According to the measurement principle of viscometer, in order to obtain accurate and reliable measurement data, the following points must be noted:
　　
1、 The performance indicators of the instrument must meet the requirements of the national metrological verification regulations. Instruments in use should undergo periodic calibration, and if necessary (if the instrument is frequently used or in a qualified critical state), intermediate self inspection should be carried out to determine whether its metrological performance is qualified and the coefficient error is within the allowable range. Otherwise, accurate data cannot be obtained.
　　
2、 Pay special attention to the temperature of the liquid being tested. Many users overlook this point and believe that a slight temperature difference doesn't matter. Our experiments have shown that when the temperature deviation is 0.5 ℃, the viscosity value of some liquids deviates by more than 5%, and the temperature deviation has a significant impact on viscosity. As the temperature increases, the viscosity decreases. So special attention should be paid to keeping the temperature of the measured liquid constant near the specified temperature point, and not exceeding 0.1 ℃ for measurement.
　　
3、 Selection of measuring container (outer cylinder). For a dual tube rotational viscometer, it is necessary to carefully read the instrument manual. Different rotors (inner tubes) should be matched with corresponding outer tubes, otherwise the measurement results will have significant deviations. For a single cylinder rotational viscometer, the principle requires an infinite outer cylinder radius, but in actual measurement, it is required that the inner diameter of the outer cylinder, i.e. the measuring container, is not less than a certain size. For example, the NDJ-1 rotary viscometer produced by Shanghai Tianping Instrument Factory requires a measuring beaker or straight cylindrical container with a diameter of not less than 70mm. Experimental results have shown that, especially when using the first rotor, if the inner diameter of the container is too small, it can cause significant measurement errors.
　　
4、 Correctly select the rotor or adjust the speed so that the displayed value is between 20 and 90 grids. This type of instrument uses a dial and pointer to read, with a combined stability and reading deviation of 0.5 grids. If the reading is too small, such as around 5 grids, it can cause a relative error of more than 10%. If the appropriate rotor or speed is selected to make the reading at 50 grids, the relative error can be reduced to 1%. If the reading is above 90 grids, the torque generated by the balance spring will be too large, which can easily cause creep and damage to the balance spring. Therefore, it is necessary to choose the rotor and speed correctly.
　　
5、 Frequency correction. For domestically produced instruments with a nominal frequency of 50Hz, and the current power supply frequency in China is also 50Hz, we use a frequency meter to test the variability, which is less than 0.5%. Therefore, frequency correction is generally not required for measurement. However, for some instruments in Japan, Europe, and America with a nominal frequency of 60Hz, frequency correction is necessary to avoid a 20% error. The correction formula is: actual viscosity=indicated viscosity x nominal frequency ÷ actual frequency
　　
6、 The depth of immersion of the rotor into the liquid and the influence of bubbles. The rotational viscometer has strict requirements for the depth of immersion of the rotor in the liquid, and must be operated according to the instructions (some dual tube instruments have strict requirements for the amount of liquid to be tested and must be measured with a graduated cylinder). During the process of immersing the rotor in liquid, there are often bubbles present. After the rotor rotates for a period of time, most of them will float up and disappear. Bubbles attached to the lower part of the rotor sometimes cannot be eliminated, and the presence of bubbles can cause significant deviation in the measurement data. Therefore, tilting and slowly immersing the rotor is an effective method.
　　
7、 Cleaning of the rotor. The rotor (including the outer cylinder) used for measurement should be clean and free of dirt. Generally, it should be cleaned in a timely manner after measurement, especially after measuring paint and adhesive. Attention should be paid to the cleaning method, which can be soaked in suitable organic solvents. Do not use metal tools or other hard scrapes, as severe scratches on the rotor surface can cause measurement deviation.
　　
8、 Other issues to be noted. 1. Most instruments need to be leveled, and attention should be paid to leveling issues at all times after replacing the rotor and adjusting the rotor height, as well as during the measurement process, otherwise it may cause reading deviation or even inability to read. 2. Some instruments require protective brackets, carefully read the instructions and install them according to regulations, otherwise it may cause reading deviation. 3. Determine whether it is an approximate Newtonian fluid. For non Newtonian fluids, the rotor, speed, and rotation time should be specified after selection to avoid misunderstanding as inaccurate instrument. In summary, although the rotational viscometer has a simple structure and is easy to use, if not used correctly, a qualified instrument cannot obtain accurate measurement results, which affects product quality.