Help with Buoyancy Math

[don Francisco]

There are 2 calculations needed here. First the fresh to salt conversion, assuming you'rre neutrally weighted in either is 2.5%, or 1# per 40, of your total weight, including equipment and ballast. You could take your weight, add 2.5% plus 1-1/2-2# for equipment and ballast (60-80#s)

For the wetsuit change, I'd estimate you'll need to shed about 5#s. You can get a better estimate by putting the suit in a mesh bag and experimenting to find out how much weight is needed to sink it. Subtract slightly less than 2/3s of that amount (3mm vs. 1mm). This isn't super precise because the different brands exact thickness vary somewhat, but it'll be close.

Net the 2 adjustments to get a good starting weight, and dial it in on your dives.

BTW- for future reference, find out your weight requirements without any exposure protection, and for your current 3mm suit, and it'll be easier to extrapolate the required weight for different wetsuits. Have a great trip. dF

 

[SoggyPretzel]

Tell the divemaster that you have new gear and you need to do a boyancy check before the first dive. He or she will help and you can do it right next to the ladder, no worries. It will take just a minute or two. Then you know for sure and you can get the most out of your dives. Anything esle is just guessing and if you don't guess right you run the risk of hosing up an excellent dive opportunity. Also no need to buy more weight if you need it, most boats have some on board for you to use.

 

[scubadiver888]

I don't know when I will have time to do that, but for what it's worth - I did think of that first. Unless the charter folks are willing to let me play around for a minute before we start the dive.

Hopefully they will allow me a minute. If not, I guess I will just have to make a best guess for the first tank and correct on the second tank. Assuming 6 extra lbs for the tank, would that mean I need to use 5-6 for the saltwater, and 6 for the tank? Taking my total to 22lbs (Minus my guess of 4 for the suit, someone else said 6, so, 16 lbs, ish?)

We're getting into Destin around midnight Thursday, diving 8 am Friday. Otherwise I'd just hop in the surf at low tide and check it out, but I don't think I will have time.

I usually find if you let them know well ahead of time that you'll need a moment to adjust your buoyancy because of new equipment, most dive operators are okay about it. The only exception to this is if the water is choppy; most guys would rather be under the water than (a) in the water on the surface or (b) on a rolling boat.

If it is a shore dive, let everyone know so you don't have a bunch of guys gearing up and waiting for you to adjust. I usually drop 2lb weights in my pockets so I can easily toss them on shore if I'm over weight. Or you could go the other route and leave your pockets empty so you can add weights should you be light. I'm usually overweight in my guess (dive operators tend to be overweight on a guess as welll; better to be overweight and sink than underweight and float).

 

[scubadiver888]

I'm 5'9" - 185-190 depending on the week, lol.

Sounds like you are a little thinner than me, i.e. less fat more muscle. I'd revise my guess to 12-14 lbs. As everyone else has said, if you could weigh yourself with the 1mm suit in a pool then add 6lbs you'd be set.

 

[Charlie99]

If it is a shore dive, let everyone know so you don't have a bunch of guys gearing up and waiting for you to adjust. I usually drop 2lb weights in my pockets so I can easily toss them on shore if I'm over weight.

If you are checking your buoyancy with a full tank, you need to compensate for that -- otherwise you'll have a hard time maintaining a safety stop with a near empty tank.

80 cubic feet of air is about 6 pounds. For example, if you are using a full AL80 and you have found the amount of lead that just allows you to hold a 10' stop with no air in the BCD, then you need to add 6 pounds because that's how much lighter you will be with a near empty AL80.

In real life, you can add a bit less, because when you first jump in with a dry wetsuit to do a weight check it takes a while for it to fully saturate. Many people notice that it is easier to descend on a second dive, even with identical gear and weighting. IMO, it is because their wetsuit and BCD are fully wetted at that point.