Hydrometer vs Refractometer

How each works

Buoyancy against light

A hydrometer is a weighted glass float. Denser liquid is more buoyant, so the float sits higher, and a scale on the stem reads specific gravity. It measures density directly, which means it is agnostic about what is dissolved -- sugar, alcohol, anything.

A refractometer measures how much a liquid bends light. Sugar solutions bend light in proportion to their concentration, so a scale calibrated in Brix (or dual-scaled to specific gravity) reads off a couple of drops on a prism.

That difference in mechanism is the entire source of the post-fermentation problem.

The alcohol problem

Why a refractometer lies after fermentation starts

Ethanol refracts light differently from water, and it refracts it in a way that makes a solution look more concentrated than it is. So once fermentation has produced alcohol, a refractometer reads a gravity that is too high -- often substantially so.

The reading is not useless; it is correctable. There are published correction formulas that take the original Brix reading and the current Brix reading and estimate the true final gravity, and most brewing apps and calculators include one. But a correction is an approximation layered on a measurement, and it relies on your original reading being right.

A hydrometer has no equivalent problem because it measures density, and density is density regardless of what produced it.

The practical division of labor

Use each where it is strong

• Brew day: refractometer. Checking first runnings, pre-boil gravity and post-boil gravity costs two or three drops each and takes seconds. That makes mid-process checks practical, which matters because pre-boil gravity is actionable -- you can still adjust the boil length or add extract. A hydrometer reading costs a cupful and a minute, so people skip them.
• Original gravity: either. Both are accurate before fermentation starts. Take it with whichever is to hand.
• Final gravity: hydrometer. No correction formula, no approximation. This is the number your ABV calculation depends on, so measure it directly.
• Fermentation progress: a floating sensor, if you have one. Two hydrometer readings 48 hours apart is the classic test and requires you to draw two samples two days apart. A floating bluetooth hydrometer gives you the same evidence as a continuous curve with no samples drawn.

Getting good readings

Four things that go wrong

• Hydrometer temperature correction. Hydrometers are calibrated at a specific temperature, usually 60°F. A reading taken on a hot wort sample without correcting reads low. Cool the sample or apply the correction.
• Refractometer calibration. Zero it on distilled water at the start of every session. It drifts, and an uncalibrated refractometer is confidently wrong.
• Hot refractometer samples. Even with automatic temperature compensation, a drop of boiling wort on the prism reads unreliably until it cools. Let the sample reach roughly room temperature on the glass.
• Bubbles on a hydrometer. CO2 bubbles clinging to the float add buoyancy and read high. Spin it gently in the jar to shed them before reading.

Related: holding the right fermentation temperature is what makes any of these readings meaningful -- fermentation temperature control. And a stalled fermentation shows up as a gravity that stops falling, which is exactly what these instruments are for.