by Brain Dixon (Brewing Techniques)
Even if you don’t have access to high-end instrumentation, you can use standard solutions to calibrate your hydrometer and make your specific gravity readings more accurate.
If you’re like me, you care about how accurately your measurement equipment measures what it is supposed to measure. Many small-scale brewers use low-end hydrometers that cost from $ 3–10, and a healthy concern for their accuracy is appropriate and natural. Combined with accurate temperature correction, a little calibration work with your hydrometer can go a long way towards helping you obtain accurate readings over a broad range of specific gravities and temperatures.
The best way to calibrate a hydrometer is to compare the reading to an actual wort solution with a known gravity and a sugar profile similar to the beer (or wort) you are testing, but this is impractical for most brewers, and that level of accuracy is not really necessary or even attainable with floating glass hydrometers. The best approximation for many brewers is thus derived from using simple table sugar, which is desirable for making standards because it is closer to wort in its characteristics (surface tension and meniscus thickness) than other solutions such as those made with salt (NaCl), sulfuric acid, or other dissolvable compounds. Making a series of solutions of various gravities and determining the correction necessary at each gravity allows you to construct a calibration curve, so that you can apply the appropriate correction at any point in your hydrometer’s range.
To make a standard solution, you will need an accurate scale. The scale should be one that can measure a wide range of weights, say up to 5 or 6 kg (10 or 12 lb) or so, but is still able to accurately register small differences in weight. My electronic scale, which cost around $ 45, can weigh up to 5 kg, and measures accurately in 5-g increments over its whole range.* In 5 kg of solution, a measurement error of about 2.5 g (half the smallest increment) results in an error of less than half a specific gravity point (0.0005). Although this is probably fine for home brewing equipment, you want to use the finest scale that you can find if you either desire better accuracy or are calibrating a hydrometer for professional use. It’s probably also worth mentioning that the highest accuracy is obtained by using the best water, that is, water that has the correct density (1.000). You can achieve this by degassing your own water; simply bring it to a boil, then cool it.
*You might argue that for the price of a sufficiently accurate scale, you could buy a better hydrometer. Indeed, you could, but to cover the range of gravities that you can cover with a cheaper, but calibrated, hydrometer, you’d have to buy a few narrow-range hydrometers, or invest in a refractometer, a pycnometer, or a spectrophotometer. A scale like mine is suitable for many other uses. I use mine for baking and canning and for weighing out my grain before milling it.
Make a standard solution by carefully weighing out a predetermined amount of sugar, then adding clean water until the correct weight of solution is reached. For example, a 12 °P solution contains 12 g of dissolved sugar in every 100 g of solution. To make 100 g of a 12 °P solution, you first measure 12 g of sugar into a container, then add enough water to bring the total weight to exactly 100 g. To make a solution of a certain specific gravity, you first convert the desired specific gravity to °P, then create the solution as described. See the box “Understanding and Converting Density Measurements” on page 44 for instructions on how to do the conversion.
Most scales measure most accurately over just one portion of their range, and this is something you should try to discover about your scale prior to making your standard solutions. Larger (heavier) solutions tend to minimize the effect of the scale’s measurement error by making it a smaller percentage of the solution’s total weight. But if the upper end of your scale produces unacceptably large measurement error, then you should use a smaller (lighter) solution. Table I gives sugar and total solution weights for six standards that span the range of specific gravities that most brewing hydrometers cover. (It is advisable to take measurements at several different data points because many hydrometers will have different correction factors at different gravities.) Weights in the table are given for both 1 kg and 1 lb of solution. You can multiply or divide these amounts as necessary to produce the amount of solution you desire.
Once your standard solution is made, you need to adjust its temperature to the temperature that your hydrometer was calibrated at by the manufacturer (it should say so right on the hydrometer’s scale). Temperature correction is necessary because the density of water varies with temperature, and wort is mostly water. Most hydrometers are calibrated at 60 °F (15.6 °C), but some are calibrated at other temperatures such as 58 °F (14.4 °C), 68 °F (20 °C), or even 70 °F (21.1 °C). At these temperatures, 5 °F (2.7 °C) too warm reduces the apparent specific gravity by about half a point (0.0005) and 5 °F too cool increases it by about a quarter of a point (0.00025), so it’s better to err on the cool side. Of course, to correctly judge the temperature, you will need an accurate thermometer. A thermometer is easier to calibrate than a hydrometer because (assuming it covers the range from freezing to boiling) you can check its accuracy by seeing what it measures in a mixture of crushed ice and water and also at boiling. Then you can adjust the readings in between accordingly.
Determining your hydrometer calibration is now very easy. Just fill your hydrometer test flask with the standard solution you made, verify the temperature, and carefully measure the specific gravity with your hydrometer, using the guidelines outlined below. The difference between your measurement and the actual specific gravity of the solution is your correction factor. Take this factor into account for all readings at similar gravities. Repeat this procedure with solutions at several gravities, plot the results on a piece of graph paper, and you’ll have a calibration curve that will give you the correction factor at any gravity.
You should take all your hydrometer readings at the temperature at which the hydrometer was calibrated. If you take readings at a different temperature, you will have to apply a correction factor, which varies with the temperature and with the specific gravity of the solution. At 80 °F, for example, the readings on a hydrometer that was calibrated at 60 °F will be low by about 3 points for a medium-gravity beer. Your hydrometer may come with a correction chart, but be sure the chart takes gravity into account; if it does not, seek out a published table of gravity-specific corrections (see Table II for an example).
All of the guidelines mentioned above will do you no good if you are not reading your hydrometer in a correct and consistent way. First of all, readings should be taken at the same point each time, optimally at the top edge of the meniscus. Also, because you generally have a rough idea of what the specific gravity should be, you can gently lower the hydrometer into the the solution to approximately the right point or reading, and then let it go, giving it a spin as you do to release any bubbles that may be clinging to the hydrometer. If you hold your hydrometer higher before releasing it to float in the solution, then it bobs too deeply into the liquid, causing liquid to cling to the stem above the meniscus, adding extra weight to the stem. This in turn causes your hydrometer to float too deeply, consequently giving you a falsely low reading. If you wait 10 or 15 minutes before reading the hydrometer in such a case, you’d be surprised at how much liquid will coalesce on the stem of the hydrometer. You should always carefully rinse your hydrometer before using it, and the ASBC recommends that it also be rinsed in the fluid being tested.
The hydrometer itself can introduce error if it is improperly designed. If the stem of your hydrometer is not straight, for example, or if the printed scale inside your hydrometer is crooked, then the meniscus will cut the scale at an angle. Similarly, if the glass in the stem is not of a consistent thickness, if the stem itself is not of a consistent diameter, or if the weight in the bottom of the hydrometer is not evenly distributed across the bottom of the tube, your readings will be off. Sadly, many low-end brewing hydrometers have at least one of these problems. I have taken digital calipers and micrometers and made many measurements on several hydrometers, and the results are disappointing, sometimes to the tune of 7 or 8 gravity points difference at higher gravities.
The bottom line is that calibration can help you obtain more accurate readings from your hydrometer, but if you are looking for accuracy greater than 2–3 points, you will want to buy a more precise hydrometer(s) which might be calibrated for narrow ranges. Also keep in mind that when you are taking readings on fermented wort, the alcohol, which has a density less than that of water, will skew the reading. Thus, other methods must be used to obtain accurate readings at this point in the brewing process. A good-quality refractometer, prism, and scale will cost you from $ 100 to $ 200. Pycnometers and spectrophotometers are extremely accurate but require some knowledge of lab techniques.
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