Weighting and Buoyancy Characteristics

[Casarez]

So part of what I am looking at with the weight I need is considering everything else I have on. Exposure protection, BCD, etc, basically everything that is both negative and positive in buoyancy. I am having issues finding some information though and hope someone out there can help.

I currently dive a Seaquest balance BC. I know it is positively buoyant but I am not sure just how buoyant it is. Does anyone know a place that either lists this or has a technique to find out?

One of the reasons I am inquiring is that when I finally switch to a BP/W system with hog harness I am wondering if I would shed a small amount of weight just by the change in BCs. The balance does have a lot of material in comparison. But I am not sure this would actually be the case.

Aside from BC should I also take into account 'extras' like dive light etc? For example if I am diving 10 pounds in seawater with a full 3mm wetsuit but I take a backup light which weighs 1 pound would I only need 9 pounds then or does the light have characteristics which negate that?

Any ideas/help would be appreciated.

 

[meesier42]

its much easier just to do a weight check as they taught you in your o/w course than it is to speak to specific numbers

 

[Casarez]

its much easier just to do a weight check as they taught you in your o/w course than it is to speak to specific numbers

Um, thanks but that is not what I was asking. Besides it only helps once I am at 500psi or less. If I add or change gear I would not know the difference until the end of the dive. I would like to try the not so easy way and see if I can figure things out beforehand. Sort of like researching tanks so you know the positive or negative characteristics.

 

[Charlie99]

I currently dive a Seaquest balance BC. I know it is positively buoyant but I am not sure just how buoyant it is. Does anyone know a place that either lists this or has a technique to find out?

Put it in a mesh bag. Throw it into a pool. Tie weights to the mesh bag until it sinks. (The amount of lead needed will be so small that saltwater vs freshwater corrections won't matter.)

One of the reasons I am inquiring is that when I finally switch to a BP/W system with hog harness I am wondering if I would shed a small amount of weight just by the change in BCs.

You may several pounds, particularly if you use a 6# or more steel plate as the backplate.

Aside from BC should I also take into account 'extras' like dive light etc? For example if I am diving 10 pounds in seawater with a full 3mm wetsuit but I take a backup light which weighs 1 pound would I only need 9 pounds then or does the light have characteristics which negate that?

There are two things to look at --- one is whether it is attached, like a can light on a waist belt, or something that may be dropped, like a reel. Droppable items like a reel shouldn't be considered when figuring out your lead requirements for making deco/safety stops at the end of a dive ---- if you don't have it with you at the end of your dive then you may have problem holding your stop depth. Something firmly attached like the battery cannister of a cannister light can legitimately be counted since you aren't going to drop it unless you are purposefully ditching it to get more buoyant while on the surface.

The second things is that it is the buoyancy that counts, not the weight. A 1 pound light isn't anywhere near 1 pound negative buoyancy.

 

[dkktsunami]

The light will have a buoyant force when immersed in the water so the difference will be less than one pound. For example, my 6 cu. ft. pony bottle and regulator weigh appx. 7 pounds but I remove only two pounds of weight from my bp/w to off-set the bottle.

 

[Casarez]

Put it in a mesh bag. Throw it into a pool. Tie weights to the mesh bag until it sinks. (The amount of lead needed will be so small that saltwater vs freshwater corrections won't matter.)

OK, figured it would be something like that.

You may several pounds, particularly if you use a 6# or more steel plate as the backplate.There are two things to look at --- one is whether it is attached, like a can light on a waist belt, or something that may be dropped, like a reel. Droppable items like a reel shouldn't be considered when figuring out your lead requirements for making deco/safety stops at the end of a dive ---- if you don't have it with you at the end of your dive then you may have problem holding your stop depth. Something firmly attached like the battery cannister of a cannister light can legitimately be counted since you aren't going to drop it unless you are purposefully ditching it to get more buoyant while on the surface.

I know if I go SS I will be dropping weight due to that. I was curious about the other factors. The note about droppable items though is something that makes a lot of sense. Thanks.

The second things is that it is the buoyancy that counts, not the weight. A 1 pound light isn't anywhere near 1 pound negative buoyancy.

That was what I was trying to say but did not say it quite right.

 

[Casarez]

The light will have a buoyant force when immersed in the water so the difference will be less than one pound. For example, my 6 cu. ft. pony bottle and regulator weigh appx. 7 pounds but I remove only two pounds of weight from my bp/w to off-set the bottle.

Exactly the sort of information I was looking for. Thanks.

 

[don Francisco]

Weight in air is meaningless since it will be offset by the displacement when in water. For your purposes you need to know the net weight/bouynacy in water, which you can determine by experiment.

Go to a local pool or lake to do the following tests. For floating items like a negative bouyancy BC, add lead weight until it just starts to sink, or barely float just at the surface. That'll give you the bouyancy.

For items that normally sink, take a 2 quart soda bottle & calibrate it lengthwise in eighths. Cap it and tie it to your test object by the neck and toss both into the water. Two quarts of displacement equals 4#s of lift. Since only the submerged portion of the bottle is providing displacement, you'll know by your marks how much your test object weighs in the water. For example if your bottle floats exactly 1/2 out of the water the object weighs 2#s.

You can use this method to get reasonably close values for the bouyancy of everything except a drysuit. You can even account for the air in your tank (80cuft weighs 6#s). By knowing what all the individual pieces of your equipment weigh separtely, you'll be able to calculate adjustments to your ballast, assuming you knew what you needed for any one configuration.

I hope that answered your question, have fun. dF

 

[b1gcountry]

Most BCs are between one and two pounds positive. I recommend you take it at that, as anything more accurate is:
A- pretty hard to measure, and
B- not very useful in real life.

If you're interested in how heavy things are in water, another useful tool is using a fish scale. You can measure how heavy something is by hanging it off the hook underwater, or you can measure how buoyant something is by doing it upside down. You can even weight yourself with a full tank by weighing yourself pre-dive. In an AL80, you should be about 6-8lbs heavy with empty lungs.

Tom

 

[Peter Guy]

My Balance (large?) is just about 3 lbs positive. (Tested in a pool by the simple means of emptying the bladder and then putting a bunch of weight on it so that it became as waterlogged as it was going to get -- then letting it float and adding weights until it sank.)

My can light is not quite 1 lb negative.