The hydrometer is a glass body which is dipped into the sample. After a short equilibration time it will swim at a certain level (when the mass of the hydrometer is equal to the buoyancy effect). The higher the density of the sample, the less the hydrometers will sink. The level of equlibration reads the density on the calibrated scale.

Advantages:	Disadvantages:	Main applications:
simple method	breakable glassware	quick control of a "rough" density value, mainly for process control
quick measurement (*)	long and difficult thermostating	suitable for the measurement of the same kind of sample (wine, beer) due to the limited measuring range
inexpensive instrument	very small measuring range (requires several hydrometers to cover a wider range, typically 20 units)	not suitable for expensive samples (large volume required).
special instruments with direct readings in Alcohol%, Sugar% (BRIX) or other density related values available	operator dependent readings, therefore limited accuracy
(*) if no thermostating required	no measurement protocol (not suitable for GLP)
	large sample volume required
	difficult to clean and dry