RANDOLPH, Vt.—Sugarmakers should use hydrometers carefully, according to University of Vermont Extension.

A hydrometer is a thin glass tube with printed graduations that floats in syrup to measure density. Cold syrup makes it float higher, while hot syrup makes it sink lower.

Most hydrometers have “hot test” and “cold test” lines. The cold test line shows the minimum legal density at the calibrated temperature, while the hot test line helps estimate density in syrup coming directly from the evaporator.

Knowing the syrup temperature is critical. Syrup that has cooled can appear denser than it actually is, resulting in syrup below the legal density. In Vermont, the legal density is 66.9 degrees Brix.

Hydrometers are delicate and should be checked periodically for accuracy. When buying a new one, producers should look for hydrometers tested and certified by the state of Vermont.

Read the point where the syrup crosses the stem, not the curved surface known as the meniscus. Lower the hydrometer gently into the syrup rather than pouring syrup over it.

Keeping the hydrometer clean is essential. Syrup, sugar crystals or niter clinging to the glass can affect readings. Even half a gram of extra weight can throw a reading off by about 2 degrees Brix.

Mary Fogle Douglass of Sugar Bush Supplies in Mason, Mich., said the most common problem is the paper scale slipping inside the glass. Hydrometers can also break if dropped too hard into the test cup.

Douglass recommends rinsing hydrometers often with distilled water, permeate or vinegar during a boil to prevent sugar sand buildup.

“You want it to be a nice clear glass,” she said.

Longer hydrometers are recommended because they have more detailed graduations.

“The more marks, the more accurate you will be,” Douglass said.

Overall, hydrometers are reliable and can last for decades.

“Hydrometers for their cost are very accurate,” she said. “It’s the go-to way to check your density.”