Weight and Buoyancy

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knucklebusted

Contributor
Messages
110
Reaction score
36
Location
Vette City, USA (Bowling Green, KY)
# of dives
25 - 49
Kinda long but I am one for details. Hopefully it will avoid lots of minor questions.

I was recently certified and had some very minor issues with buoyancy. I think I got it all sorted out but I'm really curious and want to understand what was going on.

First, when we arrived at the resort on Maui, I started the cert with the first dive and I used a 3mm shorty with 10lbs of weight. It was all sort of a blur but I didn't have any trouble getting down and staying down. Came back with 500PSI in an AL63

Second dive one day later, same weight and suit but I couldn't get off the surface without forceably having to swim down. My BCD was completely deflated and I could not vent any additional air from any of the vents. None from the hose, none from the shoulder and none from the kidney. The instructor stuck 3lbs more (total 13) in my BCD and I was fine. Had 700psi in an AL63.

Third dive with 12lbs and 3mm shorty was great. No buoyancy issues. Had 1000PSI in a AL63.

Fourth dive next day, 12lbs since it worked, same 3mm shorty suit. Couldn't get off the surface without trying to swim down and constantly rose. I had to completely expel all the air from my lungs to get neutral. Any breath in and I'd start rising. Instructor clipped another 2lbs on me and it was good. Returned with 1000PSI in the AL63.

I was certified after the fourth dive. I was also on vacation and eating a good bit. I gained 3-4 lbs in the first week. From my 5'10" 157lbs I got up to 162lbs. Would 5lbs make that much difference?

Fifth dive was a scooter dive and weight was the same 14lbs. Scooter sort of threw it all off. Lots of air left, too.

Sixth dive was a night dive and the light added negative to me. 14lbs and a 3mm shorty worked fine.

Seventh dive was a day dive and with 14lbs and the 3mm shorty, I sunk like a rock and finally felt my weight was right on. Fin pivot was good on the bottom and I could control up/down simply by breathing. Came back with 1500psi in an AL80 for a 45 minute dive. Instructor said my trim was good and it felt right.

Eighth dive was a carbon copy of dive 7. Had to swim around a bit more, trying to stay out of the way of a bunch of kids on intro dive so I burned a little more air but still had 1200psi in an AL80 for a 45 minute dive.

So, I'm feeling good about my trim and air consumption but I'm just wondering why lighter weight was good at first and I slowly over the week I needed more?

Thanks
 
I'd think its a combination of things. Could be the actually fill you had in the tank combined with plumping up a bit (or being slightly bigger at different times of day) combined with just being new and not noticing the finer details of your bouyancy combined with slightly different rental gear (i.e. different bc of same type). Even something like gloves or air trapped in your shortie. Those things could account for a pound or two of wieght.
 
I agree with RICoder and might add....

Were these commercial weights or local hand cast items? Some of the dive resort weights are crap and your initial ones may have been oversized and heavier than you thought.

When you went from the AL63 to AL80 you would need to add 2 pounds by definition due to the increased buoyancy of the larger cylinder. See the Empty Buoyancy column for this.
http://www.huronscuba.com/equipment/scubaCylinderSpecification.html

You seem to be on the right track with observations like the weight of the dive light. Congratulations on you certification and welcome to the sport. Oh and remember, diving isn't just a vacation thing.

Pete
 
spectrum:
I agree with RICoder and might add....

Were these commercial weights or local hand cast items? Some of the dive resort weights are crap and your initial ones may have been oversized and heavier than you thought.

When you went from the AL63 to AL80 you would need to add 2 pounds by definition due to the increased buoyancy of the larger cylinder. See the Empty Buoyancy column for this.
http://www.huronscuba.com/equipment/scubaCylinderSpecification.html

You seem to be on the right track with observations like the weight of the dive light. Congratulations on you certification and welcome to the sport.
Ah, that's what I was looking for. The weight belt was the same for each weight. I wore the same 10lb belt each time, then when I went to 12lbs, it was a different belt and the 14lb belt was the same. They had each size with a different color belt and I always got the same one from the previous day. The weights were color coded, blue, green, yellow and so on. The BCD was of the same type and size but I don't know that I got the same one each day. Same for the shorty wetsuit, it was the same size but don't know it was the same one.

I'm guessing the tank was what made the biggest difference now that I know the difference. While sitting around the pool, I observed them pretty much using only AL63's on the intro divers. I'm guessing that is to keep the weight down or the dive time shorter. ;)

spectrum:
Oh and remember, diving isn't just a vacation thing.
There is no LDS around here. It closed up some time early this year. I don't own any gear and I'm pretty much not interested doing this if it is too cold, too long a trip to go anywhere local (oxymoron) and too low viz to make it enjoyable. I may look some folks up but everyone I know at work (3 folks) that are divers don't fool with diving locally because they say the rivers and lakes here are muddy and cold.

Thanks for the feedback. I think it all makes sense for now.
 
Knucklebusted,

I had a similar experience on my first dive trip to Cozumel.

I had done all of my dives in a 7mm full wetsuit with rented BC in a cold local lake. I only needed 10 lbs on my weight belt.

So, when I went to Cozumel with only a 3mm rented shorty, I thought I would certainly use less weight even with the increased density of the salt water. I was wrong. I decided to use 10 lbs on my first dive just to be safe, even though I thought I would be overweighted. I couldn't descend.....I added four pounds and had no problems.

The next day I had a 3mm shorty, but different than the one the day before. I also had a different rented BC. I went with the 14 lbs and sunk like a rock!

Every day with different gear was a new experiment with the weight belt.

One more reason to get your own gear, if you can afford it.
 
It varies a little from day to day I think. You notice this variation most right at and near the surface.

If you empty your BCD and exhale and you still float like a cork, you need more weight. However if you do in fact sink underwater, I'd say just swim down a little bit (be careful to equalize). After 10 feet or so you'll find it much easier to descend.
 
rjpv:
If you empty your BCD and exhale and you still float like a cork, you need more weight. However if you do in fact sink underwater, I'd say just swim down a little bit (be careful to equalize). After 10 feet or so you'll find it much easier to descend.
Be careful about beginning a dive underweighted. Remember, by the end of the dive, you will have tossed several pounds overboard (as bubbles), and if you're several pounds buoyant at at stop at the end of the dive, you're not going to be having a great time.
 

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