30 replies to this topic

[Harleyguy1963]

I was wondering if charters carry different sizes of air cylinders to compensate for the differnt SAC rates of a diverse set of divers. Being a large begining diver, I imagine that an 80cf cylinder will not last very long and I would hate to feel like I am the one that would be cutiting a dive short for someone else.

Thanks for any info.

Gary

[BeachJunkie]

Most of the cylinders I've rented around my area have been steel 100's. When I go down to florida to dive, its usually AL 80's. Most shops carry multiple sizes and I'm sure if you were so inclined you would be able to get a larger tank if you requested one. Also, many of the dive boats have time limits on their dives. Often much sooner than you would normally need to come up for NDL or being out of air. Unless your SAC rate is very bad (say 25-30 mins) I can't really imagine it being too horrible. Next time you rent a tank/do a charter, see if you can get the bigger tanks like a 100 or 120. Can't hurt to ask.

[Harleyguy1963]

Most of the cylinders I've rented around my area have been steel 100's. When I go down to florida to dive, its usually AL 80's. Most shops carry multiple sizes and I'm sure if you were so inclined you would be able to get a larger tank if you requested one. Also, many of the dive boats have time limits on their dives. Often much sooner than you would normally need to come up for NDL or being out of air. Unless your SAC rate is very bad (say 25-30 mins) I can't really imagine it being too horrible. Next time you rent a tank/do a charter, see if you can get the bigger tanks like a 100 or 120. Can't hurt to ask.

Thanks for the info;

[Bubble2Bubble]

I like BIG Tanks whenever available, you can call ahead of time and see what they offer..

BeachJunkie advise is on Spot

Mike

[divzac]

For what it's worth the dive operation that I work at in the West Palm Beach, Fl. area has different tanks available if you ask.
remember, though if you are renting a steel "100" it is only "100" at 3500psi. If your operation fills all tanks the same, which most do, you may want to go to an aluminum "100" which I think is actually more volume. On the other hand the steel allows you to drop some lead and that may improve your sac rate as well. For me, it's about the FUN factor. The length of a dive is not directly related to the FUN... IMHO...

[shadragon]

Wenchie might be the expert to answer this Q as she is the most widely travelled of us all in the dive field.

Most of the tanks I have seen are AL 80 with a few AL100 every now and then. I suspect cost has something to do with it. At $145 (roughly) an AL80 is half the cost of a S100 at $345 or so. So when you are a dive op you can buy twice as many tanks for the same money. I always try to get the biggest tank I can along with a Pony for redundancy.

However, I think the main reason dive ops use AL are for safety. It is hard to out-dive an AL80 and end up in deco. I am sure there are some out there who breathe O2 in teaspoons who could prove me wrong, but us mere mortals would be challenged to exceed NDL on an 80. Plus, smaller tanks means shorter dives meaning they can go on more trips per day.

[peterbj7]

Actually I go into deco before emptying my tank on almost every dive. I don't like aluminum tanks very much, but down here in Belize that's almost all that's available. A benefit here of aluminum is that they don't corrode nearly as much as steel in this environment. When I closed my dive center I kept all my steel tanks (and sold all the aluminum ones that weren't manifolded), but lost two when they were condemned on test.

I wonder whether people here, mostly American, know how tank sizes are measured? This is one example where I think the American way of doing things is simply crazy, and I can't begin to imagine how it arose. Note that I don't feel that way about all, or even most, American ways of doing things where they differ from the rest of the world.

In America and satellites (including the Caribbean) the volume of air at standard surface conditions that can be compressed into the tank is what is quoted. Clearly the pressure needs to be specified, and that is the rated pressure of the tank. Unfortunately this is far from standard even for tanks that look virtually identical, and sometimes it can be quite difficult to find what the true capacity of a tank is. Then of course the actual fill pressure has to be allowed for, and pro-rated with this difficult-to-find rated pressure. A further complication is that many tanks can be "overfilled" by 10% beyond their "rated pressure" (seems nonsense to me), but there is again no standardisation as to whether the pressure used for computing the rated capacity is with or without the extra 10%. I've come across many people who say I'm wrong in this, but I can show you the tanks to prove it (all manufactured in the USA, yet to completely different conventions).

