Weighting Rules of Thumb

[Nudgeroni]

Howdy,

I ran a quick search on the topic of weighting and found a lot of information, but no thread that put a bunch of the 'rules of thumb' together (if there are any). A number of posters can rattle off quick proper weighting estimates for divers, and I was wondering if you would be willing to share what is behind your estimates.

For instance, it seems that an AL80 requires about 5# of weight, a steel 72 is fairly neutral and a HP steel 100 takes a pound or two. (I may very well be corrected here).

* What is a good assumption for the buoyancy/weighting needs at surface for a full 7mil wetsuit, 3mil, and dry suit?
* What is the buoyancy of a medium framed unequipped person (not that I plan to scuba naked) :54:

etc. etc. --you get the idea. I know 'it all depends,' but again, I think some general rules of thumb could be helpful for us new folks trying to get a sense of the effect of equipment choices.

-Nudgeroni

Thanks for your input.

 

[Blackwood]

For instance, it seems that an AL80 requires about 5# of weight, a steel 72 is fairly neutral and a HP steel 100 takes a pound or two. (I may very well be corrected here).

I think you are confusing concepts. A tank may be neutral at some specific state, but its characteristics will change throughout the dive (hence the weight requirement).

Weighting as it relates to tanks is a function of capacity and density. Put as much on your belt as the gas in your full tanks weighs (such that, if you breathe them down to empty, you have compensation for the consumed gas).

As far as I'm concerned, that's the only hard rule with respect to weighting. Beyond that, it depends entirely upon the person. (Sorry if that seems like a copout).

 

[dmblack58]

First, let me say there is no formula that will tell you exactly how much or how little lead a diver will need to be neutral during a safety stop with about 500psi in their tanks.

I do remember a post from a while back that suggested using percentages of a divers body weight as a starting point and then fine tuning it from there. I'm doing this from memory but its was something like
· 7mm farmer john or drysuit - 10% of body weight (not sure why 10% was taken, might be because its a nice round #)
· 3-5mm - 5% of body weight (logic was that the neoprene was half as thick so you'd only need half as much lead)

Depending on your body density and the amount of 'bioprene' you have you may find this will either over or underweight you.

The best way to figure it out for you personally is to get a buddy and your usual tank (with 500psi) and do a weight check so that you float at eye level with an empty BC. If you find you're too light add weight in 2 lbs increments. If you’re too heavy subtract weight in 2 lbs increments.

 

[Nudgeroni]

Thanks for the input, Blackwood. My interest was to understand the range of buoyancy that might exist for a typical diver given certain equipment.

The amount of lead for neutral is established at the surface; but the diver becomes negatively buoyant at bottom (say 100'), particularly if the diver is wearing thick neoprene. As the tank(s) empty, buoyancy becomes more positive again.

In extreme cases, the diver can become too heavy at bottom or will be unable to maintain depth during a safety stop. DAN had an interesting article on buoyancy trouble--

http://www.diversalertnetwork.org/medical/articles/article.asp?articleid=28

To quote the article:

There is some debate about whether divers should weight themselves to be neutrally buoyant at the surface or in shallow water. Divers who are neutrally buoyant at the surface may be significantly positively buoyant in shallow water towards the end of the dive when their cylinders are nearly empty...

 

[String]

Weighting rules of thumb are pointless - there arent any.

 

[Nudgeroni]

Weighting rules of thumb are pointless - there arent any.

That may be, but when initially trying out an amount of lead, I'd rather go by a rough calculation that can be adjusted, than do it by *feel* (which I don't have as a beginning diver).

 

[Charlie99]

Rules of thumb are approximations. They aren't perfect, but it sounds like you are looking for some starting points.

Rather than starting afresh with each new configuration, it is more accurate to start with a known configuration and estimate the changes.

The changes in buoyancy from changing the type of tank is very predictible. Simply compare the empty buoyancy numbers on a website such as http://www.huronscuba.com/equipment/scubaCylinderSpecification.html
If you are only changing a tank, just add or remove weight per the difference in empty buoyancy of the two tanks.

Changing from freshwater to saltwater or vice versa is also very predictable. There is about a 2.5% difference in density, so going from fresh water (such as a pool where you have adjusted your weighting) to saltwater, you add 1 pound of lead for every 40 pounds of TOTAL dive weight. Total dive weight includes you, and all of your gear including tanks and lead. For a typical diver of 180 pounds and 60 pounds of gear, this means that you add 6 pounds of lead when going to saltwater, and remove 6 pounds when going to freshwater.

The amount of lead needed when changing wetsuits is harder to approximate. Obviously a small wetsuit will have less buoyancy than a large wetsuit of the same thickness. Different brands of wetsuits have different buoyancies. While the numbers vary widely, for my L sized wetsuits the buoyancy is roughly 2.5 to 3 pounds per mm of thickness.

Obviously, the best way to check weighting is to fully gear up, get in the water and check it. The next best is to check weighting in a pool, and then do a freshwater to saltwater adjustment.

As far as how much lead needed for you, a simple test is to go into fresh water (such as a pool) and see if you sink or float with full lungs. Then exhale fully and see if you sink. If you need to fully, fully exhale before you sink, then your body has about 5 pounds of buoyancy.

When estimating, it is best to be a few pounds overweight than a few pounds underweight. Being overweighted causes you to have excessive air in the BCD, which makes it more difficult to control buoyancy, particularly when shallow, so overweighting is something to avoid if possible. OTOH, if you are underweighted, you will be struggling to do a shallow stop at the end of a dive.

Others may jump in with rules of thumb like 10% of body weight for a 7MM or other similar ones, but I haven't seen that sort of rule as being accurate enough to be of much use.

Experienced DMs in tropical resort areas are used to divers showing up not knowing how much weight to use with thin wetsuit, and many are pretty good at looking at a persons physique and size and coming up with a pretty good guess. After you make your own guesses, ask for some 2nd opinions from people such as DMs and instructors.

Charlie Allen

 

[mred]

One thing you should do is log your configuration. What wet suit you were wearing, gloves, hood, tanks, conditions and weights. Make notes as to how much weight you used, and if it was right, or if you were a little light at the end of the dive. In this way, you will develop approximations that are good for you.

 

[spectrum]

The defining moment of the dive is at the end when your cylinder contents are depleted and you are approaching the surface.

At that time your cylinder will be at maximum buoyancy (though it may still be a negative value) and your wetsuit will be rebounding and returning to full buoyancy. Since you always need to be in control your weighting needs to be just enough to let you stay down in these conditions.

Since wetsuit compression is a very rapid phenomenon in terms of depth to cause a change the bobbing at eye level at the surface while plumb with still feet and a near empty (~500 PSI) cylinder will serve you quite nicely as a test.

Up to that point when your air cylinder is full or you are at depth with compressed neoprene you should expect to have air in your BC to compensate for the negative condition you will be in. This is why BCs exist and are so named. To describe these conditions as being "over weighted" is a misnomer. The diver may require compensation but is correctly weighted.

Pete

 

[Nudgeroni]

Great. Thanks for the help. I think I have enough here to have a give myself a starting point when I change gear.