Rotary viscometer is widely used in the determination of oil, paint, coatings, plastics, food, pharmaceuticals, adhesives and other fluids of the dynamic viscosity. The instrument is simple, inexpensive, convenient and practical, and therefore popular. In the long-term test of such instruments, we found that many users of testers in the course of the existence of many problems, we often check the instrument performance is better than the national metrological verification procedures, but the user data deviation in the test sample is large . Now on how to correctly use this type of instrument to obtain accurate and reliable measurement results are as follows.
First of all, a brief introduction of the measurement principle of this type of instrument:
Rotary viscometer first boot to check the zero position, this operation is usually carried out without the rotor installed, and then in the outer radius of the outer cylinder R1 installed R2 inner radius, filled with viscous fluid between the synchronous motor Rotate at a steady speed, successively dial, and then through the balance spring and the shaft drives the inner cylinder that is the rotor rotation, the inner cylinder that is subject to fluid-based viscous moment role, the greater the role, the balance spring with the resulting counterbalance The greater the torque, so the pointer on the dial scale, the greater the scale. Multiplying the reading by a specific factor gives the dynamic viscosity of the liquid.
According to its measurement principle, in order to obtain accurate and reliable measurement data must pay attention to the following points:
1. the performance of the instrument must meet the national measurement regulations requirements. The instrument in use should be periodically checked, and if necessary (the instrument is used frequently or in a critical state), self-check is performed to confirm that its metrological performance is acceptable and the coefficient error is within the allowable range; otherwise, accurate data can not be obtained.
2. pay special attention to the measured liquid temperature. Many users ignore this point and think that the temperature difference does not matter. Our experiments show that: when the temperature deviation of 0.5 ℃, some liquid viscosity deviation of more than 5%, the temperature deviation of the viscosity of a great impact, the temperature increases, the viscosity decreased. Therefore, we should pay special attention to the measured liquid temperature constant in the vicinity of the specified temperature, the best measurement should not exceed 0.1 ℃.
3. measuring the choice of container barrel. For binocular rotational viscometer to read the instrument manual, different rotor inner cylinder to match the corresponding outer cylinder, otherwise the measurement results will be great deviation. For a single cylinder rotary viscometer, the principle requires an infinite radius of the outer cylinder, the actual measurement requirements of the outer cylinder that is measuring the inner diameter of the container is not less than a certain size. For example, the NDJ-1 rotary viscometer produced by Shanghai Balance Instrument Factory requires that the diameter of beaker or straight-shaped container for measurement be not less than 70mm. Experiments show that especially in the use of a rotor, if the container diameter is too small cause greater measurement error.
4. the correct choice of rotor or adjust the speed, so that the value of 20 to 90 grid.
This type of instrument using dial plus pointer readings, the stability and reading deviation combined with 0.5 cell, if the reading is as small as 5 cells near, causing the relative error of 10% or more, if you choose the appropriate rotor or speed Reading at 50 cells, then its relative error can be reduced to 1%. If the show value of 90 or more, so that hairspring torque is too large, prone to creep, damage hairspring, so we must correctly select the rotor and speed.
5. frequency correction. For the nominal frequency of domestic equipment at 50Hz, while China's current supply frequency is 50Hz, we use the frequency change test less than 0.5%, so the general measurement does not require frequency correction. However, for some instruments in Japan, Europe and the United States, the nominal frequency must be adjusted to 60Hz at a frequency of 20Hz. Otherwise, a 20% error will occur. The correction formula is:
Actual viscosity = indicated viscosity × nominal frequency ÷ actual frequency
6. the rotor immersed in the depth of the liquid and the impact of bubbles.
Rotary viscometer on the depth of the rotor immersed in the liquid have strict requirements must be operated in accordance with the instructions of some double-cylinder instrument to test the amount of liquid has strict requirements, the amount must be measured cylinder. When the rotor is immersed in the liquid, bubbles are often present. Most of the bubbles float up and down for a period of time after the rotor is rotated. Bubbles attached to the lower part of the rotor sometimes can not be eliminated. The presence of bubbles causes great deviation in the measured data. Slowly immersing the rotor is an effective way.
7. the rotor cleaning. The rotor to be measured, including the outer cylinder, should be clean and free of dirt and must be cleaned in time after the measurement, especially after the paint and adhesive have been measured. Pay attention to the cleaning method, soak with a suitable organic solvent, do not scratch hard with metal knives, etc., because the rotor surface with a serious scratch will bring deviation of the measurement results.