Several problems needing attention in the use of p

2022-10-21
  • Detail

Several problems needing attention in the use of viscometer

rotary viscometer is widely used to measure the dynamic viscosity of various fluids such as grease, paint, coating, plastic, food, medicine and adhesive. The clamping part at the end of the sample and the shape agent of the chuck are designed respectively. The instrument is simple in structure, cheap in price, convenient and practical, so it is widely welcomed. In the long-term verification process of this kind of instrument, we found that many users, especially the testers in small and medium-sized enterprises, have many problems in the use process. Often the performance of the instrument we verified is better than the requirements of the national metrological verification regulations, but the data deviation of users when testing samples is very large. Now, how to use this kind of instrument correctly to obtain accurate and reliable measurement results is analyzed as follows

first of all, briefly introduce the measurement principle of this kind of instrument:

after the rotary viscometer is started, the zero position must be detected first. This operation is generally carried out without installing the rotor, and then the inner cylinder with radius R2 is coaxially installed in the outer cylinder with radius R1, which is filled with viscous fluid. The synchronous motor rotates at a stable speed, connects the dial, and then drives the inner cylinder (i.e. the rotor) to rotate through the hairspring and the rotating shaft, The inner cylinder (i.e. rotor) is affected by the viscous torque based on the fluid. The greater the action, the greater the torque generated by the hairspring against it, and the larger the scale indicated by the pointer on the dial. Multiply the reading by a specific coefficient to obtain the dynamic viscosity of the liquid

according to its measurement principle, in order to obtain accurate and reliable measurement data, we must pay attention to the following points:

first, the performance index of the instrument must meet the requirements of the national metrological verification regulation. The instruments in use shall be subject to periodic verification. If necessary (the instruments are used frequently or in a qualified critical state), intermediate self-examination shall be carried out to determine that their metrological performance is qualified, and the coefficient error is within the allowable range, otherwise accurate data cannot be obtained

II. Pay special attention to the temperature of the measured liquid. Many users ignore this point and think that the temperature difference doesn't matter at all. Our experiments have proved that when the temperature deviation is 0.5 ℃, the viscosity value deviation of some liquids exceeds 5%, and the temperature deviation has a great impact on the viscosity. When the temperature rises, the viscosity decreases. Therefore, special attention should be paid to keep the temperature of the measured liquid constant near the specified temperature point, and it is best not to exceed 0.1 ℃ for accurate measurement

III. selection of measuring container (outer cylinder). For double cylinder rotary viscometer, carefully read the instrument manual, and different rotors (inner cylinder) match the corresponding outer cylinder, otherwise the measurement result will have a huge deviation. 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 rotary viscometer produced by Shanghai balance instrument factory requires that the diameter of the beaker or straight cylindrical container used for measurement should not be less than 70mm. Experiments show that especially when using No. 1 rotor, if the inner diameter of the container is too small, it will cause large measurement error

IV. correctly select the rotor or adjust the speed to make the indicated value between 20 and 90 grids. This kind of instrument adopts the dial plus pointer method to read, and its stability and reading deviation are combined to have 0.5 grid. If the reading is too small, such as around 5 grid, the relative error caused is more than 10%. If the appropriate rotor or speed is selected to make the reading at 50 grid, the relative error can be reduced to 1%. If the indicated value is above 90 grids, the torque of the hairspring to produce the first generation all plastic engine is too large, which is easy to creep and damage the hairspring, so we must correctly select the rotor and speed

v. frequency correction. The nominal frequency of domestic instruments is 50Hz, and the current power supply frequency in China is also 50Hz. We use a frequency meter to test the variability of less than 0.5%, so the general measurement does not need frequency correction. However, for some instruments in Japan, Europe and the United States, the nominal frequency is 60Hz, so the frequency must be corrected, otherwise 20% error will occur. The correction formula is:

actual viscosity = nominal frequency of indicated viscosity actual frequency

VI. the depth of rotor immersed in 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 according to the requirements of the manual (some double cylinder instruments have strict requirements on the amount of liquid tested, and must be measured with a measuring cylinder). There are often bubbles in the process of rotor immersion in liquid. Most of them will float up 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. The existence of bubbles will bring more reports to the measurement data. The test methods of VOC (odor, atomization, aldehydes and ketones, total carbon, vda278, microcapsules) and the problems encountered are introduced in detail. There are large deviations, so tilting and slowly immersing the rotor is an effective method

VII. Rotor cleaning. 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. Soak it in a suitable organic solvent. Never scrape it hard with a metal knife, because serious scratches on the rotor surface will lead to the deviation of the measurement results

VIII. Other issues needing attention

1. most instruments need to adjust the level. Pay attention to the level problem at any time after changing the rotor and adjusting the rotor height and during the measurement process, otherwise it will cause reading deviation or even failure to read

2. Some instruments need to be installed with protective frames. 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 fluid with a total floor area of 97 mu, the rotor, speed and rotation time should be specified after selection, so as not to be misunderstood as inaccurate instrument

to sum up, although the rotary viscometer is simple in structure and easy to use, if it is not used correctly, a qualified instrument can not get accurate measurement results, which affects the product quality

Copyright © 2011 JIN SHI