Determine weighting empirically?

[geoff w]

I've been wondering about this for a while, and had a chance to try this out when helping with a rescue class recently. After the rescue scenarios, we're heading back to the beach. I had a HP80 tank, and most specs say an 80 cu ft tank will become about 6 lbs more buoyant when it goes from full to empty. On the surface with a nearly full tank, I took my 6 lbs of weight off placed it in a float, deflated my wing and was able to submerge to the bottom without swimming down. I was diving wet

Since I took off an equivalent amount to the difference between a full and empty tank, for the gear configuration that day, is this a reliable indication I was definitely not underweighted? (I might have still been overweighted, but I only had 6 lbs of lead, so couldn't drop any more weight) I haven't heard of this as a method for checking to make sure you aren't underweighted, but it makes sense. Am I missing something?

Any suggestions for how to do the same empirical test with a dry suit? I still don't have my weight dialed in with my dry suit, and this seems like a way to get close with high confidence I'm not underweighted

 

[rongoodman]

(Never mind--comment entered without enough coffee on board.)

 

[Akimbo]

Here is the best techique I have found so far:

http://www.scubaboard.com/forums/basic-scuba-discussions/344735-minimum-diving-weight.html

 

[geoff w]

Here is the best techique I have found so far:

http://www.scubaboard.com/forums/basic-scuba-discussions/344735-minimum-diving-weight.html

If I read this right, a boat is required. Correct? We were diving off the beach when I did my weight test

Also, I wasn't going for minimum diving weight with this experiment, just checking to make sure I had enough weight to stay down with an empty tank. Even here in So Cal with a 7mm full suit, hood, gloves diving wet with steel bp/w and a steel HP80, I only had 6 lbs of lead on me, so that's all I could take off. Conveniently, 6 lbs is the same as the difference between a full and empty tank (or at least close enough). Before I start trying to carry less lead, I wanted to make sure I wasn't underweighted to begin with

If I can validate this, then I can start to get a bit more precise on weighting with my dry suit

 

[TSandM]

Yes, I would say that, if you can descend with your weights less the expected amount of gas you are going to use, you know you aren't underweighted. It works the same with a dry suit, except I'd make sure that you can lie on the bottom with a comfortable amount of gas in the suit. Checking from an upright posture on the surface will only ensure you have enough weight to work with a totally empty suit, and it's hard to get a suit that empty while you are actually diving.

 

[fisheater]

What I do:

At the end of the dive during which I want to do a through weight check, I do a safety stop at 15' over a sandy bottom (shore dive).

I signal my buddy that I'm starting the weight check and purge my octo to get my tank down to 500 psi. I dump all the air from my wing and dump any excess air from my drysuit.

I remove a few pounds and place on the sand and then try to hover in horizontal trim with normal breaths. If I sink, I remove more weight. If I can't stay down, I retrieve some weight. When I get I the Goldie Locks amount ("just right"), I make note of the lead in the sand and then subtract that amount from lead I use from then on in that gear configuration.


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[Akimbo]

If I read this right, a boat is required. Correct?...

Boat, pier, edge of a swimming pool…

 

[3D diver]

Or do it the slow way...

Take/add a pound or two off/to your belt & see how it feels at the end of the dive

Repeat on the next dive till you know how much you need

Keep track of the numbers in your log book if the dives are separated by any significant period of time.

 

[Bob DBF]

I'm with Fisheater, I use a pocket weight belt and weights down to 1# for that very purpose. The only way to actually know the weight is to test it under actual conditions. Anyway it works for me, and apparently Fisheater as well.

If the bottom is crap, you can drop the weights in a game bag on the bottom for the same result, along with any gear you don't want in your calculation.



Bob
---------------------------
I may be old, but I'm not dead yet.

 

[Diver0001]

Here is the best techique I have found so far:

http://www.scubaboard.com/forums/basic-scuba-discussions/344735-minimum-diving-weight.html

I don't like the idea of weighting for the safety stop. I often do free-floating deco stops (actually swimming mid-water back in the direction of the shore while we do the stops) and weighting for the safety stop, while always a bad idea in my mind, can also be dangerous.

And like a couple of the other guys, the only way I've found to do a buoyancy check that works is to do it in the water. I've tried making calculations like the OP suggested and as an experiment even made an elaborate spreadsheet that calculated the amount of steel/aluminium in the tank and accounted for the volume and density of the metal in the tank, the salinity of the water etc etc and never managed to get satisfactory results from it. It's just one of those things that has to "feel" right.

R..