Accuracy of refractometer?

[ADK_XJ]

I've had a brix refractometer for a few years now, I only use it occasionally but it always seems that we have high sugar % compared to the average. Not complaining but wondering about the accuracy of such a device...I bought a relatively inexpensive one off Amazon.

Our average yield (mostly from hard maples) is often 2.5 and even in the 4s and 5s when I test from the tank. Is there a way to test for accuracy or to "zero" the device?

[SeanD]

You can zero it out with distilled water.

[johnallin]

I have a Misco refractometer which can be zeroed out with a glycerin product available from Misco.
I use distilled water.
Did you receive a manual or instructions?
They need to be calibrated every now and then.

[Russell Lampron]

There should be an adjustment screw that's probably hidden under a small cap. Use distilled water for calibration fluid and adjust it to zero. You'll need a small Allen wrench or screwdriver to make the adjustment. Most refractometers are automatically temperature compensating (ATC).

When you use the refractometer you need to clean it very well between uses to get an honest reading. I find my refractometer to be very accurate.

[DrTimPerkins]

The other key thing in refractometer measurements is that the solution being measured and the instrument be isothermal (the same temperature). If you put a cold drop of sap on a warm refractometer (or vice versa), you'll end up with an erroneous reading, even with an instrument with automatic temperature compensation (ATC). Give it a minute or so to stabilize before taking the reading.

Some good info at: http://www.omafra.gov.on.ca/english/food/inspection/maple/mple-guidance-S1-1-4.htm