In the last “How do I do that?” we learned how to use a hydrometer and convert the resultant specific gravity readings to percentage alcohol by volume. This method is pretty accurate, and pretty easy as well as being relatively cheap in that hyrdrometers aren’t particularly expensive instruments. So why would anyone need to resort to purchasing a more expensive one that ultimately does the same thing?
The answer is efficiency.
Hydrometers — though relatively accurate — require a sizable sample of wort to take a reading. Brewers conscientious of sanitation know that it’s best to discard the sample once a reading has been taken. This wastes precious wort, especially in the later readings used to determine whether the beer is fully attenuated. Let’s say – conservatively – that a sample size uses three ounces. If four samples are taken, a whole 12 oz. beer is wasted.
Another issue is temperature. An accurate hydrometer reading requires wort to be cooled to a temperature of 60° Fahrenheit. And while we know from the previous, hydrometer focused posting that temperature can be corrected using equations or brewcalcs, it still takes time without access to efficient ways of chilling wort to cool it enough that a reading can be taken safely. Boiling wort can be dangerous, even if it’s just three or four ounces.
So what does all this have to do with Refractometers? That’s the nice part — they don’t need more than a drop for a sample, and that drop will cool enough to handle much more quickly than the hydrometer sample would. Beyond that, if your refractometer automatically temperature corrects (ATC) — most decent ones are indeed ATC — then you get an accurate reading regardless of how hot or cold the liquid is.
How does it work? This is where things take a turn toward the complicated.
Refractometers contain a prism that when exposed to sun light, and calibrated properly can measure the relative sugar weight of a few drops of liquid placed between the light source and the prism. It reacts differently to light depending upon the amount of sugar that is available in the liquid sample held between the prism and a plate that protects the sample called a daylight plate. The reading is displayed on a scale viewable through the refractometer’s eyepiece.
Calibration usually involves using pure, distilled water to essentially “zero out” the device. Since distilled water should read as zero Brix — the unit of measure associated with refractometry — it’s a baseline against which readings of less pure liquids can be made.
Instructions on how to properly calibrate your device should come with it. As refractometers aren’t designed the same universally, I will refrain from explaining how this is done. Of course, you should always read instructions or manual before using any new device.
But how do we use one?
To use a refractometer, place a few drops of liquid on the prism assembly, making sure there are no air bubbles to obscure and distort the reading. Close the protective daylight plate, and point the device toward a natural light source; the sun is always your best bet. Fluorescent lights will not provide an accurate reading, and should be avoided. Incandescents — while not ideal — can still give a usable reading.
Assuming the device is properly calibrated, a few drops of wort — either cooled to 68° F. or automatically temperature corrected (ATC) — will display the relative sugar weight (Brix) of your wort.
Again, you can convert the Brix notation to specific gravity at brewcalcs, and once an original gravity reading (OG), and a stable, final gravity reading (FG) are obtained, you can convert to percentage alcohol by volume, or weight or whatever measurement floats your boat.
The refractometer’s main advantage lies in the fact that it provides a less wasteful means of calculating ABV. If you can swing $89 or more for a decent specimen with ATC, I highly recommend it. Because even an ounce of wasted beer is one ounce too many.
Tags: Brix to specific gravity, Calculating ABV, how to use a refractometer, Refractometry