Fresh vs Saltwater weighting

[Dam0]

Recently passed my OW course in freshwater with a dry suit. In a few weeks ill be diving in salt water for the first time.

In freshwater used 8kgs of lead, when i take my first dip in saltwater should i start with 8kgs or up it slightly?

Ta

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[fisheater]

If you weigh yourself and all your gear, and then multiply by 2.5%, you'll get the amount of extra lead you'll need for saltwater.


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[DivemasterDennis]

The proposed formula from fisheater is a general statement that does not account for variables like gender, body type, and etc. The short answer to your question is that you will need more weight in salt water. add 2 kilos to do your first weight check and adjust from there. You may need to add (or subtract) a little more. You will be more buyant in salt water than in fresh water.
DivemasterDennis

[DevonDiver]

I don't mean to sound pedantic... but wasn't this explained clearly on your OW course?

[Dam0]

I don't mean to sound pedantic... but wasn't this explained clearly on your OW course?


Was what clearly explained? That salt water is more buoyant and fresh water [Yes] or how to calculate the lead difference between the two? [Not that I recall, hence the question. However I'm more than happy to reference my OW course material]

Dennis, thanks I'll try that.

[Splitlip]

The proposed formula from fisheater is a general statement that does not account for variables like gender, body type, and etc. The short answer to your question is that you will need more weight in salt water. add 2 kilos to do your first weight check and adjust from there. You may need to add (or subtract) a little more. You will be more buyant in salt water than in fresh water.
DivemasterDennis
I think fish eaters formula is a much more logical place to start than picking an arbitrary number. It does take into account variables. '

Speaking of body types I was helping a new diver on another forum. He weighed in at close to 400# without his gear. With he was close to 500#. Using fish eaters he started his weight check with an additional 14 pounds. He never would have noticed 4#.
Lets say OP weighs in at 180 and his gear comes in at another 50. 230 x .025 = 2.6 kilos. Close to your 2, but this is much more logical.
An obese person needs more ballast than a muscur one and this formula accounts for that. It's actually simple physics.

[sam1]

The proposed formula from fisheater is a general statement that does not account for variables like gender, body type, and etc.
Although two people of the same body weight (and gear) may/will require different lead based on body type (fat being less dense than muscle), body composition would not affect the formula for the lead differential for a given person (and gear) going between fresh and salt water.

[Hawkwood]

Was what clearly explained? That salt water is more buoyant and fresh water [Yes] or how to calculate the lead difference between the two? [Not that I recall, hence the question. However I'm more than happy to reference my OW course material]

Dennis, thanks I'll try that.




Were you taught how to do a weight check?

[chrpai]

In my O/W class the 'book' answer was 'about 7lbs' for estimating purposes. Obviously a neutral bouyancy test is required to get an exact answer.

The rest is.... :popcorn:

[Steve50]

fisheater is correct - gender etc has nothing to do with it.

[decompression]

Although two people of the same body weight (and gear) may/will require different lead based on body type (fat being less dense than muscle), body composition would not affect the formula for the lead differential for a given person (and gear) going between fresh and salt water.


Body composition has little to do with buoyancy, other than extreme or abnormal cases, due to the fact that most elements of the body are composed of water (tissue, muscle, fat etc).

[fisheater]

AFAIK, body composition, gender, etc. were already compensated for when the fresh water proper weighting was determined. The only difference is the increased density of the salt water. Thus, an additional 2.5% of the total diver+gear weight is needed.


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[Dam0]

Were you taught how to do a weight check?

Yes, of course. My question is only asking for advise on my initial lead loading as I've not dived in saltwater before, afterwards ill adjust as required.

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[Hawkwood]

Yes, of course. My question is only asking for advise on my initial lead loading as I've not dived in saltwater before, afterwards ill adjust as required.

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Great, then Fisheater gave you the best advice to use as your starting point. Remember to include your 8kg in the initial total.

[windapp]

Fisheater is mostly right.. The density of salt water varies depending on where you are in the oceon. His formula works if the relative density is 1.025. However, you can check this on the internet, and adjust the formula accordingly. The reason why the formula works so well is that if you are neutrally buoyant, you weight exactly as much as the weight of water you displace. To adjust for seawater, you are simply adding weight (assuming of course that the lead adds negligable volume compared to its weight) so that you weigh as much as seawater you displace instead of freshwater. The formula actually accounts for all variables. Just step on a scale with all your equipment when it is completely dry, and calculate away. You should do this with an empty tank, as you should be neutral with empty tank, and not a full one.

If, howerver, you are changing your equipment (like wearing a wetsuit instead of a drysuit), this formula won't work. In that case, I suggest you check your buoyancy with the equipment you will be using in a pool, and the apply the formula.

[Splitlip]

AFAIK, body composition, gender, etc. were already compensated for when the fresh water proper weighting was determined. The only difference is the increased density of the salt water. Thus, an additional 2.5% of the total diver+gear weight is needed.


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Absolutely. You got to the point better than I did in me first post.

The required ballast carried by a properly weighted diver in freshwater accounts for any variables associated with body type etc.

[Agility]

In addition be sure to take into account changes in equipment.
In Europe steel tanks are standard in fresh water, moving to the Red Sea and aluminium tanks has to take into account the difference in bouyancy of the tanks in addition to the heavier salt water.

[DevonDiver]

Was what clearly explained?



That your weighting requirements will differ between fresh and salt water - specifically that salt water diving will require increased weighting.


That salt water is more buoyant and fresh water [Yes]

Buoyancy is dictated by the weight of the water displaced by an object.

Salt water water weighs more than fresh water:
Fresh water approx 1kg per litre.
Salt water approx 1.03kg per litre (depending on salinity).

Therefore, a given volume of object (i.e. diver plus equipment) displaces more 'weight of water', when accounting for the added salt.

So, in essence... a 2-3% increase in buoyancy, requiring a proportional percentage increase in weighting (as others have mentioned... 2.5% should hit the spot).

In my classes, I take a glass of water and the class agree it has a weight. I then add a volume of salt to that glass, which the class also agree has a weight. Thus, they can see that the weight of water has increased (water + salt).


....or how to calculate the lead difference between the two? [Not that I recall, hence the question. However I'm more than happy to reference my OW course material]


This really is down to the instructor. I teach the difference - that'd be especially so when conducting classes in a fresh-water environment, because the student is likely to complete post-qualifications in a salt-water environment.

This is PADI's Weighting Guidelines...

http://scubatechphilippines.com/scuba_blog/wp-content/uploads/2011/07/weighting-guidlelines.jpg
Whilst body composition doesn't matter - body size does have a direct correletion with the amount of material used in your exposure protection. More neoprene/bigger drysuit... more buoyancy.

More details on this, and other weighting factors here:
Scuba Buoyancy Masterclass 4of9 - Assessing Your Weight RequirementsScuba Tech Philippines (http://scubatechphilippines.com/scuba_blog/2011/07/scuba-buoyancy-masterclass-4of9-assessing-your-weight-requirements/)

[chrpai]

In my classes, I take a glass of water and the class agree it has a weight. I then add a volume of salt to that glass, which the class also agree has a weight. Thus, they can see that the weight of water has increased (water + salt).


The glass of water with the salt added is also now taking up more volume. To an average person, this demonstration probably gets the point across but it's not really complete. But to an above average person they probably already understand the underlying concepts of weight, volume and density so it's probably good enough.

A better example might be to get to identical glass jars and fill one with water and one with salt and ask the students which jar weighs more. Then ask them if the same jar willed with salt water would weigh more then the jar filled with fresh water.