same for both an Al.80 and a steel HP.80 (ignoring the actual 77.4cf capacity of the Al.80), slightly more for the HP100 due to the increased weight of a larger amount of gas, or again, identical if you were to underfill to the same amount of gas (in CF).

WaScubaDude
11-06-2007, 16:32
Thank You. I was looking for all the really smart math guys to weigh in with yes 85cf = 85cf. More for the new divers who may be confused about what CF means. I explain it in terms of how big is the "phone booth" the air fits into at the surface. I think this is a good analogy. Others?

How about one more tank challenge...It doesn't matter that an Aluminum 80 ends up bouyant at the end of the dive, it's the Delta, or difference between weight in water full and weight in water empty that matters. Then it's just a question of how many lbs. on the "belt' vs. lbs on the back.
Your thoughts??

Not true. You have a significant net reduction of weight carried with the steel tank.

Steel HP80 = 28.0 lbs
Steel HP100 = 33.0 lbs
Alum. 80 = 31.6 lbs

The tanks weigh about the same on land (actually, a little more with the HP100 but still under 2# difference, and 3.5 LESS with the HP80) and yet you have to carry more lead on your belt with the Al.80 (approx 6-7 lbs) due to the buoyancy characteristics. (-3 for HP80, -2.5 for HP100, +4 for Al.80)

So going from the Al.80 to an HP80, you drop 7 pounds due to the different buoyancy characteristics, and then the tank itself weights 3.6 pounds less, for a net weight reduction of 10.6 pounds... that's weight on your feet on dry land. For the HP100, that's 6.5 lbs for the buoyancy, but add ~1.5 for the tank weight, so a net reduction of 5 lbs.

The buoyancy swing during the dive would be roughly same for both an Al.80 and a steel HP.80 (ignoring the actual 77.4cf capacity of the Al.80), slightly more for the HP100 due to the increased weight of a larger amount of gas, or again, identical if you were to underfill to the same amount of gas (in CF).

I am confused. You say they have roughly the same buoyancy swing in the last paragraph, but in the paragraph above you say there is a 7lb dif due to buoancy characteristics. I am a little slow, so if you could have another go at it I would be greatful. i think there are some physics that I am not understanding. Thanks. Again I thought the Delta in water is what matters.

CompuDude
11-06-2007, 16:47
Thank You. I was looking for all the really smart math guys to weigh in with yes 85cf = 85cf. More for the new divers who may be confused about what CF means. I explain it in terms of how big is the "phone booth" the air fits into at the surface. I think this is a good analogy. Others?

How about one more tank challenge...It doesn't matter that an Aluminum 80 ends up bouyant at the end of the dive, it's the Delta, or difference between weight in water full and weight in water empty that matters. Then it's just a question of how many lbs. on the "belt' vs. lbs on the back.
Your thoughts??

Not true. You have a significant net reduction of weight carried with the steel tank.

Steel HP80 = 28.0 lbs
Steel HP100 = 33.0 lbs
Alum. 80 = 31.6 lbs

The tanks weigh about the same on land (actually, a little more with the HP100 but still under 2# difference, and 3.5 LESS with the HP80) and yet you have to carry more lead on your belt with the Al.80 (approx 6-7 lbs) due to the buoyancy characteristics. (-3 for HP80, -2.5 for HP100, +4 for Al.80)

So going from the Al.80 to an HP80, you drop 7 pounds due to the different buoyancy characteristics, and then the tank itself weights 3.6 pounds less, for a net weight reduction of 10.6 pounds... that's weight on your feet on dry land. For the HP100, that's 6.5 lbs for the buoyancy, but add ~1.5 for the tank weight, so a net reduction of 5 lbs.

The buoyancy swing during the dive would be roughly same for both an Al.80 and a steel HP.80 (ignoring the actual 77.4cf capacity of the Al.80), slightly more for the HP100 due to the increased weight of a larger amount of gas, or again, identical if you were to underfill to the same amount of gas (in CF).

I am confused. You say they have roughly the same buoyancy swing in the last paragraph, but in the paragraph above you say there is a 7lb dif due to buoancy characteristics. I am a little slow, so if you could have another go at it I would be greatful. i think there are some physics that I am not understanding. Thanks. Again I thought the Delta in water is what matters.

Buoyancy characteristics: The only important number (at the recreational level) is how buoyant the tank is when empty, because that determines how much lead you need for weighting yourself.

Buoyancy swing: 80cf of air weighs ~6 pounds (thus 80cf in an Al.80 weighs the same as 80cf in an HP100 or 80CF in an HP80). Since the size of the tank doesn't change, your buoyancy will change over the course of a dive by this amount, since the tank weighs less and less as you consume air and bubbles leave your vicinity. It's easy to go down at the beginning of a dive with an extra 6 pounds of gas on your back, the tricky part is staying down once most of those 6# have bubbled away, thus the actual empty buoyancy of the tank takes on more importance from a weighting standpoint, and you can ignore the "full weight" since it's less important.

