Measuring must weight or the Oechsle scale can be done with several procedures, including must weights and refractometers. Must weight is the number which shows how many grams a litre of must is heavier than a litre of water at 20 °C. Must weight is therefore a density measurement. A correctly displaying density meter is therefore a reference device here. A refractometer, also one which is officially standardised, is not a reference measurement device in our view because it does not directly measure density. One should note that in order to compare the measurement results of a refractometer and a must scale, one must use must, and not a sugar-water solution. It must be assumed that there is a certain, ideal acid ratio in the must. If the acid ratio changes - with the same sugar contents - then the refractometer’s results change, because the acids can detectably influence the refractometer’s measured results. The results of a must scale, on the other hand, are not affected by various acid compounds. This is therefore the main difference between refractometer and hydrometer must results.

Brix (=%mass) is the sucrose share of a 100 gram sucrose-water solution. One commonly uses ‘sugar’ to mean ‘sucrose’. The underlying conversion formula is defined by ICUMSA on the basis of the refractive index (nD).

The refractive number (often also referred to as the refractive index) is an optics term. It identifies the refracting of light when transmitted through a transparent material, and is the ratio between the phase speed of light c0 in a vacuum and its phase speed c in a respective medium:
n = ------

The term “refractive index” comes from the term ‘refraction’ and its appearance in the Snell’s law of refraction.  This physical quantity has no units, and is therefore expressed as a number. It can also be given as a complex number, and then also identifies optical density, mostly however only the real portion, through which the light speed is determined in a medium.

Refraction can also be defined by radiation optics. According to the above Snell’s law of refraction, n corresponds to the sine ratio of entry and exit angles. Thus the angle of the “light beam” which passes through the media interface, are related to the perpendicular to this.

Thus one must note that refraction through most materials depends upon wavelength, which is described as dispersion. 


 Refractive index



Air (near the ground)   










As a rule, you can easily exchange the daylight plate. There are various daylight plates. It’s important, therefore, to tell us what model you have when ordering a prismatic cover. If you can't name the model, please send us a picture by email. The current refractometers are models within the ATAGO Master series.

The Master series covers, such as the Master 53 alpha, are attached with cotters. The covers of previous series were attached with screws and nuts.

Please ensure that the cover (no. 4) is placed without a gap. Should it not lie flat, move the prism bracket (no. 1). Shift the prism bracket in such a way that the screws (no. 2) open to the right and left of the adjustment screw and move the prism bracket in the lengthwise direction.

The value measured depends upon all dissolved materials. Wine contains alcohol and acids in addition to sucrose. The measured value is influenced by these materials. Therefore, one cannot directly determine the sucrose contents.

Water has penetrated the refractometer. Slowly heat the device with a hair dryer, not exceeding 50 °C. If this is unsuccessful, please send the device to Customer Service. 

Refractometers measure correctly at + 20 °C. Many of our refractometers have an automatic temperature correction. Since the measured temperature strongly influences the results, refractometer results which are measured at another temperature must be corrected. The appropriate corrective data can be directly read on some of our refractometers on the prism cover. Depending upon the temperature, the corrective values must be either added to or subtracted from the results. No other corrective table is needed.

When looking through the eyepiece, one can see the boundary line when water cuts the scale at -1% Brix. A correction value of +1% when the prism cover is folded up. 

The corrected measurement result is therefore -1% + 1% = 0.

A rule of thumb for temperature correction of refractometers with refractive index scales: Subtract 0.0001 to 0.0004 nD per °C with a device temperature of less than 20°, if the temperature is higher, add that number. Since the effect of temperature on the refractive index is different with every kind of medium, with very precise measurements, the appropriate exact correction value must be obtained from reference books and the like. This also applies for refractometers which are only outfitted with a thermometer.

Automatic temperature-correcting refractometers automatically perform the temperature correction (primarily between +10 and +30 °C).

The refractometer is properly adjusted when delivered. One must nevertheless regularly check the refractometer. It can, in particular, become maladjusted after a shock. If the refractometer has a beginning value of 0% Brix, it can be adjusted with distilled water. The measured temperature (especially the device temperature) should be 20 °C. The boundary line must therefore be set to 0% Brix. If the result read is not 0, the boundary line must be set by turning the adjustment screw to 0% Brix.

For medical refractometers and for Item no. 2350, the value must be adjusted to 1.3330 nD, because pure water at 20 °C has this refractive index.

In order to adjust refractometers which do not have a starting value of 0% Brix, we can provide a test liquid (Item RH9000) with a defined Brix or nD value (additional cost). This liquid is measured with the refractometer. For this, the refractometer must have a measurement temperature of about 20 °C. If the refractometer does not show the target value, the boundary line must be re-set with a screwdriver to the displayed target value. The test liquid is removed with soft paper and alcohol or rinse liquid/soap from the prism and from under the prism cap (water alone is insufficient).

One can easily and quickly determine the active ingredient concentration using a refractometer. Adherence to a specific active ingredient concentration significantly influences the operating behaviour of the cold lubricant.

