After the thermometer, the hydrometer is probably the most important measuring instrument for the brewer. Used well, it lets you know fairly precisely how much sugar is in the wort.
What is a hydrometer for?
A hydrometer measures the relative density of a liquid: the ratio between its density and that of water. On the scale, distilled water reads 1.000; any denser solution shows a higher value, any less dense solution a lower one. The density of a wort before fermentation most often sits between 1.040 and 1.084.
Some hydrometers are graduated in degrees Plato (°P), which express the percentage of extract by weight: 1 °Plato equals 10 g of sugar per kilo of wort. Handy, because the brewer then reads the amount of sugar present directly.
Careful, fragile!
The hydrometer is a very fragile instrument. It must never be plunged directly into boiling wort: the thermal shock would shatter it.
The influence of temperature
Commercial hydrometers are generally calibrated at 20 °C — so that’s the ideal temperature to measure at. In practice, the wort isn’t always there: colder, it’s more viscous and the hydrometer sinks less; warmer, it’s the opposite. These deviations are corrected using the tables supplied with the instrument. For example, at 30 °C a reading of 1.040 actually corresponds to 1.043. Some models include a thermometer that shows the correction directly.
When to use it?
The hydrometer is an ally at several key stages:
- Before the boil — measured on the wort collected after lautering, the density tells you about the efficiency of your setup. If the target isn’t reached (or is exceeded), you can adjust the wort’s concentration.
- Before fermentation — the density of the cooled wort, before adding the yeast, gives the original gravity (OG): the amount of potentially fermentable sugar.
- During and after fermentation — the density at the end of fermentation gives the apparent final gravity. After distilling off the alcohol, you obtain the real extract.
How to read it correctly
- Choose a graduated cylinder (250 or 500 ml), set on a level surface.
- Fill it with cooled wort (~20 °C) until it overflows: this clears the foam.
- Note the temperature in your logbook.
- Check that the hydrometer is clean and dry, then lower it slowly into the wort.
- Once it floats, spin it on itself as you let go, and keep it from sticking to the wall (which would skew the reading).
- A concave meniscus forms: read at eye level, facing the cylinder. Clear liquid → read at the bottom of the meniscus; cloudy or dark liquid → at the top.
- Note the value, rinse the hydrometer with water at 20 °C, wipe it gently and store it upright in its case.
Checking the accuracy
First measure the density of distilled water: it should read 1.000. Then prepare a solution of 100 g of white sugar in 900 ml of water at 20 °C: it should read 1.040. If your hydrometer drifts slightly, carry that offset over to your future readings.
Converting between °Plato and density
The conversion isn’t quite linear, but for convenience we take 1 °Plato ≈ 0.004 density points. For more precision:
- Degrees Plato = 259 − (259 / SG)
- SG = 259 / (259 − degrees Plato)
