HYDROMETER
Basics
The following illustrates the versatility of the hydrometer (with Oechslescale).

A hydrometer with Oechslescale looks like this

But one could also
look like s would be 200 minus degrees on the hydrometer (with Oechslescale) if it had a minus scale long enough (coloured scale).
1 litre water = 1 kg (1 000 grammes)
1 litre alcohol = 800 grammes

10% alcohol/water - mixture

100 ml alcohol x weight 0.8 = 80 grammes

900 ml water = 900 grammes

80
900
980 grammes

980 grammes is 20 grammes less than 1 kg = minus 20 hydrometer (with Oechslescale) degrees