the factor depends on the salinity of the water. see http://icp.giss.nasa.gov/research/ppa/1997/oceanchars/salinity.html
Am I missing something?
- Thread starter Altamira
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2.5% is a reasonable ballpark figure for the difference. As this is in the OW materials, I'm kind of surprised that someone teaching that class wouldn't realize that.
Wouldn't you be able to just take an extra pound or 2 down the first dive and if heavy, get rid of it during the SI prior to the 2nd dive?
beanojones
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It is in the PADI OW materials? I cannot find it in any of the manuals I have on hand. Not that I am looking real hard right now.
It's (Total dry weight of diver)* 2.5%= Difference between Fresh weight and Salt weight? What's the factor change for different exposure protections?

Correct. (Assuming "Total dry weight of diver" includes diver and all kit.)
There is none. For each different kit you need to determine the appropriate amount of weight in one medium before doing the math for the other.
beanojones
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http://www.scubaboard.com/forums/basic-scuba-discussions/488316-how-much-weight.html
see that thread, good instructions there
see that thread, good instructions there
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Sometimes you can take a pretty simple question and totally overthink it and try to make it science. So....you're switching jacket BCs with somewhat different inherent weights, the latter being lighter. No other variables are being changed. I don't think a predive check will help you much, the only thing that really matters is holding your SS with an empty BC and tank. Personally, I would take the same weight and see how you do. What's the worst thing that will happen, you will tend to bob up on a no stop dive? You could fix that. If you're worried, take a couple of extra pounds as per MrChen and discard them if you are too heavy at the SS.
You didn't say what wetsuit is being worn. With 14#, I'm assuming a 5mm as this would be a lot weight for a 3mm. This must be an Al 80 cylinder
You didn't say what wetsuit is being worn. With 14#, I'm assuming a 5mm as this would be a lot weight for a 3mm. This must be an Al 80 cylinder
K
KeithG
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Maybe not?
I just re-read your original post and as I hinted at before, my experience is the opposite of what you expect. Which is also the opposite of what I expected. I got fooled. So I vote for you to perform the bouyancy test that you proposed. You can do it in your bath tub. No pictures please!
I got surprised on my check out dive after the purchase of the my spiffy new shinny ultra deluxe light weight travel bcd. But no big deal. It was a checkout dive from a dock in Bonaire to make sure our weight was good. The dive served its intended purpose. We ended up spending a few extra minutes at the start of the dive as I had to randomly splash and flail about on the surface until I grabbed an extra 4 lbs. All was good.
Returning home, I did some actual real research since I felt a lot stupid by being surprised on something that basic. You have foreseen the future. You are smarter than me. Turns out my "light weight" travel BCD was actually a few LBs heaver than my ancient BCD. And made out of thicker / heavier material (deniers?). WTF? And unfortunately, it also contains a whole pile of very buoyant padding. But, as stressed in all the marketing fluff, it was much lighter & thinner than current standard BCDs. Seems that since my original purchase way back in the 80's they started to make BCDs tougher, stronger and heavier. Likely to make them last longer? My dive buddy recently bought a "normal" replacement BCD and it is a huge heavy monster compared to our original BCDs**. I now refuse to carry her dive bag.
Big negative issue during the belated research phase was that nobody provided any information regarding "buoyancy" of their BCDs, just the raw weight. Raw weight is great info for the plane ride, not much use for the actual dive. The real stat you (I!) are interested in is the buoyancy. This concept is well known for tanks, not so much for BCDs.
So sink that sucker in your tub and then be ready to splash into the big pool on your vacation.
** Maybe it is time to start thinking about moving to those new fangled plate things that people talk about all the time....
---------- Post added July 30th, 2014 at 12:16 AM ----------
I repeat: No pictures please!
beanojones
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If that is for me, I am really only wondering about the 2.5% of total body weight equaling the fresh versus salt thing. Does that really work for some, regardless of the exposure protection etc?
I know it won't work for me because I don't use weight the very few times I am in fresh water, so this is all news to me. All my weight checks for me and students are done at the end of the dive on an "empty" tank at a safety stop. But then again we start in the ocean, so it's all figured out there.
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