Refractometer correction calculator

How to use it

1. Read it before fermentation. Put two or three drops of the mixed wash on the prism, close the flap, and read the °Brix scale against the light. Keep this reading - you'll need it every time.
2. Read it again later. Same way, at any point during or after fermentation. Wipe the prism clean and let the sample come to room temperature first; most refractometers correct for temperature themselves.
3. Set the correction factor. 1.00 for sugar washes. For grain or malt washes, 1.04 is the usual figure; the exact one for your refractometer is the ratio of its reading to a hydrometer's on the same unfermented wash.
4. Read the result. The true gravity is what a hydrometer would show; the strength follows from it. For a quick check before fermentation, choose Before fermentation.

How it works

A refractometer measures how much a liquid bends light. Sugar bends it, so before fermentation the reading gives the sugar content - in °Brix, which converts directly to gravity. But alcohol bends light too, so as sugar turns into alcohol the reading falls more slowly than the sugar does, and a fermented wash looks far sweeter than it is.

Because the original reading tells you how much sugar there was, the two readings together can be untangled. The calculator uses Sean Terrill's correction, fitted to hundreds of measured fermentations, to find the true final gravity, then works out the strength from the original and final gravities as the hydrometer calculator does.

Worked examples

A sugar wash read 17.0 °Brix before fermentation and reads 6.5 °Brix now. Taken at face value, that suggests plenty of sugar left; corrected, the true gravity is 1.008 and the wash is about 8.5% ABV.

The same readings from a grain wash, with a correction factor of 1.04, mean a gravity of 1.008 and about 8.1%.

Questions

Why is my refractometer reading wrong after fermentation?

Because alcohol bends light as well as sugar does. Once there is alcohol in the wash, the reading can no longer be taken at face value; it has to be corrected using the reading from before fermentation.

What if I didn't take a reading before fermentation?

Then the correction can't be made, because it depends on knowing the original sugar. Use a hydrometer, which reads a finished wash directly.

Is a refractometer or a hydrometer better?

A refractometer needs only a drop and is ideal for quick checks during fermentation; a hydrometer is more trustworthy for the final reading of a very dry wash. Many distillers use both.

What is °Brix?

The percentage of sugar by weight in a solution: 17 °Brix is 17 g of sugar in 100 g. °Plato is practically the same scale.

Do I need to correct for temperature?

Most refractometers sold today correct automatically (ATC). Let the drop come to room temperature anyway - it only takes a few seconds on the prism.