Weighting question

[David P]

I have about 50 dives yet I have a weighting question. I use a Dive Rite bp with a 7lb lead weight attached to the top of the plate and a PST 119. Im 6'4" and weigh about 270,when I dive wet I carry 10 lbs in my weight belt but I KNOW I am over weighted. I once made a dive and at the very end of the dive I started feeling a little light, I then noticed I forgot my weight belt. I have never been able to come up from a dive anywhere close to only having 500 psi left in my cylinder, I think my minimum is 1500 psi. I recently purchased a TLS drysuit. I guessed at needing 10 additional lbs of lead when I dive dry. I put very little air in my suit and actually very little in my bc with 20lbs in my belt. Heres my question, I decided to clean my pool today...while wearing my drysuit...I swam around for about an hour (ok its much faster cleaning a pool from to top side) but just couldn't suck down air from my half empty tank fast enough. So I purged out all but ~400 psi and did a weight check. I had no lead on my hips and could easily hover just off the plaster bottom in the shallow end. There is no easy way to convert weighting from fresh water to salt water is there?

 

[neil]

The difference is 2.5% of your total weight. Weigh yourself and all your gear that you used in fresh water. Multiply by 2.5% and add that to your salt water setup.

Example:
You and all your gear (including weights) weigh 340 lbs.
340 x 2.5% = 8.5 lbs. to add for salt water.

 

[David P]

you mean .025?... ok I know that "generally" the specific gravity of salt water is 1.025 (unless your treating for cryptocarion with a hyposalinity) so I just multiply total dry weight by the difference in SG... cool thanks! Anyone have an extra heavy duty scale I can borrow?

 

[venturediver]

Just weigh yourself, then weigh all the gear you had on in the pool and add the two together then do the multiplecation.

 

[neil]

you mean .025?... ok I know that "generally" the specific gravity of salt water is 1.025 (unless your treating for cryptocarion with a hyposalinity) so I just multiply total dry weight by the difference in SG... cool thanks! Anyone have an extra heavy duty scale I can borrow?

Yep, .025. It's that simple.

 

[TeddyDiver]

The difference is 2.5% of your total weight. Weigh yourself and all your gear that you used in fresh water. Multiply by 2.5% and add that to your salt water setup.

Actually it's 2,5% of your total volume*freshwater density but close enough calculated with weight..

 

[neil]

Actually it's 2,5% of your total volume*freshwater density but close enough calculated with weight..

I'm no math whiz, but here goes.
Neutral diver @ 340 lbs = 5.448 cubic feet displaced in fresh water.
5.448 * .025 * 62.4 = 8.49

Same result, different method, and more complicated. Which can only mean one thing: you must be an engineer!
Confess!

 

[jsmharley]

and this is with your tank weight also? do you add extra for the positive bouyancy at the end of a dive with an alum. 80? ( 5lbs?)

 

[Charlie99]

Actually it's 2,5% of your total volume*freshwater density but close enough calculated with weight..

Unless you are overweighted, the two are going to be the same at the end of the dive. Now if you really want to get fancy, figure in the fact that adding 1 pound of lead doesn't add 1 pound of negative buoyancy.

In practice, one can get pretty close by just guessing that a single tank rig is about 60 pounds, adding that 60 pounds to your body weight to get a reasonable estimate of total dive weight. Then either multiply by 0.025 or equivalently, figure 1 pound of lead added/dropped for every 40 pounds of total dive weight.

In your case, 270 pounds of body weight + a gross estimate of 60 pounds of gear weight = 330 pounds. 330/40 = about 8 pounds.

Since being a pound underweight is a lot worse than being 1 pound overweight, I'd add 9 pounds going from fresh to salt.

-------------------

p.s. A lot of people go through contortions to get a nearly empty tank to do a weight check. It's a lot easier to just figure out the weight of the remaining air in the tank and adjust for that extra weight. For an AL80 I just use the approximation of 6 pounds for a full tank, so therefore 1 pound for every 500psi. The similar approximation for your PST119, I'd assume about 9 pounds of air or 1 pound for every 400psi. For example, if you have 1200psi of air left in your tank and you remove enough lead to be neutral with no air in the BCD, then your proper weighting is 3 pounds (1200/400) more than you have left on the belt.

That technique is an easy way to check your weighting at the end your first salt water dive, even if you still have half a tank left.

 

[donacheson]

Actually it's 2,5% of your total volume*freshwater density but close enough calculated with weight..

" ... your total volume *freshwater density) but close enough calculated with weight..." Not only close, but exact: Archimedes had that one figured out a few millenia ago. If a diver is neutral, the volume of water he/she displaces equals his/her weight.

I know: picky, picky. <G>