Home winemakers take a variety of measurements as they make and condition their wine. From the sugar content of grapes (in °Brix) to the specific gravity of fermenting wine to the volume in a tank, accurate information helps improve the quality of the finished product. However, if you haven’t calibrated your measuring devices, all these measurements could be off.
In this article, I’ll show you how to calibrate many of the most important winemaking tools.
Refractometers and Hydrometers
Two of the most useful tools in a home winery are refractometers and hydrometers. With either of these, you can determine the density of sugar in grape juice — or a mixture of grape (or other fruit) juice and sugar or water. Measuring the sugar content of your to-be-fermented juice allows you to estimate what the final alcohol content of your wine will be.
Once fermentation starts, you can measure its progress with a hydrometer. Measuring the final specific gravity of the wine can confirm that the fermentation has gone to completion and will give you some sense of how sweet or dry your wine will taste.
Both of these instruments can be checked simply with a single-point calibration — just see if they give the right value when testing a sample of pure water. This is all most home winemakers will ever do, or ever need to do. However, if you are concerned that your refractometer or hydrometer is giving off readings, a two-point calibration can be performed. Before jumping into how these instruments are calibrated, it pays to review the concept of density.
I Am Your Density
Density is the weight of an object divided by the volume it occupies. For example, 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 column, I’ll mostly be using metric units and will only give conversions to English units if that knowledge is useful.)
At 4 °C, water is at its maximum density. If you heat it above this temperature, it expands slightly. Oddly enough, when you cool it below this temperature, it also expands. So, the density of anything dissolved in water — in our case, sugar — is going to vary slightly with temperature. The weight of the sugar will not change when heated or cooled, but the volume of the liquid it is suspended in will. Almost all measurements of juice or wine will be made at “room temperature.” And, for everyday purposes, any reading you take in this range will be accurate enough. Be aware, though, that if you desire a high degree of accuracy, your refractometer or hydrometer reading should be taken at the appropriate temperature (which is usually written on the device).
When we use a refractometer or hydrometer, we are measuring the density of sugar in our juice or wine, but each returns this information in a different scale.
Most refractometers give a reading in °Brix, which is the weight of sugar in solution divided by the weight of the solution. If you added 10 grams of sugar to 90 grams of water, you would have a 10% (weight/weight) solution or, equivalently, a 10 °Brix solution.
Hydrometers most frequently give their readings in terms of specific gravity, which is the density of a liquid relative to pure water. Liquids that are equally as dense as pure water have a specific gravity of 1. If a solution is more dense than water, it will have a value over one. If its density is less than that of water, it will have a number between 0 and 1.
Home winemakers usually express specific gravity to three decimal places. Using that convention, the specific gravity of a liquid that was as dense as water would be 1.000. Because specific gravity is the density of a liquid relative to that of water, specific gravity has no units. In other words, the specific gravity of pure water is 1.000, not 1.000 followed by a weight and volume (such as kg/L or lbs./gallon).
A Single-point Calibration
To calibrate your refractometer, take a drop of pure water and place it on the prism, just as you would with a sample of juice. (Pure water means distilled water or water treated by reverse osmosis.) Point the meter at a light source, look through the barrel and take your reading. If your refractometer is properly calibrated, it should read 0 °Brix. If it does not read 0 °Brix, you must adjust the zero point. On most handheld refractometers, this is done by turning a screw near the prism end of the device. Often, the refractometer case will contain a small screwdriver for this purpose. (Most refractometers also allow the user to turn the barrel to focus on the screen. This does not change the zero point.) That simple procedure is all you need for a single-point calibration of your refractometer. Checking your hydrometer is also very simple.
To check if your hydrometer accurately measures the specific gravity of water, simply float it in pure water at the correct temperature. Spin the hydrometer to dislodge any bubbles that may be clinging to it and bring the test jar up to eye level.
You will see that, in the middle of the test jar, the water will be level. However, it will climb up the sides of the test jar, making the liquid surface look like a “U” or smiley face. The curved surface of a liquid in a container is called a meniscus. When reading your hydrometer, take your reading from the lowest point of the meniscus — the point where the liquid level intersects with the hydrometer scale gives you your reading.
If you’re lucky, your hydrometer reads 1.000 at the specified temperature. If it reads either higher (1.001 or more) or lower (0.9999 or less), simply add or subtract the amount of error from your readings in juice or wine. For example, let’s say your hydrometer reads 0.998 in pure water at 60 °F/16 °C (its calibration temperature). This means that it’s reading two “points” low and you should subtract two “points” from any reading you take. In other words, if a mixture of raspberry juice and sugar measured 1.090, your corrected reading would be 1.092.
