a refractometer to measure
the sugar concentration of sap and syrup
you ever looked at sunlight passing through a glass of clear water?
What does it look like? Does it look different if you fill the same
glass with iced tea? With orange juice? With chocolate milk? As
you have probably noticed, sunlight passes through different solutions
(liquids like water and milk) in different ways.
imagine two glasses of water. A pinch of sugar is placed in one
glass, and a whole tablespoon of sugar is placed in the other. Both
glasses are mixed until the sugar is completely dissolved, and then
they are placed in a sunny windowsill.
our eyes do not notice it, light passes through each glass in a
slightly different manner - like what you noticed with the iced
tea and milk. This is because the sugar concentration of the water
causes light to "refract," or scatter when it passes through
the water solution. The glass with more sugar would cause the sunlight
to refract more than the glass with only a pinch of sugar.
refractometers can determine sugar concentrations by measuring how
light is refracted in a sugar solution. Maple sap and syrup sugar
concentrations can be measured with these types of refractometers.
producers want to measure the sugar content of sap (1-6% sugar),
syrup (66-67%), and concentrated sap (between 6 and 66%), refractometers
are available with several different scales. Some refractometers
can measure the low concentrations of sugar present in fresh sap,
while others measure the higher concentrations found in syrup.
refractometers scales are 1) 0-36%, 2) 28-55%, 3) 54-70%, and 4)
68-92%, but the scales may vary. A refractometer with the low scale
would be useful for measuring sap, while a higher scale would be
necessary for syrup measurements.
you have obtained a refractometer, you may be interested in measuring
the sugar content of sap from maple trees in your yard or schoolyard,
or you may want to measure the sugar content of maple syrup you
have purchased or made yourself. You may even be interested in participating
in the Cornell Sugar Maple Tree Improvement
can be obtained from most maple syrup supply companies, and vary
from $100 to over $300. Some companies to consider include Waterloo/Small
or Dominion and Grimm.
about the scale you might have at home in your bathroom. Before
getting on it, do you check to see if it is pointing to zero? If
it is not at zero (for example, if it is pointing at 4 pounds),
you would probably turn the knob to adjust it. When you adjust a
bathroom scale to read zero when nothing is on it, you are calibrating
it. A refractometer needs to be calibrated in the same way.
calibrate a refractometer, you will use a drop of water containing
no sugar. As with the scale, you will adjust the refractometer to
read 0 when it is measuring the water sample. The refractometer
should read 0 for a water sample because there is no sugar in the
sample - much like the scale should read 0 when nobody is standing
calibrate your refractometer, place a drop of water (preferably
distilled) on the dark circular or rectangular area and close the
cover. A shadow or dark area is visible on the scale inside the
eyepiece. Turn the calibration screw until the shadow falls on the
zero mark. Open the refractometer cover and dry the cover and glass
prism using soft tissue paper or a cotton cloth.
components of a refractometer change slightly at different temperatures,
so it must be calibrated as temperatures change. You should check
the calibration of your refractometer (see if it reads 0 for water)
after every sap or syrup sample. Once it is stable (you have not
needed to adjust it for several samples), you will only need to
check the calibration after every fifteenth reading, or after each
cleaning. If you are waiting a long time before making measurements
(more than a couple of minutes), you should calibrate before and
after each sample.
If in doubt,
check the calibration. Your values will be meaningless if your refractometer
is not properly calibrated.
are now ready to measure the sugar concentration of a sap or syrup
sample. Only one drop is needed for a sample, though it is important
that the sample is very fresh. Old samples (for example, a droplet
that has been hanging on a spile for several minutes) will have
lost water to evaporation, so their refractometer readings will
a drop of sap on the refractometer, close the cover, and quickly
read the scale (the line at the top of the darker area). Readings
should be taken to the nearest 0.1 percent. The refractometer should
be dried with tissue paper and rinsed with water after each reading.
If you are working rapidly, dry the refractometer after each measurement,
and rinse and dry it after 10 to 15 measurements.
refractometers auto adjust for the sap or syrup temperature, some
may come with a chart that requires mathematical adjustments. Check
the documents that come with your refractometer to see whether you
need to adjust for temperature.