how do i mature a Hydrometer

[Otolith]

Put water of some salinity in it (nearish marine but doesn't have to be precise) and let it set for a few hours, rinse it out with fresh. (Always rinse it out with fresh after a use) What I've always been told and works for mine.

 

[Pufferpunk]

Still need to calibrate a refractometer, so buy the calibration fluid too.

 

[nolapete]

Get a Seatest hydrometer. No maturing necessary. Rinse with tank water a couple times, then fill. Tap on it to free any bubbles. Sit it on a flat surface. Read result. Empty. Rinse with freshwater. Store upside down so all the water drips out.