Hydrometer: Handle carefully, as it is fragile; it must be clean and dry.

Measurement vessel: Glass is best; the interior must be at least 1.5 cm wider than the hydrometer (except for the “OPTIMUM”, see below); it must be clean and dry, and must be plumb.

Test liquid: Must be free of contamination, free of bubbles, average sample.

Workplace: Must be vibration-free, bright and glare-free.

Measurement procedure: Pour the test liquid slowly into the measurement glass. Carefully mix before measurement,

gently tap on the lower part with an areometer with a built-in thermometer which is graduated at 0.1 or 0.2 °C, because the mercury column sometimes does not have a completely uniform course (especially when temperatures are falling).

Hold the areometer at the thin end above the measurement scale - tip - and slowly dip into the liquid until it floats; dip it at most 5 mm deeper than its natural float level. The rest of the areometer’s surface must also be clean and dry when dipped (no fingerprints, no grease or similar).

Wait at least 1 minute, but better 2 minutes (the larger the lower part, the longer one must wait). Read the “top” or “bottom” result (see areometer) with/without a magnifying glass; the view direction must be horizontal. The areometer must float calmly and approximately in the middle during reading.

The target measurement temperature = reference temperature (as a rule = 20'C; see the areometer). Use a table to correct for temperature if it is different than the reference temperature.

If the surface tension of the test liquid varies significantly from the underlying surface tension in the adjustment (see areometer), make a correction of the precise measurement. Areometers are basically adjusted for a defined surface tension, or for surface tension classes L (low), M (medium) and H (high), or for the surface tension of a defined liquid.

Optimal side distance to the cylinder wall: 7.5 mm or greater.

Always conduct 2 to 3 sequential measurements, and calculate the mean.

Standardised areometers (best with a standardisation slip) must be used if the precision specifications are high (such as with warranty measurements). In an emergency, one can use standardisable as well as normal devices with an official or manufacturer’s test slip. One must assume that measurements are more or less imprecise with areometers without an official test slip.