Puffer Fish
11-06-2007, 17:38
Thank You. I was looking for all the really smart math guys to weigh in with yes 85cf = 85cf. More for the new divers who may be confused about what CF means. I explain it in terms of how big is the "phone booth" the air fits into at the surface. I think this is a good analogy. Others?

How about one more tank challenge...It doesn't matter that an Aluminum 80 ends up bouyant at the end of the dive, it's the Delta, or difference between weight in water full and weight in water empty that matters. Then it's just a question of how many lbs. on the "belt' vs. lbs on the back.
Your thoughts??

Not true. You have a significant net reduction of weight carried with the steel tank.

Steel HP80 = 28.0 lbs
Steel HP100 = 33.0 lbs
Alum. 80 = 31.6 lbs

The tanks weigh about the same on land (actually, a little more with the HP100 but still under 2# difference, and 3.5 LESS with the HP80) and yet you have to carry more lead on your belt with the Al.80 (approx 6-7 lbs) due to the buoyancy characteristics. (-3 for HP80, -2.5 for HP100, +4 for Al.80)

So going from the Al.80 to an HP80, you drop 7 pounds due to the different buoyancy characteristics, and then the tank itself weights 3.6 pounds less, for a net weight reduction of 10.6 pounds... that's weight on your feet on dry land. For the HP100, that's 6.5 lbs for the buoyancy, but add ~1.5 for the tank weight, so a net reduction of 5 lbs.

The buoyancy swing during the dive would be roughly same for both an Al.80 and a steel HP.80 (ignoring the actual 77.4cf capacity of the Al.80), slightly more for the HP100 due to the increased weight of a larger amount of gas, or again, identical if you were to underfill to the same amount of gas (in CF).

I am confused. You say they have roughly the same buoyancy swing in the last paragraph, but in the paragraph above you say there is a 7lb dif due to buoancy characteristics. I am a little slow, so if you could have another go at it I would be greatful. i think there are some physics that I am not understanding. Thanks. Again I thought the Delta in water is what matters.

Buoyancy characteristics: The only important number (at the recreational level) is how buoyant the tank is when empty, because that determines how much lead you need for weighting yourself.

Buoyancy swing: 80cf of air weighs ~6 pounds (thus 80cf in an Al.80 weighs the same as 80cf in an HP100 or 80CF in an HP80). Since the size of the tank doesn't change, your buoyancy will change over the course of a dive by this amount, since the tank weighs less and less as you consume air and bubbles leave your vicinity. It's easy to go down at the beginning of a dive with an extra 6 pounds of gas on your back, the tricky part is staying down once most of those 6# have bubbled away, thus the actual empty buoyancy of the tank takes on more importance from a weighting standpoint, and you can ignore the "full weight" since it's less important.

You are doing so well... you deserve a specialty patch for understanding tanks....Now we need you to get all the doubles techy guys to understand the same thing and they will not be buying those negative buoy. tanks.

good job!!!!

Puffer Fish
11-06-2007, 17:43
Soooo all you math wizzes. Is 85cf LP and 85cf HP with no over fill or other funny business pretty much equal?? =??
Actually....no... as there is a major weight and size and buoyancy difference... amount of air may be the same.. but everything else is different.

and I would rather have the LP85. it is really easy to get cave fills around here...


And that is why they make so many different sizes.. me, I have HP100's, HP117's and HP133's, as I can get 3,400 psi nitrox fills where I dive.

I asked an engineer, a Salesman, and an accountant this question:
Is 85cf LP and 85cf HP filled to standard pressure equal?

and here's their answers.

Engineer: Hmmm 85.00000 cubic feet compared to 85.00000 curic feet. Yes they are equal.

Salesman: Why heck no buddy. Look at that HP tank right there. It's $75 more for a reason, and that reason is added value that you can take home with you today. Will that be cash or credit card?

Accountant. (He looks over his shoulder to see if anyone is listening then says) Do you want it to be?

Thank You. I was looking for all the really smart math guys to weigh in with yes 85cf = 85cf. More for the new divers who may be confused about what CF means. I explain it in terms of how big is the "phone booth" the air fits into at the surface. I think this is a good analogy. Others?

How about one more tank challenge...It doesn't matter that an Aluminum 80 ends up bouyant at the end of the dive, it's the Delta, or difference between weight in water full and weight in water empty that matters. Then it's just a question of how many lbs. on the "belt' vs. lbs on the back.
Your thoughts??


I want the lightest setup I can... as I shore dive sometimes... boat dive if rough seas.... and am generally lazy. I also want to be safe and have some ditchable weight. I also want enough gas that my dives are limited by the no-deco table, not gas.

