I am helping my son out with his small hobby operation. We have recently acquired an optical refractometer to use in the final stage of boiling. With 66 Brix calibration solution, it reads bang on 66 with a sharply defined line. With finished an filtered syrup, it reads with a sharply defined line. However, when attempting to measure syrup from the pan, the line is a fuzzy broad range. Sometimes the range seems to shift depending on the angle one holds the refractometer or between multiple measurements.

We are cleaning the lens with water and a soft clean cloth between measurements. We apply a drop or two to the lens, close the cover, make sure the entire lens is coated, and wait a minute for the temperatures to equalize.

What are we doing wrong here?