Refractometer Correction

Rea­dings
Ergeb­nis

When you read the extra­ct con­tent of a beer con­tai­ning alco­hol using a refrac­to­me­ter, the mea­su­red value is tam­pe­red by the alco­hol. But if you know the ori­gi­nal gra­vi­ty, you can cor­rect this.

The ame­ri­can blog­ger and home­bre­wer Sean Ter­ril did exten­si­ve com­pa­ra­ti­ve mea­su­re­ments with a hydro­me­ter and Refrac­to­me­ter and deter­mi­ned the fol­lo­wing cor­rec­tion formula:

REs = 1.0000 – 0.00085683 * RAStw + 0.0034941 * RARe

REs - cor­rec­ted appa­rent final gra­vi­ty in SG
RAStw - Refrac­to­me­ter rea­ding of ori­gi­nal gra­vi­ty in Brix
RARe  - Refrac­to­me­ter rea­ding of final gra­vi­ty in Brix

The accu­ra­cy of this fror­mu­la is best near to final gra­vi­ty (atte­nua­ti­on >60%). Below the accu­ra­cy decreases.

The final gra­vi­ty in °P is cal­cu­la­ted suf­fi­ci­ent­ly accu­ra­te by:

°P = (SG - 1) * 1000/4

The ratio of appa­rent final gra­vi­ty to ori­gi­nal gra­vi­ty deter­mi­nes the appa­rent attenuation:

VGs = ( 1 - REs / Stw ) * 100

VGs - appa­rent atte­nua­tio­nin %
REs - appa­rent final gra­vi­ty in °P
Stw - ori­gi­nal gra­vi­ty in °P

The real final gra­vi­ty ca be cal­cu­la­ted from ori­gi­nal gra­vi­ty and appa­rent final gravity:

REt = 0.1808 * Stw + 0.8192 * REs

REt - real final gra­vi­ty in %w
Stw - ori­gi­nal gra­vi­ty in °P
REs - appa­rent final gra­vi­ty in %w

The alco­hol con­tent results from the dif­fe­rence of ori­gi­nal gra­vi­ty and real final garvity:

Aw = (Stw - REt) / (2.0665 - 0.010665 * Stw);
D = 261.1 / (261.53 - REt);
Av = D * Aw / 0.794;

Aw - Alco­hol con­tent in %w
Av - Alko­hol con­tent in %vol
D - Density

Sources:

  • Refrac­to­me­ter FG Results auf SeanTerril.com
  • Hubert Hang­ho­fer: Bier brau­en nach eige­nem Geschmack, S. 125