Slowly piecing together doubles?

[Cheetah223]

I'm heading slowly in the direction of technical diving, but I've got a long road to travel. For now I'm simply trying to gain as much experience in the water as possible, while slowly acquiring equipment to make the initial blast less painful when I reach an inevitable crossroads. Right now I'm diving a bp/w and al80's, but i would like to make the switch to steel tanks, both to drop some lead off my belt and slowly head into doubles. The issue I've got is that I picked up an LP72 that's out of hydro as part of a package deal, and I'm debating wether to get another and strap them together as doubles or just to sell the 72. A single LP 72 wouldn't be enough gas to last my "average" dive, or my buddy would have to cut his tanks short, I thought about diving the doubles with my friend on his 80's, but then I'd have to fill the doubles for each dive.

How do you usually handle having different size tanks from your buddy? Just call the dive when diver x runs out of air on his smaller tank? Force your buddy to get a different tank? haha

If I buy new tanks to start this process, what is a tank that works well for singles dives (I don't mind cutting my dive short if I'm on a higher capacity tank and he's on an al80..) and also fits well as a double rig? I was looking at HP80's, but I was warned that they're difficult to trim out when doubled up. I don't want or need anything huge, but I want roughly the volume of an al80 for the previously mentioned reasons..

[Gombessa]

Double LP72s can be a bit tough to trim out as well, depending on which ones they are. I'd say to shoot for LP80/LP85/HP100s. Any of those will give you the same or more air as an AL80 when you dive them as singles, and will make for great "smaller" doubles that trim out well (which I hear is a boon for tech classes) and still allow you to do two dives with single tank divers.

Btw, we used to do this with double LP72s as well, and depending on relative SAC rates, sometimes you end up being the one who needs to turn on pressure, but if your friends are diving ~80cuft tanks, it's definitely manageable.

I've also heard that HP80s are very difficult to trim out if you're taller than 5'3" or so. I don't think it would be a good idea to double those.

[BabyDuck]

cheetah, if you're diving overheads (which i think for this would include deco, but don't quote me on that part), you 'tank match'. whoever has the smallest amount of gas takes 1/3 of their fill (so if he or she has 3000psi, he or she turns when they're at 2000) and the larger amount of gas person takes the same *amount of gas* (not the same psi). this way the smaller tank has a chance of getting both out. so in caves, huge tanks aren't really an advantage unless everyone has them. a hoover with larger tanks still might be the person turning the dive.

[Cheetah223]

Well I've got almost a foot on 5'3", so it sounds like HP80's wouldn't be the way to go :P I've honestly got NO clue what I've got in this 72...It's old, original hydro is '73, and there aren't any markings that give away who made it (at least not to me, but I don't know what most markings mean anyway..)

[diveandclimb]

I would say keep the 72 and use it as a deco/stage bottle once you go tec but I am pretty sure most tec divers use AL tanks for deco/stage.

[Cheetah223]

Most of what I've seen for tech are using AL tanks for deco/stage - but obviously I'm not even in basic deco procedures, so I could very well be wrong. I'm thinking about hydroing it (last was 99..) and filling it up for a dive just to see how close I come to my buddy's turn around, because with both of us on al80's he's always the one to suck his tank low, I'll usually come up well over 1,000psi when he's under 700..

[OH-JJ]

OK, if you are 6' 2-3" like myself, then No do not double up the 72's as you will not be happy with the trim. for guys our height you will be very happy with PST LP104's. As Singles they are good, but once you double them up and get the bands adjusted right, you will not need any additional weight, and you will be dead level in the water. Then save the 72's for deco/ stage bottles, and yes there are still alot of guys useing them.

They way I went about where you are is, I watched ebay, and found a guy selling two matching PST104's with matching H-valves. I jumped on them and got a great deal as they were just out of hydro. I dove them for almost a year as singles... Then I bought my buddy's OMS doubles. I did not like the way they trimmed out, so... I broke them down and used the manifold and bands on the 104's and put the H-valves on the OMS's. Granted I could have just added the bands and isolator and have been in business, but now I still have tanks to dive singles ;) ( and he made me a heck of a deal ;)

[DA Aquamaster]

I have both HP 100's and Steel 72's set up as doubles.

They both have the same internal volume and consequently the same tank factor (ie: how many cu ft they hold per 100 psi., which we'll over later)

Trim wise I find little difference between the HP 100's and the steel 72's but the qualifyer is that steel 72 vary a bit between manufacturer and some are heavier and trim differently than others. Heights and shoulder profiles also vary so they can be a challenge to match.

