HP Steel - Tank Buoyancy Swing - How to Weight

[Landlocked123]

Bottomline if I am weighted correctly based on a full HP100 (meaning performing the corrct test with no air in BC, breath of air etc) would I not be underweighted by the amount of the swing in tank buoyancy from full to empty (or near empty at 500PSI) ?? So my HP 100 swing is goes from -8.4 lbs at full state to -1.5 empty. So if I calc my weight based on the ability to hold a safety stop with 500PSI at 15 feet am I not by definition overweighted when the tank is full given the same amount of lead? I take it it's the way it goes with HP steel tanks. Am I missing something here???

 

[DuboisP]

that's the same with an AL tank
balanced at the stop, overweighed with a full tank

 

[Landlocked123]

OK - Sanity check complete. Thanks

 

[Steve_C]

There is one other factor, your wetsuit. When you first enter the water with a dry wetsuit it displaces some water and gives some extra lift. Some of this air gets forced out so the wetsuit gives less lift at the end of the dive. You can tell the difference. A wet wetsuit is a lot heavier than a dry one.

Even more over weighted if you carry a pony. I really do not want to go positive while breathing off the pony.

 

[tracydr]

Imagine how overweighted some of the really deep technical divers are when diving open circuit. I've seen pictures of divers lugging 6-8 tanks!

 

[raftingtigger]

Not really the OP's question - which was well answered on post #2 - is that a HP steel tank and a LP steel tank have vastly (about 5#) different buoyancy.

I found this out when I borrowed a HP100 and forgot to put in my usual trim weights for my LP95. Since I was already in the water I did a quick weight check and was surprised I was right on. Later I looked it up and it made sense. My trim weights were the same as the difference in buoyancy between the LP and the HP. Later I loaned a partially full LP95 to a friend to do some mask drills before we set out on our "real" dive. He normally dives a HP100, and not knowing the difference he didn't add weights and couldn't descend to do his drills.

Finding out the full and empty buoyancy of a tank is easy. Just "Google" it. If you rent or borrow tanks it might be a good idea to keep a chart with your dive gear.

 

[Steve_C]

I carry the last few pages of my log book with me on dives in my water proof bag. The last page is a listing of the weighting for all combinations I have dove of tanks, wet suits, and whether fresh or salt. There are well over 20 combinations (shorty, 3/4, 5, 7, 10/14, dry) (HP 100, 80, LP, HD, etc) (fresh, salt)

 

[Hatul]

You weigh yourself correctly to be able to perform a stop near the surface at the end of the dive. At the beginning of the dive and at depth you're "overweighted" but that's where the BC inflation comes in.

 

[nimoh]

Bottomline if I am weighted correctly based on a full HP100 (meaning performing the corrct test with no air in BC, breath of air etc) would I not be underweighted by the amount of the swing in tank buoyancy from full to empty (or near empty at 500PSI) ?? So my HP 100 swing is goes from -8.4 lbs at full state to -1.5 empty. So if I calc my weight based on the ability to hold a safety stop with 500PSI at 15 feet am I not by definition overweighted when the tank is full given the same amount of lead? I take it it's the way it goes with HP steel tanks. Am I missing something here???

by your profile, you are PADI certified. The PADI OW system for weight check involves emptying your BC, hold a normal breath and float at eye level --- THEN add weight to compensate for the amount of air you will use during the dive.

another way, as others have suggested is to check at the end of a dive with a near empty tank and hovering at 20 feet with no gas in the BC.

Both accomplish the same thing, but the latter is a bit more precise.

 

[dfx]

by your profile, you are PADI certified. The PADI OW system for weight check involves emptying your BC, hold a normal breath and float at eye level --- THEN add weight to compensate for the amount of air you will use during the dive.

Interesting. I've heard that one many times before (never did it myself because I didn't think it was too useful), but always without that last part.