Standardized EUROPE Hydrometers
ISO 387 - NF B 35-510 (Tropical Countries)

Density Hydrometers for General Purpose .

They are calibrated according to the following surface tensions:
from 0.600 to 1.000 g/ml= 16 at 35 mN/m (low tension)
from 1.000 to 1.300 g/ml= 55 mN/m (medium tension)
from 1.300 to 2.000 g/ml= 75 mN/m (high tension)
Reading below meniscus.

The reference temperature for density hydrometers is 20°C.
In special circumstances and in tropical countries, it is necessary to operate at an ambiente temperature above 20°C.
In the standard ISO 387 we accept the use of 27°C for standardized instruments and up to 27.5°C for non standardized instruments.