weighting- freshwater vs. seawater

[sandanbob]

Good morning!

Way back in OW, we were taught that seawater is more dense, hence would require more weight to offset buoyancy. The standard values used are 62.4 lb/cf for freshwater, and 64 lb/cf for seawater (which can actually vary, based on salinity.)

I have been thinking about that, and something that I recently read here made me decide to post. For reference, what I had read was concerning European standards that 'average' the densities, such that a dive computer based on those standards doesn't have separate settings for freshwater vs. seawater. (I am not familiar with those standards, so if I worded it incorrectly, I apologize.)

If you look at those density values and compare them, then you could say that freshwater is 97.5% the density of seawater; or, alternatively, seawater is 102.6% the density of freshwater. So, the difference is about 2.5%. If a diver is wearing 20 pounds (as an example) of weights for seawater, than calculations say you should remove 2.5% (1/2 pound) for moving to freshwater. Or, vice versa, if you are wearing 20 pounds in freshwater, you should be adding 1/2 pound for diving in seawater. If the diver is wearing 40 pounds, then the difference becomes one pound, and so on.

Other threads on SB indicate difficulty in finding 1/2 pound weights. I don't know if that is still the case. It seems that if you were weighted for diving in seawater, you would be fine (assuming the same gear, especially a wet suit,) without being considerably over-weighted in freshwater.

Anyway, just some Sunday morning thoughts.

Be safe!

Bob

 

[inquis]

If a diver is wearing 20 pounds (as an example) of weights for seawater, than calculations say you should remove 2.5% (1/2 pound) for moving to freshwater

Not quite. You add/remove 2.5% of your TOTAL dry-land mass (you + tanks + gear, etc.) A 180 lb diver with 35 lb AL80+reg, 20 lb of lead, and 5 lb of other gear should make a 6 lb adjustment (2.5% of 240 lb).

 

[James79]

@inquisit nailed the weighting difference for you, but I'll add to address the computer issue.
Nitrogen loading rates are determined by pressure, not true depth... So all computers are calculating NDLs by the pressure you are under. Different densities (salt or fresh water) make for minor differences in the depth that pressure is felt at... Hence tables always list pressure in Feet Salt Water, since analog depth gauges are calibrated in FSW.
I'm lazy and leave my computer set to salt water, because for the diving I do I don't care about the minor difference (in fresh water it reads a tiny percentage shallower than I really am).

Respectfully,

James

 

[broncobowsher]

Correct on the computers. Shearwater has 3 settings. Fresh, Salt, and some european standard that splits the difference. The computer is strictly a pressure calculating device. The pressure is converted to depth simply for user convience.

 

[CT-Rich]

Good morning!

Way back in OW, we were taught that seawater is more dense, hence would require more weight to offset buoyancy. The standard values used are 62.4 lb/cf for freshwater, and 64 lb/cf for seawater (which can actually vary, based on salinity.)

I have been thinking about that, and something that I recently read here made me decide to post. For reference, what I had read was concerning European standards that 'average' the densities, such that a dive computer based on those standards doesn't have separate settings for freshwater vs. seawater. (I am not familiar with those standards, so if I worded it incorrectly, I apologize.)

If you look at those density values and compare them, then you could say that freshwater is 97.5% the density of seawater; or, alternatively, seawater is 102.6% the density of freshwater. So, the difference is about 2.5%. If a diver is wearing 20 pounds (as an example) of weights for seawater, than calculations say you should remove 2.5% (1/2 pound) for moving to freshwater. Or, vice versa, if you are wearing 20 pounds in freshwater, you should be adding 1/2 pound for diving in seawater. If the diver is wearing 40 pounds, then the difference becomes one pound, and so on.

Other threads on SB indicate difficulty in finding 1/2 pound weights. I don't know if that is still the case. It seems that if you were weighted for diving in seawater, you would be fine (assuming the same gear, especially a wet suit,) without being considerably over-weighted in freshwater.

Anyway, just some Sunday morning thoughts.

Be safe!

Bob

You need keep mind that changing weights between fresh and saltwater isn’t about actually changing the weight of the diver. It is about changing the OVERALL DENSITY of the diver. The weight of the diver is actually irrelevant.

For example a hypothetical diver weighing 230 lbs in a 7 mm suit with a steel 100 would need 20lbs of lead. If goes into salt water, he would need 27 lbs, or an addition 35% of lead.

The same diver without the wetsuit would only need 3 lbs of lead in fresh water, but 10 lbs in salt water, an increase 230%. Why? Because the wetsuit, while making a small difference in his volume made a big difference in his density. A pound of lead will affect boyuancy different than a pound of steel, aluminum or neoprene. A diver trimmed for freshwater likely will float like a cork in seawater, depending on his kit.

