Getting a good S.G. reading that I can trust after the cap falls, and settling starts is pretty easy.
I'd like to know what procedures people use to do the initial reading. There are so many solids in the fresh must, even using the juice I get through a straining bag (especially if some non-grape fruits are used)... my level of confidence in the readings I get at the beginning is pretty low.
I did a Toka plum wine which started at like 1.130 when the must did not seem overly sweet to the taste. I don't believe the reading.
Most homebrew shops sell a funnel that has a inset at the bottom to accomodate a very fine gold filter insert. You can get various the filters in various grades of filter-a-bility. The filter is about 1.5" in diameter. This is a very useful tool for filtering must to take a S.G. reading. It should be about $10 for the whole setup I think.
If you have too many solids you probably should filter some juice with a better filter than the straining bag. When you do your measurement you should use something that really looks like juice rather than nectar. That does not mean it is clear but you probably know what I mean. Even if you get good clear juice there will still be solids that effect the reading. You could use an approach that subtracts out the effect of these solids. One such approach is given by Acton and Duncan in "Progressive Winemaking" where they use SG or G. Another is advanced by UCLA where they use Brix. I use the first of these two.
My equations derived form theirs are as follows:
% Alcohol = (Go - Gf) / F
Where Go is original gravity and Gf is final gravity and gravity is defined as
G = (SG-1)*1000.
And F is a value that takes into account the non-fermentable solids in the must and is defined
F = 7.75 - 3*(Go - 7) / 800
To get potential alcohol with this method you must guess the final G and then when it is finished you can use the actual final G to determine the true alcohol level. If you use Gf = 0, you will get the potential alcohol that is given in most tables or on the hydrometer which is not what you will really get as the result of fermentation. A good guess for final value is SG = 0.995 or G = -5.
I have checked a number of my musts that had a lot of fruit solids and compared simply straining it with a coarse wire mesh strainer and filtering through good paper filters. I found no discernable difference in the hydrometer readings if I was careful to make sure I bounced it up and down to make sure it was not solid viscosity floating the hydrometer. Therefore, I usually just push a kitchen strainer down into the must and scoop up some of the pretty clear juice. All of this is ballpark anyway, so if there's enough sugar to float it up near 1.095, I'm happy.
I agree with Greg. I have not found that suspended solids in must are that important to measuring SG. That is the suspended solids that we can see. Unless they are so thick that they impede the movement of the hydrometer, they are not going to effect the reading. After all, they are "suspended" solids meaning they have very little buoyancy and should not effect it. The solids that Action and Duncan and UCLA are correcting for are those that start out in the must and end up there after fermentation as well. They effect the buoyancy of the fluid at both ends and consequently they are not a large effect. But I correct for them anyway. ;o)
I've been relying more on refractometer readings in new must. It seems more accurate.
The other factor that I've found that affects the hydrometer readings is the simple thickness of the hydrometer and the fact that it seems to stick to the walls of the cylinder that I use to collect the sample. I'd love to find a thinner hydrometer.
My experience differs from Greg's and Ray's. I have noticed significant differences in SG readings in musts with suspended solids (up to 30 gravity points). Perhaps I am dealing with less diluted musts (thus higher levels of SS) or my straining methods are less effective. In any case, if winemakers want a reaosnable degree of accuracy in their SG reading I think it is important to check that the straining/reading procedures do provide it.
Hmm, the only time I have problems in different readings are when the solids make the solution very viscous and the hydrometer does not float properly. That usually occurs in the higher concentrated fruit musts. For these I always at least coarse strain and if still viscous (the hydrometer doesn't really bounce when tapped) I will filter through a coffee filter.
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