Away from American influence it all gets much simpler. The capacity of a tank is simply its volume, in other words the volume of water that could be stored in it. Then the standard-conditions equivalent is simply that volume multiplied by the pressure in bar. Makes computing SACs vastly easier!

Note that this has nothing to do with whether the imperial or metric measuring systems are used. I've never heard of of the water-volume of a tank being quoted in cubic feet, but I have come across compressed-volume measurements quoted in litres. Then all you need to remember is that one cubic foot equals about 28 litres.

[shadragon]

A further complication is that many tanks can be "overfilled" by 10% beyond their "rated pressure" (seems nonsense to me), but there is again no standardisation (snip)

As I understand it the rated pressure of a tank is stamped into it. If there is a '+' symbol after the rated pressure it allows you to go 10% over the stated pressure. This only holds true for new cylinders and (this part I am not sure of) only until the first hydro. Am looking for DOT documents now.

My S120's can go to 3850 PSI as a result of the 3500 + 10%

[dive_sail_etc]

*SNIP-SNAP*
My S120's can go to 3850 PSI as a result of the 3500 + 10%

Good grief, Simon!! Now I know why my ears pop down here in GA every time you go diving up there in NB. Leave a little air topside for the rest of the planet, please...

[shadragon]

Good grief, Simon!! Now I know why my ears pop down here in GA every time you go diving up there in NB. Leave a little air topside for the rest of the planet, please...

Better to have it and not need it, then to need it and not have it.

[Landlocked Dive Nut]

Better to have it and not need it, then to need it and not have it.

That's the best diver philosophy I've heard!

[VADiver]

As I understand it the rated pressure of a tank is stamped into it. If there is a '+' symbol after the rated pressure it allows you to go 10% over the stated pressure. This only holds true for new cylinders and (this part I am not sure of) only until the first hydro. Am looking for DOT documents now.

My S120's can go to 3850 PSI as a result of the 3500 + 10%

I've never seen a HP tank with a "+" rating--only LP's.

The PST and Worthington tanks are basically LP's with a HP rating; 104cft @ 2400 = 130cft @ 3445.

As for the "+' rating, you can get it Hydro'd at the "+" rating if the hydro facility knows what they are doing. However, once they hydro it without a "+" rating it's forever a 2400psi tank.

[JimG]

I don't know about Canadian laws, but in the US, the qualification for a "plus" designation is based on an engineering calculation, using the cylinder's Rejection Elastic Expansion (or REE) number. It is actually a "paper and pencil" calculation, not an actual "test", and uses data that is recorded during the hydro procedure. Contrary to popular belief, it is actually possible to get a steel cylinder re-plused at hydro if you have the REE number (and if the hydro facility is willing to do the test), but sometimes getting the necessary info can be difficult, if not impossible.

On many newer steel tanks, the REE number is stamped into the neck along with the "usual stuff". I believe that Faber does this on their cylinders, and its not uncommon to see something like "REE75" stamped into the shoulder of the tank. On older cylinders, you usually have to contact the manufacturer directly to get it. They may or may not have the info, and they may or may not be willing to give it to you.

Incidentally, the "plus" is stamped next to the hydro date, not the rated pressure. That's because it is based on a test that is done at the time of the hydro, not at the time of manufacture.

-JimG

[shadragon]

Here is a cute quote I found:

About 1990, two Russian titanium cylinders were demonstrated at the DEMA show. Their pressure rating exceeded 6000 psig. However, because titanium cylinder require welding, DOT approval was difficult to acquire as high-pressure DOT cylinders must be seamless. Titanium cylinders are now produced in Russia and sold by a US company to international military units. Although heavier than common scuba cylinders, those cylinders have exceptional capacity when compressors are available to achieve the high rated pressure.

No good for Nitrox, but still... Drooooooooooooooool...

[Landlocked Dive Nut]

If Simon ever gets his hands on one of these titanium cylinders, we'd never hear from him again!