As a rule, conventional or electronic hand refractometers are used for this. 

After adding 1-2 drops of sample and waiting for about 20 seconds (for temperature equalisation), one can see in a conventional hand refractometer a bright-dark boundary which is shown against a scale. One can read a refractometer value on this scale (as a rule, expressed as % Brix or refractive index). This refractometer value must be adjusted for temperature, as refractometric readings are strongly dependent on temperature. The temperature correction values must be calculated against the results. Then the measurement prism as well as the prism cover must be cleaned.

For an electronic, automatically processing hand refractometer, one needs only 1-2 drops of sample on the measurement prism and to press a button. About 3 seconds later, the already temperature-compensated results are digitally displayed. Thus one cannot incorrectly read a scale. In addition, refractometers are very reproducible, even for difficult samples such as emulsions. Commercial hand refractometers are difficult to read when testing emulsions, because there is seldom a sharp bright-dark boundary; rather, it is a border which can easily be misread. There is a frequent linear relationship between the refractometer value and concentration. Using a standards curve and correction factors for the various liquids being tested, one can determine a concentration value in volume-% (see Figure to the left). One can either create relevant standards curve oneself, or they are produced by the material’s supplier. Other industrial liquids can be tested with refractometers, such as liquids which are used for washing, etching and hardening. 

For some aromatics measured in aqueous solutions, the value of pure water is included in the measurement range.

Oechsle scales are set at 0 °Oe for pure water. The same applies for alcohol meters at 0% vol. or 0% mass, and for saccharimeters for sucrose solutions at 0% mass. sucrose.

In practice, these devices do not show the expected zero value even in distilled water. Oechsle scales and saccharimeters show a value over 0, alcohol meters show a 0 value. The reason for this is mentioned below as an example for Oechsle scales.

Testing such areometers can therefore only be done in a very rough manner. If one would like to ensure that an areometer is correct for important measurements, a calibrating laboratory must confirm the correctness of the meter, because they are low-cost measuring instruments, verifying their accuracy is expensive.

Standardised areometers (best with a standardisation slip) must be used if the precision specifications are high. In an emergency, one can use standardisable as well as normal devices with an official test slip. One must assume that measurements are more or less imprecise with areometers without an official test slip.

Must scales when measuring distilled water don’t display  0 °Oe, but, depending upon the geometric parameters in the glass - about 2 to 3 °Oe. Oechsle scales are adjusted at  0 °Oe to the surface tension value of pure water; this is very high and, in practice, is not achieved due to various contamination in water, such as dust or the smallest particles in the air. In addition, in the case of water, complete and correct wetting of the stem is poor, and very intense cleaning of the areometer is required.  In practice, this is not done. Both lead to an increase Oechsle value when measuring distilled water. For these reasons, must scales are not adjusted using water at the factory.

One must always assume a deviation of +/- 1 to 2 °Oe for normal must scales. One must use officially tested must scales for precision measurements.

Hydrometer: Handle carefully, as it is fragile; it must be clean and dry.

Measurement vessel: Glass is best; the interior must be at least 1.5 cm wider than the hydrometer (except for the “OPTIMUM”, see below); it must be clean and dry, and must be plumb.

Test liquid: Must be free of contamination, free of bubbles, average sample.

Workplace: Must be vibration-free, bright and glare-free.

Measurement procedure: Pour the test liquid slowly into the measurement glass. Carefully mix before measurement,

gently tap on the lower part with an areometer with a built-in thermometer which is graduated at 0.1 or 0.2 °C, because the mercury column sometimes does not have a completely uniform course (especially when temperatures are falling).

Hold the areometer at the thin end above the measurement scale - tip - and slowly dip into the liquid until it floats; dip it at most 5 mm deeper than its natural float level. The rest of the areometer’s surface must also be clean and dry when dipped (no fingerprints, no grease or similar).

Wait at least 1 minute, but better 2 minutes (the larger the lower part, the longer one must wait). Read the “top” or “bottom” result (see areometer) with/without a magnifying glass; the view direction must be horizontal. The areometer must float calmly and approximately in the middle during reading.

The target measurement temperature = reference temperature (as a rule = 20'C; see the areometer). Use a table to correct for temperature if it is different than the reference temperature.

If the surface tension of the test liquid varies significantly from the underlying surface tension in the adjustment (see areometer), make a correction of the precise measurement. Areometers are basically adjusted for a defined surface tension, or for surface tension classes L (low), M (medium) and H (high), or for the surface tension of a defined liquid.

Optimal side distance to the cylinder wall: 7.5 mm or greater.

Always conduct 2 to 3 sequential measurements, and calculate the mean.

Standardised areometers (best with a standardisation slip) must be used if the precision specifications are high (such as with warranty measurements). In an emergency, one can use standardisable as well as normal devices with an official or manufacturer’s test slip. One must assume that measurements are more or less imprecise with areometers without an official test slip.