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View Full Version : Bouyancy in Fresh vs Salt Water



kenneth.hall@voith.com
06-25-2008, 22:46
I've been OW ceritified since May 25 and have 15 dives so far. I was ok bouyant in salt water with a 5mm wet suit and 16# wts. At home in fresh water I'm either pushing off the bottom or shooting straight up and tring to dump air from the BCD, with a 3mm shorty and 10# wts. Should I just cut wt. and work harder to stay down? I know I'll get beter with pratice but is there that much difference in fresh water vs salt.

thesmoothdome
06-25-2008, 23:12
there's about a 4 lb difference between fresh and salt water. Take into consideration that you're dropping 2mil or 40% of your wetsuit thickness, I'd think that you'd have to drop at least a few more pounds... Best thing to do is jump into the pool wearing only your 3mil and have a buddy hand you a few pounds at a time until you're floating at eye level while holding a normal breath....I'd guess you'll be around 8lbs or so.....Just a guess though.

bubbletrubble
06-26-2008, 00:17
Smoothdome gives good advice. Optimization of your weighting is critical to making the dive experience as effortless as possible.

To answer your question, yes, there is a significant buoyancy difference between salt and fresh water. As you know, salt water is more dense than fresh water; therefore, a given object will be more buoyant in salt water than in fresh water. You can convert your scuba weighting requirements from one kind of water to the other...but you have to be using the identical rig to do this conversion. You must compare apples to apples, you know. Your post describes 2 different rigs: one for salt water (5mm) and one for fresh water (3mm shorty). Treat them as independent of one another. The usefulness of being able to do the conversion is that you can estimate your weighting requirements for an ocean dive...in your backyard pool.

Here's how I do a weight check in fresh water/pool:
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Gear up in your salt water rig (wetsuit/hood/gloves/fins/booties/bcd/reg/tank = the whole shbang!). Make sure that you have a near empty (<500psi) tank. Jump into the pool. Relax. Weight yourself so that you're neutrally buoyant: Wear enough weight so that you can bob vertically at eye level with the empty tank, an empty BC, an average breath, and your feet crossed (to prevent finning). Weight yourself so that a deep breath in pushes your mask out of the water and a deep exhalation sinks your mask below the level of the water. Do all of this while breathing on your regulator. Be mindful that if your wetsuit isn't "fully wet" it can retain air pockets. Work those air pockets out as best you can.
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Then, do the fresh-water-to-salt-water conversion in order to calculate your weight requirements in the ocean. The conversion is easy. Add 2.5% of your total weight (you + all of your gear) if you want to convert your "pool weighting" to salt water weighting.

For instance, let's say you do the pool weight check in your 5mm wetsuit as I suggested. You determine that you need 8lbs. of lead for proper weighting in the pool. You weigh 185lbs. and your rig weighs an additional 55lbs. (total 240lbs.). 2.5% of 240lbs is 6lbs. Add 6lbs. to your pool weighting number. You would carry 14lbs. of lead on your next ocean dive.

Make sure that you do another weight check whenever you add/subtract any gear. If a pool is not accessible, typically I will overweight myself a little and then do a proper weight check (handing off weight to my buddy) at the end of my next shore dive -- it's pretty much the same as doing the weight check outlined above. Be sure to make a note of your weight configuration in your dive log.

Hope this helps...