The secret to getting the most accurate possible refractometer readings is, in a word, "temperature." Even if a refractometer is automatically temperature compensated you will not get the most accurate results if you do not allow temperatures to equilibrate (equalize).
Temperature is one of the single most important factors influencing accurate refractometer readings and is one of the largest sources of error in measurement. Refractive index is VERY dependent on temperature. The amount of error per degree of temperature is different for every fluid and differs for different concentrations of the same fluid. Although temperature compensation relieves the user of the responsibility to measure temperature and apply a correction factor when taking readings, it is not infallible.
Whether you are using a traditional analog refractometer, a digital handheld refractometer, or a desktop refractometer, you must allow time for the refractometer temperature, the ambient temperature, and fluid temperature to come to equilibrium (within the temperature compensation range of the refractometer). Since the volume of fluid is usually only a few drops, its thermal mass compared to that of the refractometer is really rather small, allowing it to come to the temperature of the refractometer relatively quickly, in most cases. Thick viscous fluids, like maple syrup, typically take longer to come to equilibrium than less viscous material such as water.
It will take a little time for a refractometer to adjust to changes in environmental temperature. For instance, if you take a refractometer from a cool air-conditioned laboratory out onto a shop floor on a hot summer day, it will take some time for the refractometer to equalize with the temperature in the shop. Likewise, if in the winter you are carrying a refractometer under a winter coat next to your body and then pull the refractometer out to make a measurement, it will naturally take some time for the refractometer to adjust to the change in ambient temperature.
Everyone asks, “How much time?” There is no magic answer to this since there are too many variables. Each situation is different. But, providing the refractometer and ambient temperature are in equilibrium, as a general rule of thumb, you should wait about 10 seconds for every 5 °F difference between refractometer temperature and 68°F, or about 30 seconds for each 10 °C difference between the fluid temperature and 20 °C.
How do you know if the temperature is stable? For the Palm Abbe digital handheld refractometer, if the rate of change of temperature exceeds a certain limit then the refractometer will not allow you to take a reading. However, it is possible that when putting a hot sample on a room-temperature refractometer the initial rate of change will be very high for the first 10 to 20 seconds and then level off and drift slowly towards equilibrium. If temperature is drifting relative to time you can expect to see some drift in your reading as well. So, if your refractometer readings are slowly drifting in one direction then there is a chance that the temperature is not stable. The direction of the drift depends on the nature of the changing temperature. Generally, if the temperature is stable and you take five measurements in a row, the readings should not vary by more than the precision of the scale.
Another issue of concern is evaporation. Evaporation is vaporization that takes place at the surface of a liquid. The process of evaporation will increase the larger the surface area, the higher the liquid's temperature, the higher flow rate of air past the surface, and the lower its density. The Palm Abbe refractometer mitigates the effects of evaporation by limiting the fluid volume/surface area and inhibiting air flow with its Evaporation Cover. If a sample is rapidly evaporating, you can expect to see an upward drift in the refractometer reading over time.
For the most accurate possible refractometer readings, take your time and allow the temperature of the refractometer, the ambient environment, and the fluid sample to come to equilibrium. As a general rule of thumb, you should wait about 10 seconds for every 5 °F difference between refractometer temperature and 68°F, or about 30 seconds for each 10 °C difference between the fluid temperature and 20 °C.