HP 100 Steel Tanks - What am I missing?

[Slmason.72]

When figuring or testing weight needed with a particular rig, why is it always suggested to do so with the tank secured to the rig? I understand not wanting to be overweight and sink like lead shot, but in water just slightly deeper than your height, a snorkel and access to shallower water, why not weight yourself for all of your kit minus the tank? Since the weight characteristics of the tanks are well known, you could then adjust your weight from there...

For example, in fresh water 3 mil wetsuit, I need 4 lbs to maintain correct bouyancy. With an aluminum tank, I then add 3 or 4 lbs for the swing in weight due to air consumption to make up for the positive 3 lbs of bouyancy at 500 psi. If I were diving an HP 100, theoretically, I could ditch two lbs from my belt as the 500 psi rating for that tank is -2 lbs bouyancy, offsetting two lbs of my lead.

Maybe Im oversimplifying this, but is makes sense to me... Why all the trial and error?

 

[halocline]

First, I've got to ask do you really need ditch-able weight?

This is an ongoing debate. Here's the thumbnail sketch: PADI is a strong believer in ditchable weight, almost certainly because many new divers seem to have problems on the surface more often than at depth. PADI's training emphasizes the ability to easily get positive on the surface, usually simply by releasing a weight belt or pockets with one hand. OTOH, there is the issue of unexpected weight loss at depth, which could lead to a runaway ascent. Quick release pockets and some weight belts have been notorious for slipping off. If you're trying to evaluate safe practices, you have to balance this possibility and it's consequences with not being able to easily ditch weight at the surface.

Then there's the issue of how negative a diver might become at depth, should the BC fail. This is primarily a cold water issue, where a diver is wearing a lot of weight to compensate for a thick wetsuit or drysuit. In warm water with a single tank, properly weighted divers are probably not ever going to be more negative than they could easily swim to the surface if necessary. So, there are those, including myself, who think that diving without ditchable weight in this type of environment is perfectly ok.

Personally I rarely dive with ditchable weight, but I dive in warm water and I don't use much lead at all.

---------- Post Merged at 01:06 PM ---------- Previous Post was at 12:57 PM ----------

When figuring or testing weight needed with a particular rig, why is it always suggested to do so with the tank secured to the rig?

I'm not sure that it is...a related point to yours would be the issue of the 'balanced' rig in which both the diver in his/her suit and the gear separately are somewhere near neutral. If you wear lots of weight on a weight-integrated BC and then take your rig off at depth, maybe to make an adjustment or get out of an entanglement, you could find yourself hanging on upside down. Plus, you want to be able to float your rig on the surface. I saw a DM candidate try to get cute and toss her rig off the boat thinking she'd just jump in and don in-water. Instead she saw it sink out of sight and a technical diver fetched it the next day....pretty funny.

But I guess you're talking about doing a weight check without a tank on, but everything else, then adjusting for the tank. I guess you could do that, but how about the regulator, and IME the published buoyancy figures for tanks might not be entirely accurate. It's probably best just to do a thorough weight check with a near empty tank, and record the results for that type of tank.

 

[acteg]

Thanks for the info. I also dive warm water exclusively so I guess that's why I moved away from ditch-able weight.

 

[OldNSalty]

A good chart can be found at Tech Diving Limited: Scuba Cylinder Specifications from Tech Diving Limited - 928-855-9400

Oh that's a great site. According to them in a 3mil I can use just 5% of my buddy's weight...I'll have to start diving with smaller people.

 

[eelnoraa]

Who says the tank is 7 lbs heavier? AL80s are about 31.5 lbs empty, worthington X-7 is listed at 33 lbs. The tank is 7 lbs less buoyant, not heavier on land. Think of it this way, the AL80 is 31.5, but you need roughly 4 lbs to sink it, so your total on land weight is 35.5 lbs, where the X-7 weighs 33, but you can remove 2.5 in lead, leaving you with 30.5 lbs, or about 5 lbs less than the AL80. Those are approximations, not exact figures.

This is the complete explaination. Not just 6lb less buoyant, but also weight 5lb less overall on land.

Ditchable weight makes no different if everything work out fine. It is usually when things happen and under stress situation. Say you are diving HP100 with no ditchable weight and no exposure suit. At the beginning of the dive, you are overweighted by the gas inside, so 8lb. Wing failure on descent, you are swiming up 8lb. No problem there, but once you reach surface, you want to stay there until help comes with your head out of water (you will need to support more than 8lb, say 8lb gas + 10lb head because it is out of water). Without ditchable weight, you have tread water. How long you can do that with 18lb negative? I know this is extreme situation, but it is the reason why people recommend ditchable weight. It is not to get your up to surface, but rather making you stay there effortlessly once you are there.

---------- Post Merged at 01:43 PM ---------- Previous Post was at 01:41 PM ----------

Oh and wetsuit doesn't help in this case. It only makes you more negative on bottom because suit loses buyancy

 

[OldNSalty]

or if it happens on decent you could just grab the anchor line and pull yourself back up.....

 

[ams511]

or if it happens on decent you could just grab the anchor line and pull yourself back up.....

A person could also use a lift bag or SMB.

 

[halocline]

No problem there, but once you reach surface, you want to stay there until help comes with your head out of water (you will need to support more than 8lb, say 8lb gas + 10lb head because it is out of water). Without ditchable weight, you have tread water. How long you can do that with 18lb negative? I know this is extreme situation, but it is the reason why people recommend ditchable weight. It is not to get your up to surface, but rather making you stay there effortlessly once you are there.

Not exactly, remember that properly weighted means floating at eye level with a near-empty tank, so half your head is out of the water. Anyone diving with a single tank that's even close to 18lbs negative at the surface is severely overweighted. But I do agree that the reasoning behind having ditchable weight is to be able to quickly and immediately get positive on the surface. For me that's simply not a problem; I'm not overweighted, I'm a good swimmer, and I can easily doff my rig in an emergency.

PADI certifies a LOT of divers that are not in good physical shape and are not very comfortable on the water surface.

 

[diverrex]

I dive with some ditchable weight, it could be 5 lbs in cold water with steel tank and SS BP, or with just 2 lbs in warm water with an alu tank and SS BP. I could see diving without any ditchable weight. I think the idea of having ditchable weight applies more to those who dive overweighted in the first place. Once a diver starts to dive with less weight the importance of ditchable weight decreases.

 

[ams511]

I think the idea of having ditchable weight applies more to those who dive overweighted in the first place.

Well every diver is overweight at the beginning of a dive, the question is how much? On a balanced rig you would be overweighted by just the weight of your gas. Where ditchable weight really matters is if you are diving cold in a thick wetsuit with 20 lbs of weight. If you have a wing or air failure at depth and your suit is compressed then you would have a hard time swimming up the weight. You want to be able to drop it and drop it fast.