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Calibration and Conclusion

Homebrewers have a variety of tools at their disposal — including hydrometers, thermometers and pH meters — to measure important variables during the brew day. It is important that these be calibrated, so that you can have some confidence in your measurements. Fortunately, calibration is not very difficult. Here’s how:

One theme running through this collection of articles is that we can’t always easily measure what we want in homebrewing. However, there are several instruments with which we can get adequately precise measurements. In this article, we’ll learn how to calibrate the key tools in a homebrewing lab.

It’s the (Density of) Water

Density is the weight of an object divided by the volume it occupies. Water has a density of one kilogram (kg) per liter (L) at 4 °C. In other words, if you had exactly 1 L of water at 4 °C and placed it on a (properly calibrated) scale, it would weigh exactly 1 kg. Expressed in English units, the density of water is roughly 8 lbs. 5.5 oz. per gallon. (In this article, we’ll mostly be using metric units and will only give conversions to English units if that knowledge is useful.)

When we use our hydrometers, we are measuring the density of extract in our wort or beer. (“Extract” here means dissolved solids, not malt extract.) Homebrewers tend to express this in terms of specific gravity, which is the density of a liquid relative to pure water. Liquids that are equally as dense as water have a specific gravity of 1.

Single-Point Calibration

If your hydrometer is properly calibrated, it should read 1.000 when floating in pure water. Because the density of water changes with temperature, hydrometers are meant to be used at a specific temperature (either 60 °F/16 °C or 68 °F/20 °C). This temperature should be printed on the slip of paper inside the hydrometer. Tables that take temperature into account can be found in most beginning homebrew books.

Two-Point Calibration

Checking the reading of your hydrometer in pure water is a single point calibration, and this is all most homebrewers will ever do for their hydrometers. However, what if the hydrometer read correctly at 1 but the scale printed on the paper sleeve inside the hydrometer was compressed or elongated compared to what it should be?

To check to see if your hydrometer reads correctly in the range you use it in, do a two-point calibration. If you have a (calibrated) scale, you can make a sugar solution with a density equivalent to the average density of your wort. You can use this to check if your hydrometer reads correctly in that range.

The final step is knowing the degrees Plato (°Plato) is the percentage of sucrose (table sugar), by weight, dissolved in a water solution. For example, if you had 10 g of sucrose dissolved in 90 g of water, you would have a 10 °Plato solution — i.e. 10 g of sugar in a solution that weighs 100 g overall is 10% sugar (w/w).

There is a quick and dirty way to convert between degrees Plato and specific gravity — just multiply the value in degrees Plato by four to get the value in “gravity points.” Conversely, you can divide the number of “gravity points” by four to yield the value in degrees Plato. For example, the 10 °Plato solution mentioned before would have a specific gravity of 40 “gravity points — 1.040.

This “times 4” rule is only an approximation however, as specific gravity and degrees Plato do not have a linear relationship. A 10 °Plato wort really does have a specific gravity of 1.040. However, as you get farther away from 10 °Plato, this approximation gets less accurate.

So let’s say you brew mostly pale ales and porters and your target original gravity is SG 1.048. A specific gravity of 1.048 is equivalent to 12 ° Plato. (Actually 12 °Plato is 1.04838, but the difference here is only 0.38 “gravity points.”)

If you dissolve 12 g of sucrose in 88 g of water, you will have a 12 °Plato or SG 1.048 solution. (And actually, to have enough liquid to be able to float a standard-sized homebrew hydrometer, you will need a solution consisting of 24 g of sucrose and 176 g of water.)

When you make this sugar solution, you must use sucrose (table sugar), not corn sugar. Why? Because the type of corn sugar (glucose monohydrate) sold at homebrewing stores usually has water associated with it. As such, part of the weight of the sugar is due to the attached water molecule (the “monohydrate”).

If you have a refractometer, you can calibrate it in the same manner. In this case, using sucrose is required because °Plato (the unit that most refractometer read in) is defined as a weight to weight percentage of sucrose in water.

Volume

Many times during the brew day and later during fermentation, it pays to be able to accurately measure the volume of your wort. Likewise, it is worthwhile to calibrate all of your brewing vessels so you can read the volume of liquid in them anytime during the brew day.

The basic idea for calibrating brewing vessels is simple — add a known volume of water to the vessel and make a mark at that level. For example, you could pour a gallon of water into your carboy and place a piece of tape on the outside that corresponds to that level. Repeat this process four more times to mark the 2-, 3-, 4- and 5-gallon marks. The only catch to the above plan is — how do we measure exactly one gallon?

Standard kitchen measuring cups are not very accurate. (Neither are the hash marks printed on the outside of your brewing bucket.) What you need is something that measures volume accurately. For homebrewers, a 250-mL graduated cylinder will work well (and can double as a hydrometer test jar). A decent graduated cylinder will say how accurate it is. Mine says 250 mL +/- 2 mL at 20 °C. So, it’s accurate to about 1% — which should be good enough for most homebrewing applications.