Volume wise, a steel 72 overfilled about 400 psi to 2670 psi holds as much as an AL 80 at 3000 psi. More importantly, an AL 80 filled to 2800 psi (a common hot filled AL 80 pressure) holds 71.8 cu ft. - almost identical to the 71.2 that a steel 72 holds at the plus rated pressure of 2475 psi. In short a plus filled steel 72 holds the same gas as a slightly short filled AL 80 and there is just not that much difference between them.

________


For technical diving gas matching becomes important.

To determine a tank factor, take the volume of the tank divided by the pressure and then multiply it by 100. For an AL 80 that holds 77 cu ft at 3000 psi: 77/3000= .02567x100= 2.57 (rounded to a tank factor of 2.6)

For a steel 72 this works out to 2.88 cu ft per 100 psi and a rounded tank factor of 2.9. If you double the tanks you also double the tank factors so a set of double AL 80's would have a tank factor of 5.2 while a set of steel 72's would have a tank factor of 5.8. The tank factor just makes the mental math easier when gas matching.

For overhead diving it is common to plan to turn the dive on thirds (in some cases you'd turn more conservatively on 1/4's or 1/6ths) and to do this with different sized tanks in the buddy team you need to consider the smallest set of tanks in the group.

If you had 2400 psi in the steel 72, 1/3 is 800 psi. If you take 8 times 5.8 you get 46 cu ft.

In the larger double Al 80's, you take the same 46 cu ft and divide it by the tank factor of 5.2 to get 885 psi. You'd add some conservatism by rounding that down to 800 psi to get your turn pressure. However, mentally it is easier to figure it backwards by trying 8 or 9, etc times 5.2. For example: 9x5=45 plus 9x.2 = 1.8, 46.8 total (a bit more than 46) and 8 times 5.2, 8x5=40 plus 8x.2=1.6, 41.6 total (a bit less than 46). In practice 9 is ball park close enough as the missing 15 psi is going to get lost in the noise of SPG accuracy anyway.

So...the Al 80's start at 3000 psi and the diver will turn at 2100 psi (or 2200 psi if you round down to 800) giving up only 100 psi compared to an actual third in the AL 80. Which makes the point that the difference between an Al 80 and a Steel 72 is pretty minimal even when doubled -as long as the Steel 72 is not short filled.

The difference gets bigger with larger differences in tank volumes.

For example, an HP 100 holds 100 cu ft at 3442 psi compared to the steel 72's 71.2 cu ft at 2475 psi. Both have the same tank factor of 5.8 when doubled but the starting pressures differ greatly. If the Steel 72s are filled to 2400, you'd have the same 46 cu ft third which would be about 800 psi in the HP 100's (roughly 6x8=48 - easy to do in your head floating on the surface prior to the dive). So if your HP 100's are filled to 3600 psi (a common fill pressure for HP 100's) you'd be giving up 400 psi compared to your normal 1/3rd turn pressure (turn at 2800 psi rather than 2400 psi).

If the steel 72's were filled to only 2200 psi you'd round down to 2100 (first 100 psi increment evenly divisible by 3) and figure a 700 psi third. 7 times the 5.8 tank factor is 40.6 cu ft, and again 40 divided by the HP 100 tank factor of 5.8 in your head is about 700 psi (actually 700 exactly if you work out the math) so the diver with the larger double 100 cu ft tanks filled to 3600 psi would turn at 2900 psi rather than 2400 psi, giving up 500 psi and almost 30 cu ft. compared to the normal 2400 psi turn pressure. A third diver with double AL 80's filled to 3000 psi would use the same 40.6 cu ft third and divide it by his tank factor of 5.2 to get a turn pressure 700 psi below the fill pressure (since 8x5.2 is 1.6 cu ft too big of a third). In this case the double AL 80 diver will be giving up 300 psi and about 15 cu ft compared to the normal 1/3rd turn pressure. In the extreme, if a 4th diver had LP 104's (tank factor of 8.0 when doubled) cave filled to 3600 psi (288 cu ft of gas), he would still turn on the same 40.6 cu ft "third" and would turn at 3100 psi rather than 2400 psi and would be giving up 700 psi and 56 cu ft. compared to his normal third.

The idea is that if everyone figures thirds based on the smallest tank, everyone will turn on the same volume of gas and, if needed, could get out of the overhead breathing the reserve third in those smallest tanks. The bad news is that the guys with the larger tanks don't see any advantage to having them. In the real world, diving the same sized tanks with buddies with similar SAC rates allows everyone to take full advantage of larger tank volumes - but it is important to note this poentially has the effect of actually lowering the overall reserve of gas in a multi member team.

