Neutral buoyancy and DSMB

[martincohn]

If its this much of an issue, throw on a couple kgs and do a real weight check in the water. Can be done quicker than it takes to type this.

Weight check holding the extra 2 kgs, then without.

 

[johndiver999]

Um yes....but his dsmb with crack bottle and reel is negatively buoyant....so removing it reduces his mass therefore making him more positively buoyant....to regain a state of neutral buoyancy the OP would need to add ballast to make up the difference created by removing negatively buoyant gear.

-Z

yes I think that is more accurate.

 

[wetb4igetinthewater]

It sounds like I made the basic mistake of weighing the reel/SMB on land without taking into account what happens in the water (physics stuff I think) and, therefore, the actual affect it will have on buoyancy under water?

I use a luggage scale and a rowboat to see how much the scuba kits I provide students weigh.

 

[Mr.S]

Thanks for the input so far, and this has prompted me to try to solve this mathematically / scientifically. As stated in various posts, the real test is to experiment on a dive but unfortunately we don’t always have the opportunity to do so – we are just thrown off a boat and get on with the dive….

What follows may be sad and long, so apologies in advance and feel free to ignore.

Volume of SMB + Reel = 1.5 litres, or 0.0015 cubic metres

Formula for working out the buoyant force provided by the water:

Force = volume x density of water x gravity, = 0.0015 x 1000 x 9.8 = 13.7N.

This means the upward buoyant force provided by the water is 13.7N.

The downward force = mass [of reel and SMB] x gravity = 1.91 x 9.8 = 18.7N
(Reel + SMB combined mass is 1.91kg)

Difference is 5N = 510g.

Since the downward force (18.7N) is greater than the upward buoyant force (13.7N), this means the reel/SMB will be negatively buoyant with an equivalent mass of 510g.

To test this out I filled a deep bucket with water and used a luggage scale to measure the mass of the real and SMB in water, and it showed it to be about 600g [negative].

This might be simplistic as pressure will increase as you go deeper and, therefore, has some impact on volume, but this is probably minimal with the equipment we are talking about.

So we are only talking about 500 to 600g provided by the real/SMB, which on the grand scale of things shouldn’t make that much difference to my weighting. The real test will be for me to test my theory on a dive when the opportunity next arises.