We use hydrometers to gauge sugar content. Since hydrometers measure
solubles solids and soluble solids are mostly sugar in wine grapes,
that works pretty well. In researching raspberries, I found that sugar
can make up a much smaller fraction in other fruit. A clear sample of
raspberry juice might measure 9 Brix, but contain only 3 g/100g sugar.
More detail he

http://www.washingtonwinemaker.com/b...ur-hydrometer/

If we're just talking about raspberries, that may not be a big deal.
It's unusual to make wine from 100% raspberry juice - I've never done
it with more than 25%. So, for the entire must, the error is a lot
smaller. Might be a problem with other fruit though.

Erroll
http://www.washingtonwinemaker.com/