Hydrometer (alcoholmeter)

A glass float weighted at the bottom. The lighter the liquid, the deeper it sinks, and since ethanol is lighter than water, the depth gives away the alcohol content. The scale converts that into percent, so using it comes down to dropping it in and reading off.

That is the theory. In practice there are two conditions, and both get broken.

Temperature

The instrument is calibrated for one temperature, almost always 20°C. A warmer liquid reads low, a colder one reads high. A few degrees is already a few tenths of a percent, so either bring the sample to twenty, or reach for a correction table.

Sugar ruins everything

A hydrometer does not measure alcohol. It measures density, and the scale quietly assumes the liquid holds nothing but ethanol and water. Sugar raises the density, so a liqueur will look weaker than it is — with a sweet one the error runs to over ten percent. Fruit extracts, colourings and dissolved carbon dioxide in young cider do the same.

The conclusion is awkward but simple: you cannot measure a finished liqueur. The strength is set while mixing spirit with water, before the sugar goes in, and that is the figure that ends up in the recipe.

One last thing when buying: no single instrument covers the whole scale usefully. Buy the set — 0–40, 40–70 and 70–100%. A "universal" 0–100% has a scale so cramped that reading it to the nearest percent is guesswork.