BP/W and Steel Tanks

[Charlie99]

Sorry, to catch up so late, the regular scuba tank single cylinder has a swing going from about
-1.4 to a positive swing of about +2.5 at 500 psi, and when empty it's +4.4 positive, which none of us will be breathing underwater, a empty tank See here :
http://www.mdaguam.com/tankspecs.htm

The +2.5 pounds at 500psi is a typo in the chart. The common AL80 tank is +3.4 or +3.5 pounds at 500psi.

5.8 pounds total swing from 3000psi to 0psi (-1.4 to +4.4) is pretty darn close to 1 pound per 500 psi.

 

[dcs917]

WOW. Serious discussion here.

Great replies, all. Please consider the bouyancy of tanks relative to each other. So - each time you change your gear (or fresh water vs. salt). you start over and need to recalculate your bouyancy,

Dive safe.

 

[Life-Is-Good-Diver]

WOW. Serious discussion here.

Great replies, all. Please consider the bouyancy of tanks relative to each other. So - each time you change your gear (or fresh water vs. salt). you start over and need to recalculate your bouyancy,

Dive safe.

Thats correct. I have a B/P wing set up for doubles, 1 for singles, and when i'm carrying around my camera set up, Ikelite housing with a 8" dome port, and dual Ds-125 strobes, I even have to wear different weight then when diving without my rig.

I never knew how important weighing was, but it is extremley important.

Whats even more important, is not to be negitive, and not have ditchable weight for single tank diving. Theres no reason to be.

As for doubles, the same rules apply. Somone had mentioned before, DIR does not recommend diving steel tanks with wetsuits, and they are correct. Yes, i do dive a single steel 100 wet, but I have a lift bag on the bottom back of my B/P and wing set up.

A few years ago, I had a failure and had no lift for my BC, so we shot a lift bag, and inflated my BC oralley, and didn't relize until then how important weighting was.

Now I track the weighting for which wet suit /drysuit, which tank, steel ,or alum 80's, camera, or no camera and the list goes on. It's the only way

 

[Life-Is-Good-Diver]

The +2.5 pounds at 500psi is a typo in the chart. The common AL80 tank is +3.4 or +3.5 pounds at 500psi.

5.8 pounds total swing from 3000psi to 0psi (-1.4 to +4.4) is pretty darn close to 1 pound per 500 psi.

Heres a good artical on Alum 80's :

http://home.gwi.net/~spectrum/scuba_al80.html

there are a ton of charts and specs, Luxfer is different then Sherwood, and same with Catilina, etc, but looking at my charts, for my pair of alum 80's I have a positive swing to expect of around 7 lbs. A while back I carry a tank spec chart for all my tanks, in my bag, sealed so I know my weighing for different combantions

 

[Life-Is-Good-Diver]

The +2.5 pounds at 500psi is a typo in the chart. The common AL80 tank is +3.4 or +3.5 pounds at 500psi.

5.8 pounds total swing from 3000psi to 0psi (-1.4 to +4.4) is pretty darn close to 1 pound per 500 psi.

Sorry, this was the link I meant to add, it has alot of specs for tanks :

http://www.huronscuba.com/equipment/scubaCylinderSpecification.html

 

[Charlie99]

Heres a good artical on Alum 80's :

http://home.gwi.net/~spectrum/scuba_al80.html

there are a ton of charts and specs, Luxfer is different then Sherwood, and same with Catilina, etc, but looking at my charts, for my pair of alum 80's I have a positive swing to expect of around 7 lbs. A while back I carry a tank spec chart for all my tanks, in my bag, sealed so I know my weighing for different combantions

The 7 pounds mentioned in the linked article is the difference in buoyancy between an AL80 and a PST E7-80.

Figuring out the buoyancy swing in a tank is pretty easy. It's the weight of the air in the tank.