Because the density of water changes with temperature, refractometers and hydrometers are meant to be used at a specific temperature (either 60 °F/16 °C or 68 °F/20 °C). In the case of your hydrometer, your sample should be at the correct temperature. In the case of the refractometer, the meter itself should be at that temperature. (The small size of the sample drop means that it will quickly converge on the temperature of the meter.)
A Two-point Calibration
Checking the reading of your refractometer or hydrometer with pure water is a single-point calibration, and this is all most home winemakers will ever do. However, what if the prism in your refractometer wasn’t ground correctly? Or what if the scale printed on the paper sleeve inside the hydrometer was compressed or elongated compared to what it should be? In both cases, the instrument could read correctly at zero, but not at higher values.
To be absolutely certain your tools read correctly across their entire relevant range, you need to do a two-point calibration. And, in order to do a two-point conversion, you need to make a solution of known sugar content. Recall that degrees Brix (°Brix) is the percentage of sugar, by weight, dissolved in a water solution. For example, if you had 25 g of sugar dissolved in 75 g of water, you would have a 25 °Brix solution.
The Second Point
If you have a (calibrated) scale, you can make a sugar solution with a density equivalent to some °Brix value in the upper range you are likely to encounter. You can use this solution to check if your refractometer or hydrometer reads correctly in that range.
So, make your standard sugar solution. Remember that, to have enough liquid to be able to float a standard-sized home wine hydrometer, you will need around 200 mL of solution total.
When you make this sugar solution, you must use sucrose (table sugar), not glucose (corn sugar). Why? Because most corn sugar has water associated with it. The most common kind of corn sugar sold at home winemaking supply stores is dextrose (D-glucose) monohydrate. What the “monohydrate” part means is that water is complexed with the sugar and makes up part of its weight. In contrast, sucrose is just plain sucrose.
If you make up a solution with a known °Brix value, and you want to test your hydrometer, you will have to convert °Brix to specific gravity. The following formula will allow you to do this:
SG = 1.001 + (3.859 x 10-3 x B) + (1.371 x 10-5 x B2) + 3.743 x 10-8 X B3)
where SG is specific gravity and B is °Brix. In our example, 25 °Brix converts to a specific gravity of 1.105. Many winemaking books have a table in which you can look up this conversion.
You can probably guess the next step — put a drop of the solution on your refractometer or float your hydrometer in it. (Remember to apply your correction from your pure water reading, if any. In our previous example, our hydrometer read two points low at 1.000, so we will have to correct for this by subtracting 2 from our specific gravity reading.)
If your (zeroed) refractometer gives the right value — in our example, 25 °Brix — then it is reading correctly. Likewise, if your hydrometer’s scale is correct, your (corrected) specific gravity reading should be correct. If your refractometer or hydrometer is off at this second point, you will have to apply a point-slope correction to all of your readings.
The easiest way to do this is to make a graph. For our example, let's say our hydrometer read SG 1.109 in our SG 1.105 solution. Corrected, this means it read SG 1.111 (26 °Brix) when it should have read 1.105 (25 °Brix).
Graphic Gravity Adjustment
Begin by labeling the x axis, the horizontal one, from 0 °Brix (or SG 1.000) to some value at the high end of your normal range. For example, if you plan to make some moderate-alcohol reds, make the scale on the x axis go from 0 °Brix (SG 1.000) to, say, 25 °Brix (1.105). If you plan on making big, blockbuster reds, or only lower-alcohol whites or fruit wines, pick a value slightly above what your strongest wine is likely to clock in at.
On the y axis, the vertical one, label your scale from the reading of your hydrometer in pure water to the same upper value as before. Following our previous example, we would label the y axis starting at SG 0.998 and extend it to SG 1.110. Now, with a ruler, draw a straight line between the two points indicated by your pure water test and the sucrose solution test. This line is the calibration curve.
In our example, our first point is (1.000, 0.998) — in pure water, in which the value should have yielded a reading of 1.000, our hydrometer read 0.998. Our second point is (1.105, 1.111) because in a solution with a specific gravity of 1.105, our (corrected) reading was 1.111.
To get corrected readings from your hydrometer, take a hydrometer reading and do not add or subtract anything. Find the value from your hydrometer on the y axis and trace a horizontal line over to the calibration curve. Now, drop a vertical line down to the x axis — the value at which the line intersects the x axis is your corrected specific gravity. (Note: you could take the slope of the line and calculate the value using that piece of information.)
So there, wasn't that easy? The correct answer here is, "No, that was a huge pain in the gluteal region. Why would you go through all that every time you use your refractometer or hydrometer?" The answer to that is, I wouldn't. Personally, I wouldn't go to these lengths unless my equipment was way off. And if it was that far off, I'd just buy a new refractometer or hydrometer. However, performing the second point check on your hydrometer is a useful way to ensure higher accuracy.
At the crush, most winemakers will measure their sugar content (usually in °Brix) and use an acid test kit to determine their titratable acidity (or TA). Some winemakers may also measure the pH of their juice, to gain added information about the acid profile.
