"Incorrect Readings" between Brix Refractometers and Hydrometers

First published on Distillique's website in 2015 by GM Bosman

Many of our clients contact us with questions relating to the different readings obtained when using a refractometer and when using a floating hydrometer in the same liquid.

These readings can be quite far apart and we should be aware of that and the reasons behind it.

The general “truth” that more dense liquids have greater refractive indexes (i.e. sugar water vs water), and less dense liquids (i.e. ethanol vs water) have smaller refractive indexes is NOT TRUE.  

Snell’s law states that the refractive index for more OPTICAL DENSE liquids will be bigger and that less OPTICALLY DENSE liquids will have smaller refractive indexes.

The key here lies in the fact that OPTICAL DENSITY does not necessarily correlate to the PHYSICAL DENSITY of a material. 

In summary: 

• If you use a refractometer to measure the brix value in fermenting mashes, be aware that the produced alcohol will cause the Brix refractometer to over read. (i.e. easily showing 6% Brix reading when it actually should be 0% at the end of the fermentation)

• Also, if you use a hydrometer to measure the brix value in fermenting mashes, be aware that the produced alcohol will cause the Brix hydrometer to under read. (i.e. easily may read -3% Brix reading when it actually should be 0% at the end of the fermentation)

The following graphs displays the refractive indexes of ethanol/water and sucrose/water solutions.