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.