Instructions: Determining water content with the refractometer

To prevent the honey from fermenting, the water content must not exceed 20% according to the Honey Ordinance. A maximum value of 23 % is prescribed for honey made from heather and baker's honey and a maximum value of 25 % for baker's honey made from heather (In Germany: Second Annex to the Honey Ordinance, 30.6.2015).
The water content is usually measured using a refractometer (Fig. 1). The refractometer is used to measure the water content indirectly, as the water content changes the refractive index of the honey.


Fig. 1: Hand refractometer

Measurement
The refractometer used here has a scale (Fig. 2) of 13-25 % water content in honey (honey moisture). To measure, pour honey onto the problem area and press the cover plate onto the honey so that a bubble-free layer is formed. Now hold the refractometer up to a light source (ideally sunlight) and look through the eyepiece. Now turn the adjustable eyepiece (Fig. 1) until you see the sharpest possible line (Fig. 3) between a blue and a white area. Read the water content from the dividing line.


Fig. 2: Scale 13-25 % water content in honey



Fig. 3: Reading the value: here 16.4 %

Automatic temperature compensation (ATC)
As the refractive index depends on the temperature, measurements are taken in the laboratory under standardized conditions at 20 °C. Laboratory refractometers are tempered to this value. Laboratory refractometers are tempered to this value. Depending on the version, handheld refractometers have “automatic temperature compensation” (ATC). The scale in the refractometer is shifted as a function of the temperature by means of a bimetal mechanism. This allows you to measure the water content in honey at 18°C or 25°C, for example, without having to pay attention to the temperature.
Even with devices without automatic temperature compensation, the deviation is kept within limits. If you measure with such devices without ATC, for example, at 19°C instead of 20°C, you have to add approx. 0.1% to the water content. At 21 °C, 0.1 % is subtracted. Consequently, even if the temperature deviates by 5 °C, the error in water content is only approx. 0.5 % if you do not have a device with automatic temperature compensation (J.W. White, Moisture in Honey: Review of Chemical and Physical Methods, Journal of The AOAC, Vol. 52, No. 4, pp. 729-737 (derived from the measured values in Table 1), 1969).

Cleaning
When cleaning, care should be taken not to scratch the surface of the sample carrier. I carefully rinse the tip of the refractometer (sample surface) under running water and dry it with a soft cloth. However, you can also use a soft damp cloth for cleaning.

Calibration
If you handle a refractometer with care, recalibration is virtually unnecessary. The refractometers are supplied calibrated ex works.
TIP: If you still want to check your refractometer from time to time, you can take a good cooking oil when buying a new refractometer and use it to determine the value. Of course, you do not measure the water content in the oil, but actually the refractive index of the oil. Depending on the scale (use of the refractometer), this is displayed in the desired values (e.g. in beekeeping: “water content in honey”). Measure the value of the cooking oil and make a note of it. I recommend filling a small amount of this oil into a small bottle and storing it in a cool, sealed container. This oil can then be used later for adjustment. To do this, apply the oil and adjust the line as sharply as possible by turning the adjustable eyepiece. Then use a screwdriver to readjust the value measured on the scale (on the blue and white line).

Measurement tips
Depending on the literature information or the measuring method used, the specified refractive index differs, as it is measured with refractometers. As a result, a maximum error of approx. 0.5 - 0.7 % is conceivable in relation to the adjustment of a handheld refractometer for honey, which can be caused by different adjustments of two devices. Who would want to bottle a honey with a measured 19.9 %? If the result is 17.1 %, you are on the safe side. The water content determined with a hand refractometer should therefore not be regarded as a highly precise measured value, but rather as a sufficiently accurate reference value. This is mainly due to the fact that other errors are made during the measurement. It would be correct to measure three samples (or more) of honey from different combs and determine the average value. If the honey is extracted, honey with different water contents from different combs or hives will mix. The water content also changes during storage. If samples are taken from different depths of a honey container, this can also result in different water contents. If the water content is measured before extracting, the value can also change during extracting, storage and bottling.