Air weighs very close to 6 pounds per 80 cu ft ---- so the 77.4cu ft of an AL80 is about 5.8 pounds. If you want to figure the swing of different sized tanks, just do the simple multiplication, based upon 6 pounds per 80 cu ft. A 100 cu ft tank ?? 100/80 * 6 = 7.5 pounds. Simple.

It doesn't matter if the tank is steel, aluminum, or fiberglass. The swing is the weight of the air.

If the tables say different than this, then they are in error.

 

[Charlie99]

and when i'm carrying around my camera set up, Ikelite housing with a 8" dome port, and dual Ds-125 strobes, I even have to wear different weight then when diving without my rig.

Many divers choose to weight themselves independent of any gear than may be jettisoned, such as cameras, SMB, reels, pony bottles, etc.

Whats even more important, is not to be negitive, and not have ditchable weight for single tank diving. Theres no reason to be.

Some divers like to have ditchable weight about the same weight or more than the weight of the gas they are carrying. That way, if they have a BCD or wing failure at the beginning of the dive, that they can achieve positive buoyancy by ditching that weight. If you are properly weighted so that you can hold a stop, and then ascend to the surface in a controlled fashion with a near empty tank, then when you have a full tank at the beginning of the dive you will be negatively buoyant if your BCD fails.

 

[Life-Is-Good-Diver]

Many divers choose to weight themselves independent of any gear than may be jettisoned, such as cameras, SMB, reels, pony bottles, etc.

Some divers like to have ditchable weight about the same weight or more than the weight of the gas they are carrying. That way, if they have a BCD or wing failure at the beginning of the dive, that they can achieve positive buoyancy by ditching that weight. If you are properly weighted so that you can hold a stop, and then ascend to the surface in a controlled fashion with a near empty tank, then when you have a full tank at the beginning of the dive you will be negatively buoyant if your BCD fails.

You are correct. that is why it is more important at the begging of the dive, to not be negitive, as you had said which is correct, all tanks, regaurdless of steel or alum have a swing of around 6-7 lbs, but it is important to know where you start and where you end up.

Example a PST steel tank has a start of being around somwhere 8-9lbs negitive, and ends up less then 2lbs negitive. But a alum 80 can start around 1-1/2lbs neg and end up around 4 lbs positive.

As you said, it all has to be taken into consideration, for cameras, pony bottles, if your slinging tanks, and a bunch more. Simpley i cannot keep track of ALL my weighing for all the different types of diving I do. For myself, I have to chart each configuration.

I think we all agree tanks have a " swing " , but knowing where you start at the begging of the dive, when they are full, and where the tank " swing " ends up, is the most important of the weighting issues to deal with, so you dont start out dropping like a rock if you have a complete wing failure

 

[Puffer Fish]

I never knew how important weighing was, but it is extremley important.

Whats even more important, is not to be negitive, and not have ditchable weight for single tank diving. Theres no reason to be.

As for doubles, the same rules apply. Somone had mentioned before, DIR does not recommend diving steel tanks with wetsuits, and they are correct. Yes, i do dive a single steel 100 wet, but I have a lift bag on the bottom back of my B/P and wing set up.

A few years ago, I had a failure and had no lift for my BC, so we shot a lift bag, and inflated my BC oralley, and didn't relize until then how important weighting was.

And this thread was actually going along very well... at least for a while.

First off - hate to say this - but in general, blanket statements tend to be wrong...and more specifically, the Steel tank/wet suit comment is wrong.

There are some aluminum tanks that are negative when empty... some steels that are positive... you need to know the specific tank.

Wetsuit - which kind - how thick?

A far better statement would be to not dive doubles with a SS back plate, and not dive with high pressure tanks.

Example: Compare a pair of Catalina C100's (aluminum) versus a pair of Worthington LP - 108's. The Worthington's are zero when empty, the Catalina's are -.8 (two tanks). The weight difference is 16 lbs from full to empty versus 16.4 for the Catalina's... and you get more air with the Worthington's..

Several Faber FX tanks are positive when empty, and if you use them as low pressure tanks...compare very favorably with aluminum.

