Basics of Hydrometers and Refractometers

When making maple syrup, measuring its density is necessary for safety and regulatory matters. Reliable sugar bush performance and maple production can be easily accomplished with a hydrometer or a refractometer.

Difference between hydrometers and refractometers

Understanding how a hydrometer and refractometer measure differently is critical when determining the right tool for each circumstance. “They’re measuring the concentration of sugar, but they’re doing it in two different ways,” Kathryn Hopkins, Extension professor at the University of Maine Cooperative Extension, said. “The hydrometer [measures] the percent of sugar in a sugar solution. Refractometers [measure] the change in the angle and speed of light as it passes through the sugar solution.”

Targeting the right threshold

Marked with two red lines on the stem, and assuming a hydrometer is calibrated correctly, Hopkins explained that as long the measurements are taken at the right temperatures, the red marks will give an accurate density reading.

However, Hopkins explained that if you were to measure, for example, at 180 degrees Fahrenheit, or another temperature reading, you would have to do a little math to figure out the actual density. “You can’t just read right off the hydrometer stem, unless you’re right at 211 or 60 degrees Fahrenheit,” she said.

Hopkins explained that for maple producers to maximize their “hot test,” an evaporator that can be set to 7.1 degrees above the day’s boiling point of water will help them achieve a minimum of 66 degrees Brix. Although the generally accepted temperature of boiling water is 212 degrees Fahrenheit, it does often vary depending on the atmospheric pressure. It’s important to determine the baseline of the water’s boiling point, and add the 7.1 additional degrees to turn sap into maple syrup that has a minimum of 66 degrees Brix every time maple syrup is processed.

Refractometers measure the change in the angle and speed of light as it passes through the sugar solution.

Understanding a refractometer

Refractometers work differently; based on those considerations, producers can use them for similar and different sugar bush needs. “In a refractometer, it’s a prism in there. It refracts the light based on the concentration of solids inside the liquid and bends the light differently as more solids are in the liquid,” Jim Finley Ibberson, professor of Forest Resource Management Director Center for Private Forests at Penn State Pennsylvania Extension Forester, said. “So a refractometer for sap might be calibrated from 0 to 6 or 10 percent sugar [and] one for sap might be calibrated from 50 percent to 75 percent sugar.”

Finley explained how testing for sap with a refractometer can help a sugar bush operator determine which trees have sweeter sap and which trees produce sap with lower sugar levels. With different species of maple trees producing different levels of sugar, such as sugar maple trees producing more than red maples, and genetics, weather and “growth rate during the previous summer” impacting sugar – sap concentration, sap testing can help. This is where using a refractometer can help a sugaring operation determine a tree’s sap sugar content for management practices.

Finley said that this method can help sugaring operations increase productivity by identifying trees that produce sweeter sap early on.

If I go in the sugar bush today and I tested 50 trees in an hour, those that produce the best sugar in that hour are likely to continue to produce better sugar all the time. So I would say that’s a good tree; [as for] a poor tree, I’ll cut the poor tree and leave the really good tree.”

This process would be done before tapping occurs, for example, when trees are “4 inches in diameter; but thin the trees to increase the depth and width of crowns – so that they become better sugar producers.”

Finley explained because of the hydrometer’s inherent requirement for a sizeable quantity of syrup to test density, a refractometer used for sap would enable him to inject a needle into sample maples in his sugar bush and be able to test the sap’s density with a refractometer.

Calibration concerns

Regarding refractometers, temperature still impacts the density of a liquid, expanding as it gets warmer and contracting as it gets cooler. Hopkins recommended, “When you go to buy one, you should look for one that’s automatically temperature compensated. Then you don’t need the conversion card or you don’t need to do the math.”

Other ways to calibrate a handheld refractometer as Hopkins said is to see if there are “control” prisms or “certified solutions” that can provide you with a baseline reading. Used in conjunction with a screw that many often have, manufacturers give a greater opportunity for more accurate readings if calibration is doubted.

Hopkins explained that if a hydrometer is not lowered into a cup of syrup slow enough, it can sink and the stem may be coated as it bounces back up to float. With the extra syrup coating the stem, it could provide a false reading with the extra weight.

Whether testing a freshly boiled batch or conducting sugar bush management, taking time to determine what type of hydrometer is right at various times of the year can translate into some sweet sales.