Weight Calculations

[Arkman]

I realize this has been discussed many times at length. The only way to truly know is do a proper weight check. But planning to dive last weekend in much different conditions than I was used to go me wondering.

Is there a scientific calculation that could be used?

Think about it. One could figure out the buoyancy characteristics of all their gear. Many of them are probably readily available from the manufacturer. That would be a pretty good indication of your gears overall buoyancy characteristics.

The major factor for anyone is their body type. Some people have more muscle some more fat, more or less hair (see avatar). All of that stuff is, in my eyes, the wild card of buoyancy. So if one was so inclined couldn't you figure out your buoyancy of just your body? I know it would change on a day to day basis depending on lots of stuff but probably only a small amount.

So in theory you would know your ideal weighting. Sure many new divers wouldn't be able to do the calculations then put that weight on and dive, but wouldn't it give something to strive for. When you get the most comfortable in the water you would get to that weight?

Not that I'm really going to do this, because its easier to just do a quick weight check but I was just curious. Am I missing something?

 

[muddiver]

Multiply your body weight by 10% and add 4 lbs.

Works for 1/4" or 6 mil neoprene wet suits, standard gear and an Aluminum 80.

For dry suits and warm or fresh water you have to kind of wing it. It will all boil down to adjusting as you do some dives.

If you happen to change the type of cylinder you use or carry some thing heavy or buoyant, you can some times find buoyancy characteristics through the manufacturer's web site.

 

[Indian Valley Scuba]

Let's see...I weigh 270#, so that's 27 plus 4 equals 31 pounds

Reality check: In a 4 mil full suit in saltwater with an aluminum 80,
i wear a s/s backplate ( 6 pounds ) and zero additional weight for perfect buoyancy

I think the formula, while handy, may not apply in all cases

 

[Insta-Gator]

If you use 10% of your body weight for a 5mm jumpsuit in salt water, add 3 to 5 pounds for a 7mm. When you move from salt water to fresh water, subtract abut 6 pounds from what you normally use.

For cylinder information, see: Catalina Cylinders or the manufacturer of your choice.

Edit: Some folks say you can use about 3# per MM. And obviously these are only starting points.

 

[Indian Valley Scuba]

Here's a handy chart that covers a lot of cylinders!

 

[Charlie99]

Arkman, you are correct that in theory one could look up the buoyancy of everything, add it together and figure out the correct weighting. In real life, the buoyancy of things like BCDs are often not listed by the manufacturer or are incorrect.

A calculation of "differences" is usually pretty accurate, though. For example, if you know your ideal weighting with one type of tank, then looking up the empty buoyancy of that tank and the new tank and adjusting lead by the difference will get your very close.

And often you will know the difference in buoyancy between your various wetsuits, so that change can be factored in accurately.

Fresh to saltwater conversion are also pretty accurate. 6 or 7 pounds if you don't want to calculate, or add lead = 3% of total dive weight.

Another variable that you may be able to sort out ahead of time is to note differences in the buoyancy of your body. I find the easiest way to keep track of that sort of difference is to jump into a pool and find out how full my lungs are when I'm neutral in the pool. A normal adult has vital capacity of about 3 to 5 liters. So the buoyancy difference between exhaling as hard as possible and inhaling as hard as possible is 3 to 5 kg or about 6-1/2 to 11 pounds. So just knowing how much you float in freshwater is a semi-quantitative measure of the buoyancy of your body.

 

[fnfalman]

What about the "buoyancy" of one's body?

A 200-lbs fatty like myself is not going to sink like a 200-lbs athlete.

Weighting plan is a guessing game. We try to be as scientific as we can, but at most it's an educated guess. That's why we do "weight check" in the water.

 

[Indian Valley Scuba]

Great response Arkman, except for that "6 or 7 pounds" comment.

It is so unfortunate that so many divers are trained to be overweighted from day one. It is a curse that is carried, literally, on every dive. "Less is more" is the key in adding lead.

 

[Bubbletrubble]

From speaking with novice divers locally, I get the impression that many divers get basic OW certified without really knowing how to do a proper weight check. Either that...or they simply forget how to do one over time.

Conducting a proper weight check only takes a couple of minutes to do. I always do one before the first dive on a vacation even if I'm diving the exact same configuration I used on a previous trip. I still check my logbook to figure out how much lead weight I needed on the previous trip...but it's always good to verify that number. It's a safety issue in my mind.

When it comes to determining minimum weighting requirements, I don't find "rules of thumb" useful at all, with the exception of the ones already mentioned by Charlie99. Some people advocate figuring out the buoyancy of each individual piece of dive gear and then recording this information somewhere. Then, the person simply adds up the buoyancy numbers to determine weighting requirements. This method works so long as you realize that, with usage, neoprene will lose a certain amount of positive buoyancy. For example, a typical, new 7mm wetsuit will be significantly more positively buoyant than the same brand/make/thickness/size wetsuit worn on 100 dives.

 

[fnfalman]

If people were to not want to put on all the gears then do a weight check, may I suggest doing a weight check with just yourself, exposure suit and weight belt?

I did it in my Rescue Diver class and it came out just about dead on for previously calculated dive weight with all my rig on me.

Weight yourself with exposure suit (or not depends on your location) and see how much weight it takes for you to hold a lungful of breath and floats eye level. Keep that lead weight in mind.

Then go weight your gears and see what you need to do in order to achieve the same weight as the previous test. For example:

1. I weighted myself with my 7mm suit, fins, mask, gloves, hood, booties and weight belt. My dive weight was 14-lbs.

2. My dive gear consists of Dive Rite Transplate BPW with steel plate, Faber 100-cuft M-series HP tank = 6lbs of steel plate + 6.69lbs of tank buoyancy at empty + 1lbs of 500psi of air/gas = 13.69lbs.

3. My alternative dive rig consists of Dive Rite Transpac soft plate BC, same tank = 4lbs of trim weight + 2lbs of weight in utility pouch + 6.69lbs of tank + 1lbs of air/gas at 500-PSI = 13.69lbs.

With these equipment and weighting configurations, I can easily do a slow descend, a slow ascend, easy safety stop, establish neutral buoyancy without putting a lot of air into the BC at any depth.