To help in calibrating larger vessels, I like to make an intermediate calibrated vessel. A one gallon (3.8 L) milk or water jug works well for this. Pour 250 mL in it almost 16 times, and you can measure out 3.79 L or 3,790 mL (1.00 gallon). Mark the 1-gallon mark on the jug and then use it to calibrate your larger vessels.

To calibrate brewing buckets, you can use a permanent marker to write on the outside of the bucket. For carboys, labeled pieces of tape can be placed at every gallon (or half-gallon) mark. For water tanks or other vessels with sight glasses, volume marks can be painted on the sight glass. For any vessel that is not see-through, you can make a dip stick.

Scales and Balances

With a reasonably accurate 250 mL graduated cylinder, you can easily make 1 L of water — 4 X 250 mL = 1 L. Recall that 1 L of water at 4 °C (refrigerator temperature) weighs exactly 1 kg. With this information, you should be able to calibrate any scales or balances in your brewery.

Thermometer

Most homebrewers probably have a variety of thermometers. Many homebrewers may be unaware of how inaccurate thermometers can be. Cheap thermometers can be off by as much as 20 °F (11 °C). Even more expensive thermometers can be off enough to make a difference in brewing. As such, every homebrewer should know how to check and calibrate their thermometers.

Every serious homebrewer should get one good thermometer — a laboratory-grade mercury thermometer or good digital thermometer — and use this to check and adjust their working thermometers. However, even the most expensive thermometers should be checked for accuracy.

To check a thermometer, you should take the temperature of two solutions that you know the temperature of. The Catch-22 here is that, without a calibrated thermometer, how do you know the temperature of a solution? The answer is you rely on the physical properties of water to supply you with two set points.

The best place to start is at the freezing point of water. Pure water freezes at 32 °F (0 °C). If you can make a solution of ice and water right at that point, you can check if your thermometer reads right at freezing. To make a 32 °F (0 °C) solution, do the following:

Take a clean styrofoam cup and fill it with crushed ice, heaped to the top. (Technically, the ice should be made from distilled water, but using tap water won’t affect your result by enough to matter in brewing.) Don’t add any water to the ice. Put the cup in your refrigerator and wait until enough ice melts to submerge your thermometer to a depth that is adequate to take a reading. (Glass laboratory thermometers will have an immersion line showing how far the thermometer tip should be submerged.) It will take a few hours for the ice to melt to this point, so plan ahead. You want to take the temperature of a solution with a lot of ice and just enough water to take a reading.

Note that you can’t just take a (warm) cup, add (warm) tap water, plunk down a few ice cubes and expect the temperature to be 32 °F

(0 °C). Waiting for ice to melt ensures the resulting ice and water mixture is right at the freezing point as long as the amount of ice is much greater than the amount of water in the mix.

Once the ice water is prepared, take the temperature of the solution. Remember that your thermometer is warmer than the ice water and will warm the local area it is inserted into, so swirl the tip of the thermometer a bit as you take the reading. Keep the thermometer in the slush until it gives a steady reading.

The second point to measure is the boiling point. Water boils at 212 °F (100 °C) at sea level, at standard barometric pressure (29.9 inches of mercury (in. Hg)). But what if you’re not at sea level and standard barometric pressure? If you don’t know your altitude, you can find out at the US Geological Survey’s Geographic Names Information System (or the USGS’s GNIS, for acronym lovers) Their web site is located at geonames.usgs.gov (no “www”).

If you wish to take barometric pressure into account, you can find your local barometric pressure at the weather channel (www.weather.com) or from a home barometer — if it’s calibrated! Alternately, take your reading when your local barometric pressure is between 29.6 and 30.2 inches of mercury (in. Hg) and you’ll be off by a 1⁄2 degree Fahrenheit (0.28 °C) at most.

Once you’ve calibrated your best thermometer using the methods above, use it as a reference to calibrate the rest of your thermometers.

pH Meters

Handheld pH meters are popular among homebrewers. These relatively inexpensive meters give very accurate readings, but should be calibrated every time they are used. (The same goes for expensive bench-top meters.)

The meter is calibrated by placing the electrode in solutions of known pH and ensuring that it reads right. For calibrating, you will need a pH 7.01 buffer and a pH 4.01 buffer — which are often sold as a kit, along with electrode storage solution, wherever pH meters are sold.

Calibration is important because pH meters will “drift” between uses — and sometimes within a long day of taking pH readings. If you don’t calibrate them, their pH readings will change over time. Also, pH electrodes eventually wear out. Letting their storage solution run out will speed their demise. If the readings of the meter bounce around and won’t settle down, or if the meter quickly goes out of calibration, this may be a sign that a new electrode is needed. As a quick check on how stable your pH meter is, take the pH of your two buffer solutions at the end of any session of pH measurement. If the meter doesn’t give their pH values as 7 and 4, the meter has drifted and will need to be recalibrated.

Once you’ve calibrated the equipment in your brewery, you will know that your readings of temperature, specific gravity, volume and weight are accurate. This knowledge can help you to consistently brew the best beers possible.