Originally Posted by Erroll Ozgencil

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

How Far Can You Trust Your 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
Washington Winemaker