Economy Size Geek

Ok you are going to have to go along with this one. It may not make any sense to you - but I’m excited so just nod along.

Part of the process of brewing involves taking samples of beer and putting them in a long tube. Into that tube you put a hydrometer. That tells you the denisty of the beer. During the course of fermentation you can use the readings you get from the hydrometer to tell two things. First, it allows you to calculate the approximate alcohol content of your homebrew. This is useful if for no other reason than to know if you have a session or serious beer on your hands. Second, it helps you determine if fermentation is truly finished.

When I started brewing I was a simple guy. I just figured when the bubbling stopped the fermenation was done. That isn’t always the case. (I learned the hard way when I let a batch get too cold and it stopped fermenting well before the process was complete).

The point of all this is that the readings called original gravity (gravity before fermenation) and final gravity (gravity after fermenation) are very important. The problem is collecting enough beer to measure the gravity once the fermentation gets going is a pain. It is especially bad because you have to open the carboy and risk oxidation each and every time you want to take a sample.

Enter the refractometer (Picutre at the top). It allows you to get these numbers with a couple of drops of wort. I read a great article on BYO about Refractometers a while ago. I thought it sounded like the answer to all of my prayers. To get the OG (original gravity) you can approximate it by taking the brix measuerment (which is what the refactormeter measures in) and multiply it by 4. That gives you a value that is a good approximation of the value.

There are much more fancy calculations taken from here

Here is the Brix to specific Gravity (useful for the OG)

Brix to Specific Gravity

Formula was derived from the 69th edition (1988-1989) of the CRC Handbook of
Chemistry and Physics, “Concentrative Properties of Aqueous Solutions:
Conversion Tables”, Table 88 Sucrose

MathCAD was used to curvefit the data for Degrees Brix @ 20 C (% sucrose
by weight) and specific gravity @ 15 C

SG = 1.000898 + 0.003859118*B + 0.00001370735*B*B + 0.00000003742517*B*B*B

where:
B = measured refractivity in Brix
SG = calculated specific gravity at 15 C

This is all well and good. When I did it on a batch of beer I got a number that closely matched the number I was getting the hydrometer. As the fermentation progressed I continued measuring the gravity using the refactometer. The number didn’t go the direction it should. Instead of becoming less dense the tool said it was becoming more dense which made no sense. The sugar in the solution was being broken down into alcohol and CO2 which should always result in a less dense solution.

Eventually I assumed there must be something wrong with the refactometer and I went back to the old way of measuring. Since then the refactometer has sat in its case at the back on one of my drawers.

Then I stumbled on a forum post that explains that the brixing reading is effected by alcohol in a way that a normal hydrometer is not. You have to use a different formula once alcohol enters the picture.

Another formula has to be used.

Final Specific Gravity from original and final Brix

SG = 1.001843 - 0.002318474*OB - 0.000007775*OB*OB - 0.000000034*OB*OB*OB + 0.00574*FB + 0.00003344*FB*FB + 0.000000086*FB*FB*FB

where:

SG = estimated specific gravity of the sample
OB = Original Brix
FB = Final Brix

So it appears the problem was with the user not the tool. This is great news because it was so much easier to measure the wort using the refactometer! I guess the next batch I’ll be using it again to see if this time I have it right.

Now if only I could mount refractometer into the carboy so I could measure the wort at anytime without ever having to open the carboy… a boy can dream….