So in effect, it is often peer pressure that drives the selection of a larger tank, since divers with smaller tanks tend to be less popular with the divers hauling around larger tanks, particularly if the goal is longer dives or penetrations.

____

Steel 72's really suck as stage bottles - they just do not ride as well as an AL 80.

[Padipro]

FYI...

When I first got into technical diving I had a set of AL 90's I put together. I figured that the extra 20 cuft I got by using the 90's was a good thing, the more gas you have the better right. Besides I already had one 90 cuft tank so it was cheaper to just buy another one and put them together then to buy another 2 tanks. I found out pretty quick that even 180 cuft of gas wasn't nearly enough to keep up with folks diving LP 95's and LP 108's. I was always the one turning the dive. Most of the tech divers down here get "cave fills" in their LP tanks so they always had way more gas then I did. I only made a few dives on the AL 90's before I bought a set of LP 108's myself and with a good cave fill I had 240 cuft of gas which allowed me to keep up with the rest of the group and not call the dive early.

IMO don't waste your money on the 72's, they're not big enough and you'll just end up buying a set of larger steel tanks anyway. Stick with LP tanks as the trim and buoyancy swing from full to empty is much better then AL or HP steels and, if you're in the right part of the country, you can get a cave fill and you'll have even more gas. And when it comes to breathing gas more is always better. And don't use steel tanks for stages or deco bottles. By the time you strap on double steel tanks, an AL backplate, reels, lights and two stages you're already so negative that you don't want to be adding any more weight then necessary. Besides AL tanks will float more streamlined when you sling or side mount them then steel tanks.

[Cheetah223]

Aquamaster, I read the whole post, and I won't lie - you COMPLETELY lost me on the math...Maybe it's because I met up with a friend tonight and I've had a bit to drink, but I had no idea where half those numbers were coming from haha. I started to understand where you were going in the last couple of paragraphs though, haha.

Either way, it's sounding like the 72's won't fit me very well in almost any application..I'm thinking of giving the one I have a shot as a single tank, possibly drop some lead, but beyond that I should start looking into LP85s or LP95s to use as doubles? I'm pretty sure the shop I go to has LP95's on sale for 213 or something..Maybe I'll have to dump a piece of this paycheck on one this weekend..

[beanojones]

DA, I'm not sure he should even think about overfilling old steel tanks like that. That's a 2250 tank. It lost its plus hydro in 1978. Its 64 cu ft filled to rated pressure.

It's not a 2400 tank with a plus hydro to 2640. It's a 2250, with no plus.

Cheetah, sell that steel 72 to somone who wants to dive it as is. There are plenty of people who like them just fine, but for tech diving that tank is kind of out of it. In Hawaii, you could sell it for $75.00 in a heartbeat (if it is in hydro). Old steel tanks last forever.

DiveRite Express ships any order over $50.00 free, including tanks. Other places have better
prices too.

DA's math is much easier to understand if you do the tank calculation in Metric. In metric, you measure tanks by the liquid capacity, and then just multiply by the pressure to get you available air in litres. Much easier to understand that way. Because pressure is measured in atmospheres, where 1 is what is around you, instead of 14.7. Decompression math is much easier that way too. 10 m = 1 atm instead of 33ft=14.7 psi. (it's not as fun in fresh water.)

[Rick Inman]

How do you usually handle having different size tanks from your buddy? Just call the dive when diver x runs out of air on his smaller tank? Force your buddy to get a different tank?


Aquamaster, I read the whole post, and I won't lie - you COMPLETELY lost me on the math...

I wouldn't worry about the math so much right now - that will come with the technical classes you should take. The point is to get that agency thinking (back in the boat with 500psi) out of your head, because it does not translate to doubles or technical diving.

The concept of true gas management must change. I would begin my reading this article by our own NWGratefulDiver, Bob Bailey: NWGratefulDiver.com (http://www.nwgratefuldiver.com/articles/gas.html) Once you understand the gas management concept, you can then begin to understand the math, sac rates, tank factors and volumes and how to apply that to your diving.

One thing about doubles, it's perfectly ok to end a dive with lots of gas left. :)

[TSandM]

I have dived doubled LP85s and LP95s, and doubled HP100s. For me, those three trim out similarly, but the HP100s give you a bit more gas for a bit less weight. Since you are in SoCal, it's HIGHLY unlikely you will be finding a shop to give you cave fills, but it shouldn't be difficult to get a full HP fill in 100s.

