Good Morning, Alan and Kevin,
I'll describe my process, which was really quite simple. I began with measuring off a quantity of water with a calibrated photographic beaker (CC's / US Fl Oz) and taking a temperature reading since I know temp will affect density. Photography conveniently had me accustomed to working w/ 20 Degrees C / 68F, which is right where I was (and s/b as a "standard" for referencing SG, as I found in several articles online, just to be sure). So, I placed the weight upside down in a small stainless steel pot and proceeded to add the water while keeping track of the quantity just as a matter of course, until the weight was completely submerged. Although this quantity is essentially immaterial, it came out to be 825 cc's. This also allowed the PWT to be at the same temperature as the water after a few minutes rest, since it was very close to begin with (within a degree - ambient air temp).
With a permanent blue marker, I began placing dots down the inside of the pot until I finally touched the surface of the water, at which point the ink immediately flared and created a perfectly level / straight line at the boundary of the water. So as to not disturb this neat little indicator, I tilted the pot with the handle so the water level fell safely away from it then retrieved the weight, ensuring that my fingers wouldn't displace the water near the line, and gently shook of any excess back into the pot as I returned it to level. Other than damp fingers, essentially none lost.
From this point, I simply added water from the calibrated beaker to the pot until the level reached the line, and recorded the amount needed to replace the amount which the Weight had occupied. That volume was 299 cc's.
Not knowing the actual weight of my subject was the next problem. My scale only goes to 600 grams. I phoned my local pharmacy and soon arrived to have them weigh it. No go, since their digital scale only went to a few hundred grams. Same deal w/ 2 local jewellers, but I found 3 shops who advertise wanting to buy gold, and was able to get tenth-of-a gram accuracy from 2 and dwt from the other: (882.2, 881.8 and by conversion 881.78 which averages 881.92 g.) I figured it was important to get at least 3 readings since one never knows how accurate their scales are, ie, trust but verify... and taking the average as a matter of best practice.
Now, simply creating a ratio of the weight in grams over the displacement / volume (also in grams since cc water @ 20C = grams) 881.92 / 299 yields 2.9496. Figuring also that my potential for error in measuring the displaced water was somewhere about a half cc at worst, and knowing that if anything, my reading was higher, not lower (yes, Kevin, due to that pesky surface tension of the water not creating a perfect plane in the graduate), I subtracted 1/2 cc and refigured: 881.92 / 298.5 = 2.9545 SG. And confident that I'm well within a half of a percent of accuracy. The smile grew large as I realized that it was right near the low end of your observed Bacchus SG's, and I'm also accepting that there's a chance that my error may be larger, but as stated above, if anything, my figure for displacement would be high, so reducing it would only increase the SG, and move it more into range... everything else I can be 99.999% sure of.
So, there's my story. Kitchen Science at its best!
With a little patience and care, I'm sure it's repeatable, and might afford others something to go on, especially if they've got a proper scale to work with. I believe it is about as straightforward as can be, since the objective is to know the volume of the PWT and then compare it to its mass (weight). Empirically measuring the difference in weights in air & water I'm sure works fine too, though somehow to me at least, seems a step removed from simplicity. But if it is a more accurate method, I'd be inclined to stick with it too. Of course, being able to do BOTH tests should only improve the confidence beyond any doubt, since they're taking different paths to the same result.
This is fascinating stuff and nice to dust off the cobwebs for a change! Thanks. Maybe there's still a chance I've got an "unknown Bacchus" after all? Wake me when it's over...
Bruce / h-g