Getting honest readings from a hydrometer: temperature, surface tension and technique
Care & Use · 5 min read · Vitreon technical desk
A hydrometer is a precision instrument pretending to be simple. Three systematic errors cause almost all wrong readings.
01Temperature first
Density changes ~0.02–0.05 % per °C for most aqueous solutions. Read the sample temperature with a real thermometer, not the room's, and apply the scale's stated reference (20 °C or 27 °C). Combined hydrometer-thermometer instruments exist precisely for this.
02The meniscus climbs the stem
Surface tension pulls liquid up the stem; scales are calibrated for reading at the flat liquid surface, sighting through from below the plane. A greasy stem exaggerates the climb — clean with ethanol, dry, and never touch the stem below the scale.
03Spin it
A gentle spin as you release the instrument shakes off adhering bubbles — an attached bubble reads the density measurably low. Wait for the float to settle dead-still before reading; in viscous samples that takes a full minute.