The best thing with doubles is to hook up with some folks who dive them, and try several different sets. Some combinations of tank and body just don't work, and some are far more comfortable than others. (I don't like double 72s, because they put me on my head, but my husband can trim them out fine.) You shouldn't have any problem in Southern California, finding some doubles divers to hang out with.

[Cheetah223]

Thanks for the article, Rick! It explains everything a bit better for sure. I've already had the 72 listed for sale on the boards, but I thought I'd consider the options before anyone popped out with a serious interest. I'll see what I can do to get in a few sets of doubles to see what works, but I figure there will be at least a few dive curve where the entire rig will be so foreign that I won't know what's good or bad haha.

[TravisD]

DA, I'm not sure he should even think about overfilling old steel tanks like that. That's a 2250 tank. It lost its plus hydro in 1978. Its 64 cu ft filled to rated pressure.

I thought that + ratings had nothing to do with age, and were simply a factor of the hydrotester confirming that it meets the REE specs at time of testing? There's nothing that would prevent a tank that was not + rated from regaining that rating, unless it can't meet the requirements of the test.

[Gombessa]

I thought that + ratings had nothing to do with age, and were simply a factor of the hydrotester confirming that it meets the REE specs at time of testing? There's nothing that would prevent a tank that was not + rated from regaining that rating, unless it can't meet the requirements of the test.

That's correct. I'd venture to say that the vast majority of LP tanks that have "lost" their plus ratings have done so simply because they were not plus-tested during subsequent hydros, not because they failed to pass.

However, the local tech shop here (that doesn't bat an eyelid when I ask for 32-3500psi fills on LP80s) is VERY wary of overfilling our old 2250psi Lp72s past 2475. That's enough for me to not do it.

[Cheetah223]

So aside from the obvious of holding more air, what are the pros and cons of getting HP tanks instead of LP tanks? I'm noticing a lot of places only carry a couple LP tanks, but an entire line of HP or FX tanks.

[Gombessa]

So aside from the obvious of holding more air, what are the pros and cons of getting HP tanks instead of LP tanks? I'm noticing a lot of places only carry a couple LP tanks, but an entire line of HP or FX tanks.

Pros: No need to find a shop willing to "overfill" to get you to 100cf. Even if you do get short-filled, you'd probably still have ~ 80cf, comparable to an AL80, with the buoyancy benefits of steel tanks. Potentially less hassle every five years over finding a hydro facility capable of plus-rating the tanks.

Cons: Some shops (and particularly boats) may only have compressors that can fill to ~3000psi, leaving you with a short fill in your HP tanks. Also, some shops may mistake the tank for an LP and short-fill you. Higher pressures may increase the strain on your regulator's high pressure seat, requiring more frequent servicing (though I doubt more than the 1-2 years recommended by manufacturers anyways). 300bar DIN-only (only an issue if you're running singles and have yoke regs).

[DA Aquamaster]

A steel 72 is still a 3AA steel tank made to the same engineering standard as a 2400 psi service pressure LP tank. People either forget that or do not know it.

I know shops get all sweaty filling steel 72's and many will not even fill them past the 2250 psi service pressure - but will fill a 2400 psi tank that may be almost as old to 3600 psi. The thing is that if a 3AA steel tank is well maintained it will last for a century or so with no real fatigue issues. I have seen welding tanks from the WW1 era pass hydros and do it with a plus ratings.

So in short, I don't sweat filling my steel 72 doubles to 2600 or 2800 psi. You'd have to push it to 3200 psi to get the same overfill you get when pushing a 2400 psi tank to 3600 psi.

I would not try to take a single steel 72 and turn it into doubles, as it is a pain getting two of them to match and to find 6.9" or 7" bands, bt I woudl not pass on a wel matched pair or an existing set of double 72's. They make great tanks for offshore diving with nitrox in situations where the bottom times are in the 20 minute range at 150' or so.

------

I agree that it is easy to do the math in liters, but only if you are working with an SPG in bar and know your sac in liters per minute.

If I know my 100 cu ft tank is a 12 liter tank (holds 12 liters at 1 bar, 14.5 psi absolute) I can multiply the number of bars I used to get the volume used (for example 50 bar x 12l = 600 liters)

But that is the point of a tank factor as well. If I know that my tanks hold 5.8 cu ft per 100 psi it gets real easy to figure out how many cubic feet I used or have left. If I used 700 psi I just need to be able to multiply 5.8 times 7 (take the 7 times the 5 to get 35, then take 7 times the 8 to get 5.6 and add them together to get 40.6. Or if you are math impaired, just round the tank factor upt o 6 and do 6 x 7 = 42. Close enough for government work in most cases.

