Section 5 – Appendices
Appendix A – Using Hydrometers and Refractometers
Hydrometers
A hydrometer measures the difference in gravity (density) by flotation. A hydrometer is used to gauge the progress of fermentation by measuring one aspect of it—attenuation. Attenuation is the degree of conversion of sugar to ethanol by the yeast. Water has a specific gravity of 1.000, and a typical beer wort has an original gravity (OG) of 1.035-1.060. Beers typically have a final gravity between 1.015 and 1.005. Champagnes and meads can have gravities less than 1.000, because of the large percentage of ethyl alcohol, which has a specific gravity of less than 1. Hydrometer readings are always quoted to the standard temperature 59°F (15°C). Liquid density is dependent on temperature, so hydrometer readings are adjusted to state the gravity of the liquid at this standard temperature.
A hydrometer is a useful tool in the hands of a brewer who knows what wort gravity is and why he wants to measure it. Beer recipes often list the Original and/or Final Gravities (OG and FG) to better describe the beer to the reader. For an average beer yeast, a rule of thumb is that the FG should be about 1⁄4 to 1⁄5 of the OG. For example, a typical beer OG of 1.040 should finish about 1.010 (or lower). A couple of points either way is not unusual.
It needs to be emphasized that the stated FG of a recipe is not the goal. The goal is to make a good tasting beer. The hydrometer should be regarded as only one tool available to the brewer as a means to gauge the fermentation progress. The brewer should only be concerned about a high hydrometer reading when primary fermentation has apparently ended and the reading is about one half of the OG, instead of the nominal one forth. Proper yeast preparation should prevent this problem.
Beginning brewers often make the mistake of checking the gravity too frequently. Every time you open the fermentor, you are risking infection from airborne microbes. Check the gravity when you are ready to pitch the yeast, then leave it alone until the bubbling in the airlock stops. Checking the gravity in-between will not change anything except to possibly contaminate it. Also, always remove a sample of the wort to test it. Don’t stick the hydrometer into the whole batch. Use a sanitized siphon or wine thief (turkey baster) to withdraw a sample of the wort to a hydrometer jar (tall, narrow jar) and float the hydrometer in that. There is less chance of infection and you can drink the sample to see how the fermentation is coming along. It should taste like beer even though it may taste a bit yeasty.
The hydrometer temperature correction table is shown at right. Hydrometer readings are standardized to 15°C (59°F). When discussing specific gravities of worts and beers with other brewers, always quote the standardized value. Measure the specific gravity of your wort, take the temperature, and add the correction value (delta G) given in the table. The correction number is added to the specific gravity number, 1.0XX.
Example: If the wort temperature is 108°F, and the measured gravity of the sample is 1.042, the delta G value would be between .0077 and .0081. Rounding it off to the third decimal place gives us .008, which is added to 1.042, yielding 1.050 as the standardized reading.
Refractometers
The benefit of using a handheld refractometer to measure wort gravity is that it only takes a couple drops and you can get the reading in seconds. They are very handy to have at the kettle to check your extraction when you are all-grain brewing. The only problem is that they do not measure wort specific gravity.
Refractometers measure the refraction of light passing through a solution. The more dense a solution is, the slower the light will travel through it and the more the light will be refracted. Handheld refractometers are calibrated with respect to the density of sucrose solutions at 20°C (68°F). The scale in the viewing window of a refractometer is scaled in degree Brix, which is equivalent to degrees Plato. The only problem is that the wort is not made of sucrose, it is made up of several different sugars and has a slightly different refraction index than a pure sucrose solution. There are industry standard equations that allow you to convert between the various scale with a reasonable degree of accuracy.
One equation to adjust the gravity between °Brix and the °Plato of your wort is:Wort (°Plato) = Refractometer (°Brix) ÷ 1.04So a wort’s refractometer reading of 12°Brix is more closely 11.5 °Plato.
Okay, Plato, Plato, what is °Plato? Well °Plato (like Brix) is defined as the weight percent of sucrose in solution. So, a wort of 10°Plato is 10% by weight sucrose. The specific gravity of that sucrose solution is approximately 4 times the °Plato, or 1.040. But this approximation becomes less accurate after you exceed 13°Plato. Fortunately, the American Society of Brewing Chemists has published a conversion table, and I have reproduced some of that table here. See Table 31.
Figure 165 - It's best to fill the jar completely, because you want a clear view of the waterline for the measurement.
Table 30 - Hydrometer Temperature Corrections
Measure your wort temperature and add the correction (delta G) to the reading that you see at the waterline.
Figure 166 – A refractometer. You place a drop of wort on the window under the lid, then hold it up to the light and look thru the eyepiece to see the reading.
Table 31 - Specific Gravity – °Plato Conversion
Here is a quick reference chart for converting Specific Gravity to °Plato. Balling, Plato, and Brix are basically equivalent and are the preferred gravity unit of large scale brewers.
Reference: ASBC Laboratory Methods for Craft Brewers, American Society of Brewing Chemists, Inc., St. Paul, Minnesota, 1997.