Gravity: Measuring the Sugar Before and After Tells You Everything

Float the hydrometer in a tall sample jar, not in the fermenter. Spin it gently as you let go so the bubbles clinging to it release — bubbles lift it and give a reading that is too low. Read at the bottom of the meniscus, at eye level, exactly as with any volumetric glassware. And take the sample AT the calibration temperature or correct for it: most hydrometers are calibrated at 20 °C, and a hot sample reads low because the liquid has expanded. Never pour the sample back. It has been exposed, it has been in an un-sanitised jar, and returning it risks the whole batch for a few millilitres. Taste it instead — that sample is also your best early warning of an off-flavour.

A refractometer needs two drops instead of a full sample jar, works on hot wort, and gives a reading in seconds. For pre-fermentation gravity it is far more convenient than a hydrometer. But alcohol bends light differently from sugar, so once fermentation has started the reading is systematically wrong. Correction formulae exist and they need the original gravity as an input, which means a refractometer alone cannot give a final gravity. That is a textbook systematic error: repeatable, in one direction, invisible from the reading itself. Use the refractometer for the start and the hydrometer for the end, which is each instrument used where it does not lie.

A single gravity reading tells you very little. Two readings three days apart tell you whether fermentation is finished, and a series tells you how fast it went and whether it slowed unexpectedly. So log OG on brew day, then a reading every few days once the visible activity slows. The criterion for done is two or three identical readings over several days — the dry-to-constant-mass loop from the chemistry batch, applied to a fermenter. Record the temperature with every reading, and the yeast used. Those three columns together turn a brewery into something that improves rather than one that produces a different drink each time.