Proper Weighting

[boulderjohn]

I had an experience this last dive trip that brings up my question.

I did a buoyancy check recently at a local park. At fourteen pounds I was able to slowly sink when I emptied my lungs with an empty bc. When I went out on the dive boat I was using the same setup but I couldn't get down. The guide added two pounds to me but that still wasn't enough. he added another two pounds and I was able to get down.

I was slightly negative throught the dive and I forgot to do a buoyancy check at the end of that dive.

The variables are that when I was doing the original weight check at the park the salinity may have been slightly lower due to the fact that it is near a freshwater canal. The other variable was a thin hooded vest. The DM on the boat said it shouldnt really matter though. I took his word on that.

My question to you is "what's up with my weighting?"

PS I'm 6' 170lbs I wear a three mil wetsuit and sometimes a 1.5-2mm hooded vest.

Although everyone is right in that there is no perfect formula, in my mind if you are the size you say you are and you ended up needing 18 pounds to sink, somethng is wrong.

I am your height, but I outweigh you by 30 pounds. I should probably need more weight than you. I am, however, pretty well weighted in a 3 mm suit with about 8 pounds, not 18. That difference goes well beyond individual differences.

There is a good chance you had trapped air in your BCD or elsewhere preventing you from getting down. It is also possible that you were tense, which might have led you to keep a lot of air in your lungs. That is like trying to dive with two life preservers in your chest. Relax and exhlale fully when you submerge.

I once had a student in an open water checkout dive whom I had not previously taught. On our first dve, he had a lot of trouble getting down, and he kept rising to the surface while trying to dive. At one point he rose about 10 feet above the class, and we all watched while he fumbled to get his inflator hose up to dump air and join us. He found his snorkel instead. He held the snorkel high above his head, pressed down on the bottom, and sank down quickly to join us. Obviously it should not have worked. Pressing the bottom of a snorkel has nothing to do with bouyancy. But he thought he had found the inflator hose, and when he pressed the bottom, he breathed a sigh of relief. It was that sigh of relief that brought him down to us.

 

[elan]

Although everyone is right in that there is no perfect formula, in my mind if you are the size you say you are and you ended up needing 18 pounds to sink, somethng is wrong.

This is what I thought... I'm 182"/180lb and I can hold the stop with 8lb of lead in 5/4/3 suit in fresh water.

My instructor of the same height but he is around 165 -170lb he can do it with no weight at all in 3 mil suit.

That's with Al80 tank.

The first feet are the most difficult to sink, the previous posters have given all the advices necessary. One thing out of my experience when I just started diving I got a BCD that had a velcro strap fixing the hose to the BCD for convenience. This strap when used prevented the air from venting from the BC completely and I needed more weight to sink. So check if there is nothing that prevents air from escaping and also There are no air pockets under the suit.

 

[don Francisco]

Is there a specific number to increase by from fresh to salt.

Just to clarify my earlier post. Seawater is about 2.5% denser than fresh water, so assuming yiu were neutral in a pool, you'd compensate for the change to the sea, by adding 2.5% of your total weight, your rig, tank(s), weights, accessories, etc.

One of the things to record in a dive log, is what equipment you used and the weighting. This saves you having to fool around rechecking weight each time you dive.

 

[Walter]

New divers often kick when trying to sink without realizing it, Next time, cross your ankles when you're ready to descend. It's also possible you have trapped air in your BC.

 

[BabyDuck]

i'm also leaning towards trapped air, nervous fins, and a big deep breath before trying to get under.

what you need is not a finite, 'one right answer' thing. it changes with gear, conditions, and comfort - sometimes a lot!

i started using a drysuit in january. i needed 12 extra pounds for it then. now i don't need any. yes, i changed underwear slightly, but mainly it's the difference in comfort.

 

[fisherkid]

To those who said eighteen pounds seems excessive that is what I thought based upon the mate on the boat. He is a bigger guy than me and said he uses fourteen pounds.

The bcd is a rental Pro QD I3 so I don't know if it has a tendency to trap air or not but it wasn't the inflation hose. It think the most likely culprit is the air in my lungs, though I seem to recall exhaling entirely in attempt to sink. maybe not:shrug:

I'm also concerned about being under weighted at the end of the dive. I don't want to be buoyant and have a hard time holding a safety stop.

I've got a good amount to think about now and I guess the best thing to do is get out there and try again.

 

[herman]

It may not be so much "trapped air" as proper BC venting technique. I have often seen divers try to vent their BC when horizonal or when vertical but keeping the inflator hose down low or with a sharp bend in it. To properly and completely vent a BC your left should must be raised to the highest point (ie, feet slightly down and body rotated slightly with the left side up), the inflator hose removed from any holders and raised above the BC. Think about air movement, it will go to the highest point and it will not pass any areas that are lower than where it is currently at, the path to the surface must all be headed up. If for example, your rear is slightly higher than your shoulder, air will be in the rear of the BC, not at the front where it needs to be to vent or even if you left shoulder is properly elevated, if you hold the inflator slightly below your shoulder, it will not vent. You can also use rear dumps, keeping the particular valve you are using at the highest point but again proper technique must be used or it is possible to trap air in the opposite side of "U" shaped wings. The shoulder dump is a little easier to master in this respect.