How to read a hydrometer
Before using the hydrometer
- Make sure both the hydrometer and hydrometer jar are clean.
- If the liquid to be tested is not at room temperature, allow it to reach room temperature before testing.
- Pour the liquid carefully into the hydrometer
jar to avoid the formation of air bubbles. Do this by pouring it
slowly down the side of the jar.
- Stir the liquid gently, avoiding the
formation of air bubbles.
Taking a Reading
- Carefully insert the hydrometer into the
liquid, holding it at the top of the stem, and release it when
it is approximately at its position of equilibrium.
- Note the reading approximately, and then by
pressing on the top of the stem push the hydrometer into the
liquid a few millimetres and no more beyond its equilibrium
position. Do not grip the stem, but allow it to rest lightly
between finger and thumb. Excess liquid on the stem above the
surface can affect the reading.
- Release the hydrometer; it should rise
steadily and after a few oscillations settle down to its
position of equilibrium.
- If during these oscillations the meniscus is
crinkled or dragged out of shape by the motion of the
hydrometer, this indicates that either the hydrometer or the
surface of the liquid is not clean. Carefully clean the
hydrometer stem. If the meniscus remains unchanged as the
hydrometer rises and falls, then the hydrometer and liquid
surface are clean, and a reading can be taken.
correct scale reading is that corresponding to the plane of
intersection of the horizontal liquid surface and the stem. This
is not the point where the surface of the liquid actually
touches the hydrometer stem. Take the reading by viewing the
scale through the liquid, and adjusting your line of sight until
it is in the plane of the horizontal liquid surface. Do not take
a reading if the hydrometer is touching the side of the
Taking the Temperature
- Using a suitable thermometer, take the
temperature of the liquid immediately after taking the
- If there is any chance of a change in the
temperature of the liquid it is safer to take the temperature
both before and after the hydrometer reading. A difference of
more than 1°C means that the temperature is not stable, and the
liquid should be left to reach room temperature.
- If the temperature of the liquid is not the
same as that on the hydrometer scale, the hydrometer reading
should have a correction due to temperature applied.
Handling the Hydrometer
- The hydrometer should never be held by the
stem, except when it is being held vertically.
- When holding the stem, always hold it by the
top, as finger-marks lower down can affect the accuracy of the
- Always handle with care.