New Gear; New Weight and Bouyancy

Question
Mike,
    I've been diving the west coast for years. I've recently purchased a new 5/7mm Henderson full wetsuit, booties, and hood.  I recently started diving again after a small break and want your best method to establish the amount of weight I will need using a standard pool before getting wet in the ocean.
    I have read countless versions on how to get neutral and I'm confused thus my question to you the....master.
    1.  Should I get completely geared-up & jump into a pool with; a tank with 500psi-750psi; a completely deflated bc; be handed weight until I reach eye level..rising on the inhale and sinking on the exhale.  Then weigh myself, all of my gear(not the 500psi tank), including the weight used and multiply the total by .025 or divide by 40 for the salt water conversion, then add that additional weight?
    2.  Do the same as the above..all geared-up, no-air in bc; 500psi tank; enough weight to sink and sit at the bottom buddah style; then drop weight until I rise on the exhale and sink on the exhale and then do the salt water conversion after weighing everything, including myself but not the tank.?
    3.  Weigh all my gear, including myself, with a 500psi tank, determine the amount of water displacement that weight would generate; convert the water to pounds and compute the salt water conversion?
    The best method would be where I can use a standard 8 foot deep pool to approx. the weight I will need.  Your detailed expertise would be most preferred as some other instructors/masters forget to be detailed.
Thanks.  

Answer
Hi Jackie

That's a great question.  As you've noticed, there are lots of different ways to predict how much weight you'll need.  I could go into calculations which are actually more detailed than those you've suggested but you're going to quickly reach a point of diminishing returns.  Despite your best calculations, it's still just an estimate and you'll have to "fine tune" the results.  All of what you've suggested is valid.  However if you want to get even closer, you can take such things as water salinity (varies from one ocean to the next), water temperature (warm water will increase buoyancy due to a slight density change), depth (again, density changes).  You can also consider whether you're using a steel, standard aluminum or neutrally buoyant aluminum tank.

I tend to think more on the practical side.  Leave the calculations to the tekkies.  If you only have access to a freshwater pool, then add about 6 pounds to your final result for salt water.  Technically, the difference in the buoyancy between salt water and fresh water is only the weight of the salt in the amount of water actually displaced by you and your equipment.  There's not actually 6 pounds of salt in that amount of saltwater but you'll need a "fudge factor" to account for the variables we're ignoring.

Jackie, my suggestion is actually a combination of the techniques you've mentioned.  Get in the pool with all of your gear in place, an empty BC, a deflated drysuit and a 500 psi tank.  If you're wearing your 5/7 wetsuit, obviously you can't deflate it!  Have someone hand you weights (string them on a weightbelt and hold it in your hand if it's more convenient) until you just sink below the water.  That way you don't have to worry about holding a "normal" breath.  Just add weight until you SLOWLY sink below the surface.  That will do the trick.  Once you're in deeper water(15 or 20 feet), excess air will escape from your wetsuit and hood.  Nitrogen bubbles in the wetsuit material and any residual air in your BC will also compress and make you even a little more negatively buoyant.  Finally, add some extra weight for the saltwater as we've already discussed and you should be weighted well enough to make a successful dive.  During the dive, note whether you're overweighted by checking to see how much air you need to add to your BC to achieve neutral buoyancy.  When your tank gets low, if you still have a bubble in your BC any larger than about a half of a volleyball in size, then you're overweighted.  Drop two to four pounds and you should be fine.

Of course, record this weight in your logbook so you don't have to repeat the process.

Jackie, this isn't a high-tech calculation and it probably wont' impress your tekkie buddies but it works and it's one that I've used successfully for many years.  With an engineering background, I can take data points and come up with a "formula" for calculating weight.  However, I also know that these formulas won't be perfect since there are so many variables (as discussed earlier)that we haven't taken into account.  So...make your "down and dirty" estimate and go dive!!  Your results should be very close and you can leave your calculator at home!!

Oh, Jackie...thanks for the compliment.  "Master" might be stretching things a bit!!

Please write back if you have any questions about my suggestions.

Happy Diving!!

Mike Giles
Mike's Dive Center
mikescuba.com