On the other hand mixing the two systems is a problem.

To use both liters and PSI I have to know several things.

100 cu ft X7-100 = 12 liters internal volume

3442 psi = 237 bar

.6 cfm sac rate = 17 lpm sac rate

1 bar = 14.5 psi (round it to 15 to make the math easier)

But then the mental math gets really complex. So use one sytem or the other but don't mix them.

[Padipro]

Pros: No need to find a shop willing to "overfill" to get you to 100cf.

If you're diving a set of LP 95's or 108's you wouldn't need to "overfill" them to have damn near 100cuft as these tanks are 95 or 108 cuft at the rated pressure.


Cons: Some shops (and particularly boats) may only have compressors that can fill to ~3000psi, leaving you with a short fill in your HP tanks.

I dived the old style, 3500 psi, HP 100's for years and I never had a problem getting them filled to 3500. In the US anyway, you may find a problem with this other places though.



Also, some shops may mistake the tank for an LP and short-fill you.

I find it hard to believe that anyone could mistake an HP tank for an LP and short fill it. That would be like mistaking an AL tank for an LP steel. They are completely different in size, a 100 cuft HP steel is about the same size as an AL 80 where a LP 95 or 108 is the size of a small school bus.


300bar DIN-only (only an issue if you're running singles and have yoke regs).

The new HP tanks are only 3442 psi and have a 200 bar valve with a removable insert, the exact same valve you find on many newer AL tanks. You can use both a yoke or a DIN reg with these newer tank valves. The older style HP tanks, like the ones I have, are 3500 psi and have a 300 bar valve which will not work with a yoke reg. In fact the thread size on these tanks is different and it's extremely difficult to even find a valve that will fit them any more. I installed a 300 bar H halve on mine and had to special order them because only a few manufactures made them and almost no shop carried them. You can't even buy these older tanks at a dive shop any more.

[Gombessa]

If you're diving a set of LP 95's or 108's you wouldn't need to "overfill" them to have damn near 100cuft as these tanks are 95 or 108 cuft at the rated pressure.

Well sure. I was comparing the HP100s to LP80/85s, which are very close in terms of dimensions and weight. You could go to LP120s as well and have even more gas without overfilling, but as with 95s and 108s, you have corresponding drawbacks by doing so. I wouldn't recommend any of those tanks for beginning doubles users.


I find it hard to believe that anyone could mistake an HP tank for an LP and short fill it. That would be like mistaking an AL tank for an LP steel. They are completely different in size, a 100 cuft HP steel is about the same size as an AL 80 where a LP 95 or 108 is the size of a small school bus.

Again, those are obviously different tanks and I wouldn't expect anyone to make that specific mistake either. But in reality, it's unfortunately not uncommon for shops to mistake HP100s and LP80s/85s; they are nearly identical in size, shape and weight. I haven't been diving that long and it's happened to me a few times (and not just at bush league shops, either).

To tell you the truth, I'm a bit confused by this "some HP and LP tanks are dissimilar, therefore all are dissimilar" kind of logic. I don't recall anybody saying that LP95s look like HP100s.


The older style HP tanks, like the ones I have, are 3500 psi and have a 300 bar valve which will not work with a yoke reg. In fact the thread size on these tanks is different and it's extremely difficult to even find a valve that will fit them any more.

I have no experience with the 7/8" UNF valve tanks, but I'd be wary about getting those if I were planning on doubling them up, for the reasons you mention. I do see them pop up on craigslist quite often though, and have to wonder how many people end up buying them without realizing the valves are different.

[DA Aquamaster]

I agree, the Sherwood/Genesis 3500 psi HP tanks use a 7/8" valve that offers very limited manifold options and most of those are the odd Genesis angled variety that I really despise.

232 bar DIN/K valves are standard on 3442 psi tanks and as a practical matter almost no one is requiring 300 bar DIN valves on 3500 psi tanks - limitations in available valves aside.

In terms of LP versus HP most LP tanks will have a high pressure counterpart that is similar in size and weight and both tanks will hold very similar volumes at the same fill pressures. So...if you live in one of the few areas where you can get 3600 psi fill in LP tanks, they can be a very good way to go. On the other hand, if you can't get cave fills, then getting an HP tank of similar weight and dimensions to the LP tank you desire gives you the same volume as the overfilled LP tank without the problems getting an overfill. In that regard, HP tanks offer more flexibility. The downside is that an HP 130 tends to be more expensive than an LP 95.