weight calculations

[Lawman]

I'm 230lbs and wreck dive the Great Lakes in a 7mil wetsuit and a hps 100 cu ft tank, It takes 38 lbs of lead to get all that under water and keep it there with an empth tank. I'm going to Coz and will be wearing a 5 mil shorty and using an al80, Got any quick way to calculate how much weight I'll need? A starter weight of course..will fine tune.

 

[Cherry]

Check out this thread ....

http://www.scubaboard.com/showthread.php?s=&threadid=15082&highlight=weight+and+formula

a quote from it ...

The "formula" to go from fresh to salt is to add 2½ pounds per 100 pounds total weight. (this is an average and a "useable" number. The actual difference is something like 102.45% average, and salinity - and therefore specific gravity - of sea water varies from sea to sea, ocean to ocean and season to season - but 102.5% will get you close enough)

 

[Bubble Boy]

Lawman, you sound like you are over weighted for the fresh water. I remember my OW instructor saying that For fresh water with an alum tank the general rule for new divers (on check outs) wearing a 7.5mil with hood and gloves is 10% of body weight plus pool weights (usually 4 lbs).

With the steel tank on you should only have about 23 lbs. Considering that most of the great lake diving I know of involves mooring lines for ascent and descent, I would be tempted to go lower in weight.

 

[Dectek]

Lawman....If your question is to try to find a starting point to figure your buoyancy I will give you an example...

Home: Dacor 7mm Farmer john and Jacket with genesis steel 100 along with the hood, booties, and gloves. 38lbs of lead

Mexico: 3mm john and a hotskin with no jacket and the rental AL80 ... 24lbs of lead using the same BC .

I know you are not looking for lectures or being told you are not doing things correctly. I know what it feels like going to a completely different setup and not knowing how much weight to ask for. Just tell the DM you want to do a Bouyancy check well before the boat stops and they are more than willing to get you into the water fast.
Hope this helps

 

[KeyLargoBrent]

I'll second the motion for a predive bouyancy check. When you check in for your dives, just tell the DM you're not sure how much weight you'll need, and let 'em know what type of exposure suit you'll be wearing. Most DM's will be able to size you up and estimate (to within a few pounds) how much you'll need.

Rule of thumb: 10% of bodyweight, salt water, 5mm 2pc suit.

Keep in mind that the aluminum 80's gain bouyancy as they go from full to near empty. Carry some extra weight to compensate, or you may find yourself fighting to stay down, or "dorkeling" at the surface as your air supply dwindles.

 

[Rick Murchison]

What I do is sort of like homework. It takes some time at first, but then I know the answer.
First, I need a pool - in your neck of the woods this time of year you need a covered pool. In just a swimsuit, mask, snorkel and fins, I determine my body's buoyancy by seeing how much weight it takes to let me sink when I exhale about half way. Two pounds. I write that down. Next, I take my BC and see how much it takes to sink that (if anything), 1 pound, and write that down. Same for each piece of gear I have that floats, writing down the item and its weight requirement - I use regular scuba weights, and I have seven quarter-pound fishing weights as well, so I record each item to the nearest quarter pound.
For items that sink, I have a balance rigged that hangs under the diving board, where I can hang weights in a bag (under water) on one side and the item I'm measuring on the other and see how much negative buoyancy an item has, then I write that down on my list as a negative number. For tanks I measure them at 500 psi (my chosen number - some folks do 'em empty) with regulator attached (I don't figure I'll be carrying too many tanks without regulators on a dive). I end up with a list in my logbook that has the buoyancy of each item - so when I get ready to go diving, all I need to do is add up the values for me and all the gear I'm wearing/carrying, and I have my accurate fresh water weighting requirement. I then add 2.5 pounds per hundred pounds total weight (me and all the gear and the fresh water weights) to get my accurate salt water weighting requirement.
Looking at my list, for the question you asked, for me it would add up like this:
Me - 2
AL 80 with reg - 1
BC - 1
5 mil shortie - 6
Fresh water total - 10 pounds required
Total weight - 250X.025 = 6 pounds for salt water
Salt water total - 16 pounds required
Using this method takes all the guesswork out of weighting, and avoids unpleasant surprises at your safety stop - especially in Cozumel, where you'll be making them on the drift without a line to hold you down.
------------------------
Dry suit divers - it's a bit tougher as you have to measure the buoyancy of the dry suit with you in it, and with various combinations of underwear - time consuming, sweaty work. But once done, it's done and you never have to guess at weighting again.
Rick

 

[Lawman]

Thanks for all the good advice! That gives me something to work with for that first dive. I didn't have any idea where to start. This should get me in the ball park.

 

[pops]

i can tell you from bathtub experiments that a henderson medium small hyperstretch 5 mm jumpsuit requires 7 pounds to neutralize. - for what it's worth.

 

[divemistress]

"In just a swimsuit, mask, snorkel and fins, I determine my body's buoyancy by seeing how much weight it takes to let me sink when I exhale about half way. Two pounds."

By "sink", do you mean to the floor of the pool? Halfway to the bottom? Or ???

TIA

Judy

 

[Doof]

Yeah, that definitely sounds overweighted. When I first started diving I was wearing 40lbs to get under, wearing a 7mm farmer john and an al80. Now I'm wearing only 30lbs in my drysuit during an ice dive (so you can imagine how much undergarments I have on - I never got around to figuring out the thickness of my undies) and an al80.

Every spring, I get all geared up, and head to the lake wearing maybe only 20lbs. All the rest of the weights are in a catchbag being floated by a tunaball. I start with a tank (al80) that's only got about 700psi in it and then I just start gradually adding weight, keeping my lungs filled to a normal volume. When I'm floating at mask level, I exhale. If I still float, I add just a tiny bit more. If I sink, I then try removing a little bit of weight until I get it just right.

Then once I know how much weight I need, I begin the tedious process of figuriing out where it should all go. :bonk:

You're body changes every year, and even throughout the year. What worked for you last year isn't necessarily going to work this year.