That means Steel HP, Nitrox and a bigger than 80 cf tank...and then life is good.

in_cavediver
11-06-2007, 17:55
I am confused. You say they have roughly the same buoyancy swing in the last paragraph, but in the paragraph above you say there is a 7lb dif due to buoancy characteristics. I am a little slow, so if you could have another go at it I would be greatful. i think there are some physics that I am not understanding. Thanks. Again I thought the Delta in water is what matters.

Two concepts here:

First - swing or change in buoyancy. This is strictly based on the weight of the air. AL steel HP LP don't matter. Only that it's 85 cft or 100cft or 77 cf. For reference, 1cft is around 0.075 lbs. Simply multiply that for ballpark number.

Second, the buoyancy characteristics of a tank. This is strictly the weight of the water equal to the volume of the tank minus the land wieght of the tank. This will be a postive or negative number depending on the tank in question. This basically tells you whether an empty tank will float or sink and how much force it takes to make it neutral. For instance, an AL 80 has a +4lbs value and takes 4lbs of lead to be neutral. A steel HP 80 has a -3lbs value or it needs 3lbs of lead REMOVED to be neutral.

Third, land weights of tanks. This is the easiest. Its what a tank weighs on land empty. HP 80 -28lbs, Al 80 - 32 lbs.

Now, how to use the information.

First, assume we know we are properly wieghted with 20lbs in an Al 80. Now, we want to know how much weight for an steel HP 80.

Step one, ignore the swing of air. It isn't used here.

Step two, get the neutral amount of lead needed. For this, we look at the Al 80 and see it +4lbs. Since we are removing the tank, we can remove 4lbs from our weights. to make us neutral again.

We are now at 16lbs of lead but no tank.

Step three, look at the Steel HP 80 and see what lead it requires and its -3 lbs. Since its negative, we get to remove 3 more lbs of lead.

We are now at 13lbs of lead with the HP80 tank and should be properly weighted.

Note: In both tanks, we start the dive 6lbs overweighted due to air but end, with dead empty tank, neutral.

in_cavediver
11-06-2007, 17:58
You are doing so well... you deserve a specialty patch for understanding tanks....Now we need you to get all the doubles techy guys to understand the same thing and they will not be buying those negative buoy. tanks.

good job!!!!

The sad thing is its not really the tanks that are so bad. Its the volume/weight of the breathing gas carried. (and you plan to use it all weighting wise).

Dbl 104'[email protected] = 21 lbs
Al 80 Stage = 6 lbs
Al 40 deco = 3 lbs
---------------------------
= 30 lbs of GAS

WaScubaDude
11-06-2007, 18:27
I am confused. You say they have roughly the same buoyancy swing in the last paragraph, but in the paragraph above you say there is a 7lb dif due to buoancy characteristics. I am a little slow, so if you could have another go at it I would be greatful. i think there are some physics that I am not understanding. Thanks. Again I thought the Delta in water is what matters.

Two concepts here:

First - swing or change in buoyancy. This is strictly based on the weight of the air. AL steel HP LP don't matter. Only that it's 85 cft or 100cft or 77 cf. For reference, 1cft is around 0.075 lbs. Simply multiply that for ballpark number.

Second, the buoyancy characteristics of a tank. This is strictly the weight of the water equal to the volume of the tank minus the land wieght of the tank. This will be a postive or negative number depending on the tank in question. This basically tells you whether an empty tank will float or sink and how much force it takes to make it neutral. For instance, an AL 80 has a +4lbs value and takes 4lbs of lead to be neutral. A steel HP 80 has a -3lbs value or it needs 3lbs of lead REMOVED to be neutral.

Third, land weights of tanks. This is the easiest. Its what a tank weighs on land empty. HP 80 -28lbs, Al 80 - 32 lbs.

Now, how to use the information.

First, assume we know we are properly wieghted with 20lbs in an Al 80. Now, we want to know how much weight for an steel HP 80.

Step one, ignore the swing of air. It isn't used here.

Step two, get the neutral amount of lead needed. For this, we look at the Al 80 and see it +4lbs. Since we are removing the tank, we can remove 4lbs from our weights. to make us neutral again.

We are now at 16lbs of lead but no tank.

Step three, look at the Steel HP 80 and see what lead it requires and its -3 lbs. Since its negative, we get to remove 3 more lbs of lead.

We are now at 13lbs of lead with the HP80 tank and should be properly weighted.

Note: In both tanks, we start the dive 6lbs overweighted due to air but end, with dead empty tank, neutral.


Thank You! and thanks for the bit on LB air. Very clear.

WaScubaDude
11-06-2007, 18:29
Don't I look really happy in my avitar pic? I love diving!