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Rotational viscometers are widely used to measure the dynamic viscosity of various fluids such as grease, paint, coatings, plastics, food, medicine, and adhesives. The instrument is simple in structure, cheap, convenient and practical, so it is very popular. In the long-term verification process of this type of instrument, we found that many users, especially the testers of small and medium-sized enterprises, have many problems in the use process. Often the performance of the instrument we verify is better than the requirements of the national metrological verification regulations, but the user is testing the sample. The data are highly skewed. Now on how to correctly use this type of instrument to obtain accurate and reliable measurement results are analyzed as follows.

First, a brief introduction to the measurement principle of this type of instrument:

After the rotary viscometer is turned on, the zero position must be detected first. This operation is generally carried out without installing the rotor, and then the inner cylinder of radius R2 is coaxially installed in the outer cylinder of radius R1, which is filled with viscous fluid. Synchronous motor Rotate at a stable speed, connect the scale discs one after another, and then drive the inner cylinder (ie the rotor) to rotate through the hairspring and the rotating shaft. The greater the torque generated by the counterbalance, the larger the scale indicated by the pointer on the dial. Multiply the reading by a specific factor to get the dynamic viscosity of the liquid.

According to its measurement principle, in order to obtain accurate and reliable measurement data, the following points must be paid attention to:

1. The performance indicators of the instrument must meet the requirements of the national metrological verification regulations. The instrument in use should be checked periodically, and if necessary (the instrument is frequently used or in a critical state of being qualified), an intermediate self-examination should be carried out to confirm that its measurement performance is qualified, and the coefficient error is within the allowable range, otherwise accurate data cannot be obtained.

Second, pay special attention to the temperature of the liquid to be measured. Many users ignore this point and think that the temperature difference does not matter. Our experiments show that when the temperature deviation is 0.5 ℃, the viscosity value deviation of some liquids exceeds 5%, and the temperature deviation has a great influence on the viscosity. The temperature increases and the viscosity decreases. Therefore, special attention should be paid to keeping the temperature of the liquid to be measured constant around the specified temperature point, and it is best not to exceed 0.1°C for accurate measurement.

3. Selection of measuring container (outer cylinder). For the double-cylinder rotational viscometer, read the instrument manual carefully, and different rotors (inner cylinders) match the corresponding outer cylinders, otherwise the measurement results will deviate greatly. For a single-cylinder rotational viscometer, in principle, the radius of the outer cylinder is required to be infinite. In actual measurement, the inner diameter of the outer cylinder, that is, the measuring container, is required to be no less than a certain size. For example, the NDJ-1 rotational viscometer produced by Shanghai Tianping Instrument Factory requires a beaker or a straight cylindrical container to be measured with a diameter of not less than 70mm. Experiments show that especially when using the No. 1 rotor, if the inner diameter of the container is too small, it will cause a large measurement error.

4. Correctly select the rotor or adjust the speed so that the displayed value is between 20 and 90 grids.

This type of instrument adopts the dial and pointer method to read, and its stability and reading deviation are combined with 0.5 grids. If the reading is too small, such as around 5 grids, the relative error caused is more than 10%. If the reading is at 50 divisions, the relative error can be reduced to 1%. If the indication value is above 90 grids, the torque generated by the hairspring will be too large, which will easily cause creep and damage the hairspring. Therefore, the rotor and speed must be selected correctly.

5. Frequency correction. The nominal frequency of domestic instruments is 50Hz, and the current power supply frequency in my country is also 50Hz. We use a frequency meter to test the variability of less than 0.5%, so the general measurement does not require frequency correction. But for some instruments in Japan, Europe and America, the nominal frequency is 60Hz, and frequency correction must be performed, otherwise a 20% error will occur. The correction formula is:

Actual viscosity = indicated viscosity × nominal frequency ÷ actual frequency

6. The depth of the rotor immersed in the liquid and the influence of air bubbles.

The rotational viscometer has strict requirements on the depth of the rotor immersed in the liquid, and must be operated in accordance with the requirements of the instructions (some dual-tube instruments have strict requirements on the amount of liquid tested, which must be measured with a graduated cylinder). When the rotor is immersed in the liquid, there are often bubbles, and most of them will float and disappear after the rotor rotates for a period of time. Sometimes the bubbles attached to the lower part of the rotor cannot be eliminated. 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 time after measurement, especially after measuring paint and adhesive. Pay attention to the cleaning method. It can be soaked in a suitable organic solvent. Never use a metal knife or other hard scraping, because serious scratches on the surface of the rotor will cause deviations in the measurement results.

8. Other matters needing attention.

1. Most instruments need to be leveled. After replacing the rotor and adjusting the height of the rotor, pay attention to the leveling problem at any time during the measurement process, otherwise the reading will be deviated or even impossible to read.

2. Some instruments need to be installed with a protective frame, read the instructions carefully and install them according to the regulations, otherwise it will cause reading deviation.

3. Determine whether it is an approximate Newtonian fluid. For non-Newtonian fluids, the rotor, rotation speed and rotation time should be specified after selection, so as to avoid misunderstanding that the instrument is inaccurate.

To sum up, although the rotational viscometer has a simple structure and is easy to use, if it is not used correctly, a qualified instrument cannot obtain accurate measurement results, which will affect the product quality.