Am I missing something?

[Altamira]

Here is the scenario. My wife/dive buddy has been diving a jacket style bcd which weighs 8.7# dry, and with her total gear configuration (including that bc) needs 14# of lead in SW. We know that weight is correct because with any less weight she starts to float with an empty bc and with 700 psi at the safety stop. She recently bought a jacket style travel bc that weighs 5.5# and all other dive gear remains the same. Although the weight of the two bcds only varies by @3#, the lighter bc may not have the same inherent buoyancy as her 8.7# model. Although she will do a proper weight check on our next dive, which unfortunately will be on a cruise, and we hate making other divers wait while we fiddle around with weights at the start of the dive. So to put her in a reasonable ball park for start the dive weight, is is valid to do a buoyancy test on the two bcs to see how much difference in lead is required to sink both bcds, and use that difference to add or reduce the start dive weight for her new travel bcd? I suspect that the travel bc will require less lead, but I am not discounting the difference in actual bcd weight because I do not know the inherent buoyancy of either bc. Thanks for the advice.

 

[knotical]

A buoyancy check on the two empty BCs seems a very reasonable exercise, and shows that you've been thinking. I suspect it will not be much of a difference.

 

[Steve_C]

Good idea assuming that the wet suit and rest of gear are the same with both BCD. If she has been diving a 5mm and you rent a 3 mm on your trip, then things change.

 

[Altamira]

All gear will be the same except for the bcd. The only variable will be the difference in weight required to sink the 8.7# vs the 5.5# bcd. I'll be doing the test in FW so will factor in the .025 per pound difference.

 

[tbone1004]

odds are that the two bc's are essentially 1 maybe 2lbs positive in the water regardless of their weight on land. Most of them are all about the same. Could have been nice to her and gotten her a SS backplate, could have taken 10lbs off of that belt and had it weigh about the same on land

This is why it is important to know the amount of lead required to sink you wearing nothing but a bathing suit, and the amount of lead for each of your exposure configurations including boots, gloves, hood, and then it makes everything pretty easy

 

[KeithG]

do the test. you may be surprised. all bcds are not equal.

my original bcd was neutrally bouyant. wore it out and bought a new "light weight" travel bcd. it is lighter and unfortunately, very bouyant. it takes more than 3 lbs to sink that sucker in salt water.

 

[Altamira]

Sure hope that is not the case, but our main concern was reducing the weight in our luggage and carrying the gear to and from the ship and dive boats and shore dive locations.

 

[NAM001]

The .025 is moot at that weight difference.

All gear will be the same except for the bcd. The only variable will be the difference in weight required to sink the 8.7# vs the 5.5# bcd. I'll be doing the test in FW so will factor in the .025 per pound difference.

 

[beanojones]

I'll be doing the test in FW so will factor in the .025 per pound difference.

(Something is amiss with that number. FW uses a lot less weight.)

We know that weight is correct because with any less weight she starts to float with an empty bc and with 700 psi at the safety stop. .

Not to trying to criticize, but the original reasoning makes some assumptions you might not want to gloss over. If a diver is neutral, when relaxed with 700 psi in the tank at safety stop depth, then in a non relaxed situation, or with less air, they will be positively buoyant.

I am all for wearing the minimum weight and not running low on air, but when it rains it pours.

It's just something to keep in mind

 

[BRT]

(Something is amiss with that number. FW uses a lot less weight.)

.025 times whatever the object weighs. In my case I weigh 205 and my gear weighs 100 or so which makes for a 7.5lb difference. in the case of the 5.5lb bcd, well fresh to salt isn't much difference.




Not to trying to criticize, but the original reasoning makes some assumptions you might not want to gloss over. If a diver is neutral, when relaxed with 700 psi in the tank at safety stop depth, then in a non relaxed situation, or with less air, they will be positively buoyant.

I am all for wearing the minimum weight and not running low on air, but when it rains it pours.

It's just something to keep in mind

.025 times whatever the object weighs. In my case I weigh 205 and my gear weighs 100 or so which makes for a 7.5lb difference. in the case of the 5.5lb bcd, well fresh to salt isn't much difference.