The rotary viscometer must first detect the zero position after starting the machine. This operation is generally carried out without the rotor. Then, the inner cylinder of radius R2 is coaxially mounted in the outer cylinder of radius R1, which is filled with viscous fluid, synchronous motor. Rotating at a steady speed, connecting the scale discs, and then rotating the inner cylinder (ie, the rotor) through the balance spring and the rotating shaft, the inner cylinder (ie, the rotor) is subjected to the viscous moment based on the fluid, and the greater the effect, the hairspring is opposite thereto. The greater the torque generated by the counterbalance, the greater the scale indicated by the pointer on the dial. The dynamic viscosity of the liquid is obtained by multiplying the reading by a specific factor.
According to its measurement principle, in order to obtain accurate and reliable measurement data, the following points must be noted:
First, the performance indicators of the instrument must meet the requirements of the national metrological inspection regulations. The instrument in use should be periodically checked. If necessary (the instrument is used frequently or in a qualified critical state), an intermediate self-check should be performed to determine that the metering performance is acceptable, 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 being tested. Many users ignore this point and think that the temperature difference does not matter. Our experiments prove that when the temperature deviation is 0.5 °C, some liquid viscosity values ​​deviate by more than 5%. The temperature deviation has a great influence on the viscosity, the temperature rises, and the viscosity decreases. Therefore, special attention should be paid to keeping the temperature of the liquid to be measured near the specified temperature point. It is best not to exceed 0.1 °C for accurate measurement.
Third, the choice of measuring container (outer tube). For the double-tube rotary viscometer, read the instrument manual carefully, and the different rotors (inner cylinders) match the corresponding outer cylinders, otherwise the measurement results will be greatly deviated. For a single cylinder rotational viscometer, the outer cylinder radius is infinitely required in principle. In actual measurement, the outer cylinder, that is, the inner diameter of the measuring container is required to be not lower than a certain size. For example, the NDJ-1 rotary viscometer manufactured by Shanghai Tianping Instrument Factory requires a measuring beaker or a straight cylindrical container with a diameter of not less than 70 mm. Experiments have shown that especially when using the No. 1 rotor, if the inner diameter of the container is too small, a large measurement error is caused.
Fourth, the correct choice of rotor or adjust the speed, so that the indication value is between 20~90 grids. This type of instrument uses the dial and pointer to read. The stability and reading deviation are combined into 0.5 grids. If the reading is too small, such as near 5 grids, the relative error is more than 10%. If you choose the right rotor or speed, With a reading of 50 divisions, the relative error can be reduced to 1%. If the indication value is above 90 grids, the torque generated by the balance spring is too large, it is easy to creep and damage the hairspring, so the rotor and the rotation speed must be correctly selected.
Fifth, the frequency correction. For the domestic instrument nominal frequency is 50Hz, and China's current power supply frequency is also 50Hz, we use the frequency meter test variability less than 0.5%, so the general measurement does not require frequency correction. However, for some instruments in Japan and Europe and America, the nominal frequency is 60 Hz, and frequency correction must be performed. Otherwise, a 20% error will be generated. 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 bubbles. The rotary viscometer has strict requirements on the depth of the rotor immersed in the liquid and must be operated in accordance with the instructions (some twin-tube instruments have strict requirements on the amount of liquid to be tested and must be measured by a measuring cylinder). In the process of immersing the rotor in the liquid, there are often bubbles. Most of the bubbles will disappear after a period of rotation of the rotor. The bubbles attached to the lower part of the rotor sometimes cannot be eliminated. The presence of bubbles will cause a large deviation in the measurement data, so the tilt Slowly immersing the rotor is an effective method.
Seven, the cleaning of the rotor. The measuring rotor (including the outer cylinder) should be cleaned and free of dirt, and should be cleaned in time after measurement, especially after the paint and adhesive are measured. Pay attention to the cleaning method, soak it with a suitable organic solvent. Never use a metal knife or other hard scraping, because there will be deviations in the measurement results when there are severe scratches on the rotor surface.
Eight, other issues to be aware of
1. Most instruments require adjustment levels, and you should pay attention to the level problem after changing the rotor and adjusting the rotor height and during the measurement process, otherwise it will cause reading deviation or even reading.
2. Some instruments need to be equipped with a protective frame. Read the instructions carefully and install them according to the regulations. Otherwise, the reading will be biased.
3. Determine whether it is approximate Newtonian fluid. For non-Newtonian fluids, the rotor, rotation speed and rotation time should be specified to avoid misunderstanding that the instrument is not accurate.
In summary, although the rotary viscometer is simple in structure and convenient to use, if it is not used correctly, an instrument that has passed the verification cannot obtain accurate measurement results and affect the product quality.
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