The numbers I used for this example were arrive at using this buoyancy calculator:

DiveBuddy

DiveBuddy - the most extensive social network for scuba divers. Search for new dive buddies, research dive sites, share photos, post comments and more.

www.divebuddy.com

 

[Silt Life]

Thanks for the divebuddy link~!

You need keep mind that changing weights between fresh and saltwater isn’t about actually changing the weight of the diver. It is about changing the OVERALL DENSITY of the diver. The weight of the diver is actually irrelevant.

For example a hypothetical diver weighing 230 lbs in a 7 mm suit with a steel 100 would need 20lbs of lead. If goes into salt water, he would need 27 lbs, or an addition 35% of lead.

The same diver without the wetsuit would only need 3 lbs of lead in fresh water, but 10 lbs in salt water, an increase 230%. Why? Because the wetsuit, while making a small difference in his volume made a big difference in his density. A pound of lead will affect boyuancy different than a pound of steel, aluminum or neoprene. A diver trimmed for freshwater likely will float like a cork in seawater, depending on his kit.

The numbers I used for this example were arrive at using this buoyancy calculator:

DiveBuddy

DiveBuddy - the most extensive social network for scuba divers. Search for new dive buddies, research dive sites, share photos, post comments and more.

www.divebuddy.com

 

[edwants2dive]

Ok got to dispute the Dive buddy calculator. Just punched in my info being truthful about it and dive buddy says I need 20 pounds. When I dove Saturday I wore 6 and was weighted as perfectly as I have ever been. That's a big difference in what they came up with. My advice is learn your gear, note your weight each dive and adjust until you have it figured out. I have finally learned to dive weighted properly and not with the I can be overweight and correct for it method.

 

[NW Dive Dawg]

All of the pre-determined formulas may be a decent starting point, but otherwise are BS....... in my opinion. In my opinion, the procedure to determine the proper weight to carry is to dive a consistent rig and then determine the minimum amount of added weight that will allow you to hover comfortably in 15-20 ft with 300-500 psi in your tank.

PS.... I will always prefer to be a couple of lbs over weighted than under weighted.

 

[Rose Robinson]

Good morning!

Way back in OW, we were taught that seawater is more dense, hence would require more weight to offset buoyancy. The standard values used are 62.4 lb/cf for freshwater, and 64 lb/cf for seawater (which can actually vary, based on salinity.)

I have been thinking about that, and something that I recently read here made me decide to post. For reference, what I had read was concerning European standards that 'average' the densities, such that a dive computer based on those standards doesn't have separate settings for freshwater vs. seawater. (I am not familiar with those standards, so if I worded it incorrectly, I apologize.)

If you look at those density values and compare them, then you could say that freshwater is 97.5% the density of seawater; or, alternatively, seawater is 102.6% the density of freshwater. So, the difference is about 2.5%. If a diver is wearing 20 pounds (as an example) of weights for seawater, than calculations say you should remove 2.5% (1/2 pound) for moving to freshwater. Or, vice versa, if you are wearing 20 pounds in freshwater, you should be adding 1/2 pound for diving in seawater. If the diver is wearing 40 pounds, then the difference becomes one pound, and so on.

Other threads on SB indicate difficulty in finding 1/2 pound weights. I don't know if that is still the case. It seems that if you were weighted for diving in seawater, you would be fine (assuming the same gear, especially a wet suit,) without being considerably over-weighted in freshwater.

Anyway, just some Sunday morning thoughts.

Be safe!

Bob

Bob,

Makes no difference...preferably with assistance, follow your initial OW training, in the shallows, using various weights, based on your existing gear, whatever that may be, take the time to find your neutral buoyancy.

Your narrative, involves far too many variations, to perform a calculation, and dive in the water without a pre-performed buoyancy test.

Don't ever forget the basics, you were taught them for a reason.

LOL,

Rose.

 

[CT-Rich]

Ok got to dispute the Dive buddy calculator. Just punched in my info being truthful about it and dive buddy says I need 20 pounds. When I dove Saturday I wore 6 and was weighted as perfectly as I have ever been. That's a big difference in what they came up with. My advice is learn your gear, note your weight each dive and adjust until you have it figured out. I have finally learned to dive weighted properly and not with the I can be overweight and correct for it method.

I used the buoyancy calculator to illustrate a point. I have used it once or twice over the years and for me it would get me within a few to several pounds of where I wanted to be, which is why I even knew of it.