Refractometers, like any tool, have to be used correctly to give good results. Low power may lead to erratic readings. Temperature of the syrup and the instrument have to be the same and NOT changing quickly (either cold syrup or hot syrup or cold/hot instrument). Trying to get a good quick measurement from really hot (or really cold) syrup is an exercise in futility and frustration.

One thing I don't like about the Hannah unit is that it doesn't have a cover. If you put hot syrup in the well, there can be some evaporation of water from hot syrup before the temperature stabilizes.

In comparing hand-held optical and digital units...they're all fairly close in terms of accuracy ASSUMING THEY ARE USED PROPERLY. Just make sure to use an automatically-temperature-compensated unit (ATC) or take measurements only at room temperature after the syrup and the instrument have cooled/warmed to room temperature.