“Blow test?”

How do you determine the point at which your syrup is done? How do you measure the density of your finished product? How do you determine that it’s within Maine’s required standard? The current Maine density standard for maple syrup is, I have read, a Brix value—sugar content—not less than 66° Brix, or more than 69° Brix. I suspect most producers, interested in maximizing syrup production, aim for the minimum standard of 66° Brix which, of course, requires precise measurement and flirts with being outside—less than 66° Brix—the lower end of the acceptable range. But how do you measure it? What are the measurement methods?
Well, producers have options: a simple thermometer is one. A syrup hydrometer, a hydrotherm, and a refractometer are common. And more methods exist. Pick your poison. None are accurate. All involve a “moving target.” But most choices, used carefully and prudently, and with consideration for their limitations—some methods are more consistent and accurate than others—will produce reasonable results, i.e. “close enough.” (Actually, 67° Brix looks and tastes noticeably better than 66° Brix.)
First, a thermometer. Syrup at 66° Brix boils at 7.1 degrees Fahrenheit above the temperature of boiling water. Water boils at 212° F at sea level. At any other elevation the temperature of boiling water must be determined somehow (moving target). So, unless you have an automatic drawoff that continually adjusts for barometric pressure, good luck. (If you can afford such an instrument why are you using a thermometer to determine the drawoff point in the first place?)
Second, syrup hydrometers. Syrup hydrometers have pretty-much the same problem. In this case, a change in the syrup temperature (moving target) is cause for a change in the target hydrometer reading. Syrup hydrometers are calibrated in Vermont for hot (211° F) and cold (60° F) syrup with a red mark. Vermont, however, is not Maine. Vermont standard syrup is 66.9° Brix. (As I just said, it tastes better at a higher sugar content.) The NAMSC Maple Syrup Producers Manual alleges that syrup hydrometers are quite accurate—when properly calibrated. Good luck if you make syrup in Maine.
On the other hand, the Ayres hydrotherm, invented in the 1930s by Henry Fairfax Ayres of Addison, Vermont , graduate of the U.S. Military Academy at West Point in 1908, employs a column of red liquid that indicates the point at which syrup is at standard Brix at any syrup temperature. No need to calibrate it. Just wait thirty seconds for the column to adjust. Ayres, however, calibrated his hydrotherm at 65.8° Brix, a dite under Maine’s standard and more than a dite under Vermont’s. (again, moving—but consistently so—target). For Mainers, this bit of target practice with Colonel Ayres’ hydrotherm requires the top of the red liquid column to rise slightly above the surface of the syrup, perhaps to the bottom of the meniscus rather than the top. What Vermonters who use the hydrotherm do, I don’t know. Experienced producers, however, defend the hydrotherm as a simple, quick, and consistently accurate device at any temperature.
A hand-held refractometer is also relatively simple to operate. But refractometers, though they allow precise measuring to within 0.1° Brix, are also inaccurate, especially so with hot syrup. The NAMSC in its Maple Syrup Producers Manual suggests that “the use of refractometers with boiling syrup is not advised.” Something to do with continued evaporation of the syrup while the test on one drop is being conducted. Research by the Proctor Maple Research people confirms a variation in refractometer readings using the same sample. Personally I haven’t gone near a refractometer since I submitted two samples of the same batch of syrup to a contest. The two density readings differed by 1.5° Brix, a fairly substantial amount I thought. So I stay away from refractometers.
So there you are. Pick one. Does anyone use the blow test? As for me I use a scoop. Lift a bit of syrup out of the pan with a scoop and watch it sheet off the edge. If it looks like syrup and flows like syrup, it usually is syrup. I verify with a hydrotherm. (See Maple News June 2012 for more on the hydrotherm.)

John Hodgkins