The refractometer is one of those useful tools but not essential for brewing that often sooner or later end up being part of the collection of "gadgets" of the home brewer: the cost is limited (a few tens of euro), the use is simple and the actual usefulness in different situations.

The instrument is used to measure the sugar content of the must, like the densimeter but based on a completely different principle: not the density but the refraction of light through a liquid.

The refractometer could of course also be used during fermentation to check its progress and determine whether stable FG has been achieved.

Here, too, the advantage of the refractometer would be to be able to use only one drop of must, thus avoiding the loss of a certain quantity of must that occurs when measuring with the densimeter when it is preferred not to re-enter the sample into the must.

In this case, however, the problem is that the reading of the refractometer is distorted by the alcohol content of the fermenting must / beer.

Fortunately this distortion of the measure is predictable, ie there are formulas that allow the exact correction of the reading based on alcohol.

Even if the percentage of alcohol is not known (we would need the FG to determine it!) In reality it is possible to express the formula for the correct value of FG based on the reading made and OG of the beer that has been measured at the time.

There are quite complicated formulas (I think also because an OG-Plato conversion formula different from the one shown above is used [1]) but there is a fairly simple and sufficiently precise formula for our applications: FGb = 1.53 * RI - 0.59 * OGb [2 ] Where RI is the reading of the refractometer, and FGb and OGb are the FG and OG expressed in Brix (if desired they can then be converted to Plato and then to "density")

Source: LINK