Question about general info....?

Sean C · Nov 14, 2003

Hello there, I have a simple question.

When I read a dive publication and they tell me how much weight I need, if something is negatively or positively bouyant or suggest the change in bouyancy of a given article according to depth, are they talking about Salt or Fresh Water?

This may be obvious to you but I do not have a clue......

Walter · Nov 14, 2003

An example would be helpful. Salt water averages 64 lbs/cu ft while fresh water averages 62.4 lbs/cu ft.

Sean C · Nov 14, 2003

Hey Walter, perhaps I wasn't clear, here is an example:

I was reading RSD (Rondale's ScubaDiving) and in an article they were giving figures for bouyancy. One of the suggestions was that for every pound a wetsuit weights it will require two - three pound of weight to counter its inherent bouyancy. They gave several other examples including the bouyancy of an empty vs. full tank. (I going from memory).

My question is, when they issue their recommendations or theories are their numbers based on Salt or Fresh Water, they never indicate this factor and I want to know if these things are usually based on one substance or another, in magazines atleast.

Rick Inman · Nov 14, 2003

I think you're asking a good question and I don't know the answer, but I'd guess that they're talking salt water. However, we're talking less that 3% difference between salt and fresh water weight, and most of the numbers I've seen are vague enough to fall into both. But I could be missing your point.

Sean C · Nov 14, 2003

Hey Rick, I probably don't have a point......but I live in an area dominated by cold fresh water that would cause shrinkage on a whale. So if someone 200 lbs and fat, not lean, is wearing a thick wetsuit 7+ mm in cold fresh water with an tank filled to 500 psi trying to hover at 15 feet the different weight required for Fresh and Salt Water could be fairly high. (6-7 pounds???)

By no means am I suggesting following a magazines advise but having an idea of what they use for a base would be helpful to a new diver like myself.

P.S. I do weight 200lbs but am not fat, not that there is anything wrong with that, it was just an example.

diverlady · Nov 14, 2003

Sean C once bubbled...
Hey Rick, I probably don't have a point......but I live in an area dominated by cold fresh water that would cause shrinkage on a whale. So if someone 200 lbs and fat, not lean, is wearing a thick wetsuit 7+ mm in cold fresh water with an tank filled to 500 psi trying to hover at 15 feet the different weight required for Fresh and Salt Water could be fairly high. (6-7 pounds???)

Yes, the difference in weighting could be quite large. I would guess they are using salt water if they aren't stating it but really, they should state which they are using.

P.S. I do weight 200lbs but am not fat, not that there is anything wrong with that, it was just an example.

Yes, stay politically correct!

Diverlady

BTW - welcome to the Board.

adder70 · Nov 14, 2003

Think of it this way. The easy way to tell the difference is this. About how much do you and all of your equipment weigh? When you have air in your BC to make yourself neutral, you are displacing that weight of water. 300 lbs = 4.8 ft^3 of fresh water or 4.68 ft^3 of salt water.

If an item compresses and causes a change of 20 lbs, the change from fresh to salt would be affected on the order of .51 lbs, not a significant amount.

Besides, most gauges/computers read in fsw anyway, so if your gauge says 112 ft, the buoyancy change will be as for 112 fsw, even if you are in fresh water, where you would be at 115 ft actual depth.

Question about general info....?

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