Handheld pH meters are popular among wine hobbyists. These relatively inexpensive meters give very accurate readings, but they must 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 winemaking, 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.
Take the electrode out of the electrode storage solution. In handheld meters, the cap contains the storage solution. You should never let this dry out. If you do, you should rewet the electrode bulb in storage solution overnight before using the meter. Rinse the electrode with water, preferably distilled water. Take a tissue and wick any excess water away from the electrode. Do not touch the electrode with the tissue; just bring the tissue close enough to the bulb to draw the liquid off.
Place the electrode in the pH 7.01 solution and swirl the liquid briefly, to ensure that no bubbles cling to the electrode. Turn the meter on. (The power to the electrode should never be on if the electrode is not submerged.)
Different meters handle calibration differently. On older benchtop models, you placed the electrode in the pH 7.01 solution and turned one knob until the meter read 7. Then you turned the power to the electrode off, rinsed it, placed it in pH 4.01 solution and turned another knob until the pH read 4. Newer pH meters have calibration modes. You basically just place the electrode in a pH 7.01 solution until the the meter says to switch to a pH 4 solution and the meter’s electronics does the rest. Your owner’s manual will give the details.
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.
Knowing the volume of juice or wine in your vessels can be important. If you are a kit winemaker, you need to have a bucket that has a six-gallon (23-L) mark, so you know how much water to add to the concentrate. As a grape winemaker, you’ll need to know how much juice you collect from the crush so you can calculate how much sulfite powder and yeast to add. And, no matter what form of winemaking you pursue, you need to know your volume of wine to calculate how many bottles it will require.
The basic idea for calibrating winemaking 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 fermentation bucket and mark the outside of the bucket with a permanent marker. Likewise, you could do this in a carboy and place a piece of tape on the outside that corresponds to that level. Repeat this process five more times to mark the 2-, 3-, 4-, 5- and 6-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. Scientists use volumetric glassware for this. Unfortunately, good volumetric glassware is very expensive. For home winemaking, we need something that is reasonably accurate, but much cheaper. Fortunately, just such a thing is sold at many home winemaking shops — a graduated cylinder. Chemists use graduated cylinders when they need a measure of volume more accurate than that stamped on the sides of beakers and flasks, but not so accurate that they need to drag out their expensive volumetric flasks.
For home winemakers, 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 applications.
Calibrating a 5-gallon (19-L) carboy by pouring in 250 mL at a time would be very tedious. You’d need to do this almost 75 times to get to the 18,927-mL (18.9-L/5-gallon) mark. 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 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. Or, you can mark them in increments of liters.
If, however, you ferment your wine in a stainless steel tank or condition it in a barrel, marking the outside of the tank won’t be possible. For any vessel that is not see-through, you can make a dip stick. To do this, simply add your water incrementally and mark off notches on your dipstick corresponding to the liquid level. You’ll need a different dipstick for each vessel (even if they are the same type of vessel).
Some stainless steel tanks may come equipped with sight glasses, and volume marks can be painted on the sight glass.
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 winery.
Most home winemakers probably have a few thermometers. We may have bimetal thermometers for our fermenters, glass or digital thermometers for taking spot measurements and “temperature strips” stuck to the side of our buckets or carboys. Many home winemakers, however, may be unaware of how inaccurate thermometers can be. Cheap thermometers can be off by as much as 20 °F (11 °C)! As such, every home winemaker should know how to check and calibrate their thermometers.
Serious home winemakers will probably want to 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.) 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 several 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?
Most decent cookbooks will have a table of the temperature of boiling water at various altitudes. 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, the simplest way to do this is to measure the temperature of boiling water when your local barometric pressure is between 29.6 and 30.2 inches of mercury (in. Hg). This way, you’ll be off by a 1?2 degree Fahrenheit (0.28 °C) at most. (There’s also a web site that can do these calculations for you —www.biggreen-egg.com/boilingPoint.htm.)
Incidentally, you may have wondered why we took both altitude and barometric pressure into account when we measured the boiling point of water. The boiling point of water varies depending on the amount of air pressure on the surface of the water. Air pressure (barometric pressure) decreases with altitude, of course, but some of you may have wondered why we didn’t just apply the adjustment for barometric pressure and forget about altitude. This would be the correct approach except for one thing; local weather services express all barometric pressure readings as if they were taken at sea level. Therefore, when you measure the boiling point of water at your house, you need to “decorrect” for the fact that your barometric pressure reading is corrected for altitude by the weather service. For most home winemakers, a simple check of the water at freezing point of water will suffice.
With all your winery tools and vessels calibrated, you can focus on the artistic side of the hobby, knowing the technical aspects are taken care of.
Chris Colby, a former chemistry major, is the editor of WineMaker magazine.