Empty BCD buoyancy?

[Barmaglot]

Diving with rental gear, I used to need around 3.2 - 4kg of weights (four or five 0.8kg weights on a belt). Recently, I got my own kit, including a BCD - a Cressi Travelight - and today I couldn't descend with 5.6kg of weight on me (2x1kg in each of the main pockets, 1x0.8kg in each of the trim pockets), no matter how long I held a full exhale for (within my body's limits of course), until the divemaster gave me another 0.8kg. At this point, I descended at the same rate as the rest of the group, but on the bottom I felt quite heavy, although not excessively so.
Major differences between the rental BCDs I used before and the Travelight are the lack of backplate on the latter - it's all fabric, with the tank providing rigidity while it's mounted - and the lightweight fabric it's made of, compared to heavy-duty fabric on non-travel BCDs.
Both before and now, I'm diving with a short 3mm wetsuit, though not the same one.
Does it make sense that I have to compensate for the lack of a backplate with extra weights, and that the lightweight fabric easily compresses at depth, reducing excessive surface buoyancy?

 

[Jonny Wishbone]

Different wetsuits do make a difference as different grades and types of neoprene have different buoyancy characteristics. I remember reading an article awhile ago about wetsuit buoyancy. Some neoprene has more micro bubbles within it than others do. And if the suit is new, it will be really buoyant until it loses its virginity so to speak. The BCD may have a small bit to do with it based on the buoyancy of the material used for its manufacture. Keep in mind that the best way to see if you are properly weighted is to use a near empty tank (especially if using Aluminum) for your buoyancy check. If you are barely negative enough to get down with a full Alum 80, you will be too buoyant at your safety stop when done with your dive and you may fight a little to stay at your stop. Most divers really don't bother with the empty tank check and just compensate for additional buoyancy of their tank (when near empty) based on the buoyancy characteristics of the tank they are using,, but an empty tank check is truly the right way to go about it. The BCD fabric itself really does not compress. The fabric threads will wet out and not effect buoyancy. Even when you get properly weighted, you may still need to move weights around in different combinations as you seek the best trim you can get with a given set-up. Once you get properly weighted and obtain your best trim, you will be so comfortable you will never want to walk on land again...

 

[Barmaglot]

Should have mentioned; I had no trouble whatsoever holding the safety stop - if anything, it was easier than before. The wetsuit is sort of new - I used it on about a dozen dives previously. The rental wetsuits I used before that were, well, rentals with god knows how many dives of them. I keep trying to figure out whether I'm doing something wrong, or if I really do need to put on that much weight to dive properly.

 

[The Chairman]

Every piece of gear has it's own buoyancy characteristic. That's to be expected. So add the extra weight, dive and have fun. It's all good.

 

[rongoodman]

If you're able to hold a safety stop with a nearly empty tank and little to no gas in the BC, you're weighted properly. You don't get extra points for the amount of weight you need.

 

[KeithG]

Diving with rental gear, I used to need around 3.2 - 4kg of weights (four or five 0.8kg weights on a belt). Recently, I got my own kit, including a BCD - a Cressi Travelight - and today I couldn't descend with 5.6kg of weight on me (2x1kg in each of the main pockets, 1x0.8kg in each of the trim pockets), no matter how long I held a full exhale for (within my body's limits of course), until the divemaster gave me another 0.8kg. At this point, I descended at the same rate as the rest of the group, but on the bottom I felt quite heavy, although not excessively so.
Major differences between the rental BCDs I used before and the Travelight are the lack of backplate on the latter - it's all fabric, with the tank providing rigidity while it's mounted - and the lightweight fabric it's made of, compared to heavy-duty fabric on non-travel BCDs.
Both before and now, I'm diving with a short 3mm wetsuit, though not the same one.
Does it make sense that I have to compensate for the lack of a backplate with extra weights, and that the lightweight fabric easily compresses at depth, reducing excessive surface buoyancy?

Different BCDs have different buoyancy. Unfortunately the manufacturers do not publish this info.

So if you change BCD then you may have to adjust your weights.

I suggest you dunk each new BCD in a rinse tank and then pile weights on it until it starts to sink. Some are neutrally buoyant. Some are not. It takes about 3 lbs to sink my BCD.

 

[Scared Silly]

As folks have said different gear different buoyancy. The rental wetsuits probably had zero buoyancy whereas your new suit has some buoyancy. That is probably the main difference. Once you have your own gear you will learn what you need for proper weighting.

Also remember that the salinity of the ocean is not equal. The ocean is 35 ppt, Where as in the Mediterranean Sea it is 38 ppt and in the Red Sea it is 43 ppt. So you will need more weight in the Red Sea than the Mediterranean Sea.

 

[Barmaglot]

Thank you. I'm in Gulf of Thailand right now (Koh Tao), did two more dives this morning with 8x0.8kg and all seemed fine - I guess this is what I'll stick with.

 

[Blasto]

Brand new BC tend to be a bit rigid and trap a lot of air as a result. It's likely you didn't get it close to empty on your first dive.
Try playing with it in a shallow pool, testing buoyancy as mentioned, and try squeezing all the air out of it manually (taking it off). See if that helps, see how much its buoyancy changes. The Travelight is a popular BC and it shouldn't be highly buoyant, but jackets don't empty easily to begin with.

Once you get it empty and fill it back up, practice squeezing the air out of it, using the inflator and all three of your dumps, note which dumps to use, where to push to get the air out. After some cycles, it will get softer and air will come out easier, so you'll need less weight and less effort to deflate.

 

[Barmaglot]

The BCD isn't new, I bought it used - no idea how many dives it's had; it's in a good shape, but it does have definite signs of use.