This issue of suit compression, ditchable weight... overall weighting is a not very pretty picture. Using a lift bag would not be consided by most, a proper safety response...but that should have a whole thread.. or several threads dedicated to it.

Assuming you start a dive adjusted for the air weight, meaning you are neutral when the tank is near empty (500 psi) at 15 ft. That means you are entering the water around 14 - 20 negative at the surface. At say 99 ft, and just picking a wet suit requirement for 20 lbs, you will be around 30 lbs negative... that assumes that the suit just lost 75% of it's buoyancy, but some of the newer materials, when compressed actually go negative (density of the material is heavier than water, and when the gas is compressed, will go from buoyant to sinking..you can see it with a small piece of the material, by the way). So, have a super stretch suit, and you might be 35 lbs negative or more. If you ditch all your weight, you are still 15 lbs negative..which you can swim against, unless you are in an inclosed space..

Which makes a BP/W failure not an option... as the results could be really, bad.

Could you make this situation safer? Sure... have a plastic backplate (more ditchable weight). Pick your tanks very carefully. Consider having an oil filled (light weight, non-compressable liquid addition to the tanks - I know, they don't make them, but they should.)

As a note,consider the following:

1. The weight of 500 psi varies with the volume of the tank.

2. Almost all of the mfg have math errors in their tank numbers... Luxfer is the best, but even they have some. The worst is Faber... don't trust anything they publish.. some is good, so is not.

 

[Life-Is-Good-Diver]

And this thread was actually going along very well... at least for a while.

First off - hate to say this - but in general, blanket statements tend to be wrong...and more specifically, the Steel tank/wet suit comment is wrong.

There are some aluminum tanks that are negative when empty... some steels that are positive... you need to know the specific tank.

Wetsuit - which kind - how thick?

A far better statement would be to not dive doubles with a SS back plate, and not dive with high pressure tanks.

Example: Compare a pair of Catalina C100's (aluminum) versus a pair of Worthington LP - 108's. The Worthington's are zero when empty, the Catalina's are -.8 (two tanks). The weight difference is 16 lbs from full to empty versus 16.4 for the Catalina's... and you get more air with the Worthington's..

Several Faber FX tanks are positive when empty, and if you use them as low pressure tanks...compare very favorably with aluminum.

This issue of suit compression, ditchable weight... overall weighting is a not very pretty picture. Using a lift bag would not be consided by most, a proper safety response...but that should have a whole thread.. or several threads dedicated to it.

Assuming you start a dive adjusted for the air weight, meaning you are neutral when the tank is near empty (500 psi) at 15 ft. That means you are entering the water around 14 - 20 negative at the surface. At say 99 ft, and just picking a wet suit requirement for 20 lbs, you will be around 30 lbs negative... that assumes that the suit just lost 75% of it's buoyancy, but some of the newer materials, when compressed actually go negative (density of the material is heavier than water, and when the gas is compressed, will go from buoyant to sinking..you can see it with a small piece of the material, by the way). So, have a super stretch suit, and you might be 35 lbs negative or more. If you ditch all your weight, you are still 15 lbs negative..which you can swim against, unless you are in an inclosed space..

Which makes a BP/W failure not an option... as the results could be really, bad.

Could you make this situation safer? Sure... have a plastic backplate (more ditchable weight). Pick your tanks very carefully. Consider having an oil filled (light weight, non-compressable liquid addition to the tanks - I know, they don't make them, but they should.)

As a note,consider the following:

1. The weight of 500 psi varies with the volume of the tank.

2. Almost all of the mfg have math errors in their tank numbers... Luxfer is the best, but even they have some. The worst is Faber... don't trust anything they publish.. some is good, so is not.

Well said and I couldn't agree more!!

Yes, I also agree about the lift bag, as a source of lift. I had both the OMS dual bladder wings, and the Diverite's dual bladder wings, but thats another subject, because you will recive many responces as to what's correct, and whats not, dependimng who is certified through which agency, such as DIR, Padi, and so on


Thanks for the clarifaction !