optical refractometer technique

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?

I’m new at this too but I’ve found that blurry line is a result of warm syrup. It needs to be cool to get an accurate result.


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Much better with cooled syrup. Thanks.

Yes, even with an ATC refractometer, it gives a more accurate reading with syrup that has cooled. It doesn't have to be 60 deg, but below 100? helps. I'll save a bit in a sample cup to cool, but I'll also let the amount I'm testing sit on the glass for a bit before closing the plastic shield/diffuser. That gives the refractometer and syrup a chance to reach the same temp. Sometimes the refractometer is cold.

I can also have the same problem with honey in the summer, so I've found the thickness of the sample makes a difference, too. I'll dribble some honey on the class, let it even out and air bubbles to move out of the way. Sometimes I'll use the plastic shield to push down on the sample to spread the honey or syrup out to get an even layer.

I got my refractometer a few weeks back and it did not come with a calibrating solution. I tested a few syrups and thought it was reading a little low. I then read you could use extra virgin olive oil as a calibrating solution. It’s Brix reading is 71.5. When I tested it my refractometer, it showed 170, so I calibrated it to read 171.5.

I retested some reputable syrups that had tested low, and they were now dead on at 66 Brix.

The blurred line can be caused by the 'film' that will form on the surface of those couple of drops of syrup as it cools. The unit gets calibrated at 68 F if my memory is correct so let the syrup cool to close to that temp, put the 2 drops on the glass and mix thoroughly with one of the little plastic spears that bartenders use to skewer fruit. You'll get more consistent results.