lift calculations: no, not the standard thread

[Big Toes]

hey folks:

was wondering how the mfgs. calculate their lift capacity and if there is any way i can do it here at home. now apologies in advance but did a search and wasn't able to find much on this. i'm sure it's calculations and not just a bunch of in-water tests but would still like to hear how this is derived.

i'm asking because i've got a bc that was custom made for me about 12 years ago by a company called 'dive-tek' (i believe it was a subsidiary of zeagle). and i'm think that it's got a lift capacity of around 45# but would like to measure this myself is possible. need to make some decisions regarding this bc as it's seen a whole bunch of diving in the last decade...

thanks in advance for the help;

V/R;

erik

 

[Rainer]

Set up the BC on a cylinder like you normally would. Weigh it. Now fill the BC full of water. Weigh it. Subtract the weights.

 

[aquaregia]

Fill the BC completely, chuck it in a bucket of water and toss lead on it until it sinks?

 

[adm3745]

Lift capacity is based on the volume of water that an inflated BC displaces. For instance a square box of dimensions of 1 ft on all sides. The volume of this box is 1 ft ^3. The density of water is somewhere around 62 lbs per cubic foot I believe. So your 1ft x 1ft x 1 ft box has provides 62 lbs of buoyancy.

 

[miketsp]

Lift capacity is based on the volume of water that an inflated BC displaces.
..snip..

Not forgetting of course to subtract the weight of the BC itself (or the box in your example).

I have one BC that is very heavy.

 

[Scared Silly]

was wondering how the mfgs. calculate their lift capacity and if there is any way i can do it here at home.

The proper way is to put the BCD on a cylinder - this reason is because the bladder can inflate differently once on a cylinder.

Set up the BC on a cylinder like you normally would. Weigh it. Now fill the BC full of water. Weigh it. Subtract the weights.

This technique works if you do not have a pool and is very easy to get the lift for just the bladder. However, some BCDs are inherently buoyant to start with so you may not have the overall buoyancy.

Fill the BC completely, chuck it in a bucket of water and toss lead on it until it sinks?

This technique will work but you need to put it on a cylinder that is neutral to get an accurate measurement.

 

[Big Toes]

thanks guys.

 

[vladimir]

was wondering how the mfgs. calculate their lift capacity and if there is any way i can do it here at home.

How Deep Sea Supply does it:

DSS wing are all lift capacity rated in the assembled state, meaning we determine capacity with the wing mounted to a plate with a tank secured. It's pretty common for a wing in the "free state" or un mounted to have 3-5 lbs greater capacity than when assembled.

 

[muddiver]

You really need to know the volume of the bladder to caculate the weight of displaced water that causes the buoyancy.

Two methods that can be done in the backyard come to mind:

1. Use a known volume container like a large kitchen measuring cup to fill the bladder/or empty the bladder into, one cup at a time.

2. Filly inflate the bladder and submerge it in a container of water that has dimentions that are easy to measure. I have an old plastic 55 gallon drum I use to rense my gear in. If I fill it 3/4 with water and submerge an item in it I can measure the change in depth of the water and caculate the difference with teh diameter of the drum to find the volume of water displaced. This is how body mass is mesured accuratly by the medical profession. Note this does not work if the water spills over the top of the container because you lose some of the volume without measuring it. I would say you could use a bath tub except that most modern tubs are not a uniform shape that can be easily and accurately measured.

 

[Mike Boswell]

hey folks:

was wondering how the mfgs. calculate their lift capacity and if there is any way i can do it here at home. now apologies in advance but did a search and wasn't able to find much on this. i'm sure it's calculations and not just a bunch of in-water tests but would still like to hear how this is derived.

i'm asking because i've got a bc that was custom made for me about 12 years ago by a company called 'dive-tek' (i believe it was a subsidiary of zeagle). and i'm think that it's got a lift capacity of around 45# but would like to measure this myself is possible. need to make some decisions regarding this bc as it's seen a whole bunch of diving in the last decade...

thanks in advance for the help;

V/R;

erik

Tobin at DSS has described the way he does it, somewhere on SB.

I would just add that IMO you might benefit by thinking about how much lift you NEED, rather than how much you already have. Most BC's have more lift than necessary, and a smaller bladder might improve your streamlining.