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This post was last edited by Wenxin Jiayuan on 2009-5-6 at 19:06. Rotational viscometers are widely used to determine the dynamic viscosity of various fluids such as oils, paints, coatings, plastics, foods, pharmaceuticals, and adhesives. This instrument is widely popular due to its simple structure, low cost, and convenience. Through years of working on the calibration of such instruments, we have observed that many users, especially those from small and medium-sized enterprises, encounter numerous problems during use. Often, the performance of the instruments we calibrate meets or even exceeds the requirements set by the metrological calibration standards, yet the data obtained by these users when testing samples shows significant deviations. The following is an analysis of how to use such instruments correctly to obtain accurate and reliable measurement results. First, let’s briefly explain the measurement principle of this type of instrument: After the rotational viscometer is turned on, it is necessary to check the zero point, an operation that is generally carried out without the rotor in place. Then, an inner cylinder with radius R2 is installed coaxially inside the outer cylinder with radius R1; the space between them is filled with a viscous fluid. A synchronous motor rotates at a constant speed, driving the scale disc. The spring and shaft then cause the inner cylinder (i.e., the rotor) to rotate. The rotor is subjected to a torque resulting from the viscosity of the fluid; the greater this torque, the greater the torque generated by the spring to counteract it, and as a result, the pointer indicates a higher value on the scale. Multiplying the reading by a specific coefficient yields the dynamic viscosity of the liquid. Based on its measurement principle, to obtain accurate and reliable measurement data, the following points must be taken into account: First, the performance specifications of the instrument must meet the requirements specified in the metrological verification standards. Instruments in use must undergo periodic calibration, and when necessary (when the instruments are used frequently or are at the threshold of compliance), intermediate self-inspections should be carried out to ensure that their metrological performance is satisfactory and that the coefficient errors remain within acceptable limits; otherwise, accurate data cannot be obtained. II. Pay special attention to the temperature of the liquid being tested. Many users overlook this, thinking that a slight difference in temperature isn’t a big deal. Our experiments show that when the temperature deviation is 0.5°C, the viscosity of some liquids can vary by more than 5%; therefore, temperature has a significant impact on viscosity – as temperature rises, viscosity decreases. Therefore, special care must be taken to keep the temperature of the liquid being tested constant around the specified temperature level; for accurate measurements, the deviation should not exceed 0.1°C. III. Selection of the measuring container (outer cylinder). For twin-screw rotational viscometers, it is necessary to read the instrument manual carefully; different rotors (inner screws) must be paired with corresponding outer screws, otherwise the measurement results will be highly inaccurate. For a single-cylinder rotational viscometer, the principle requires that the radius of the outer cylinder be infinite; in actual measurements, it is necessary that the inner diameter of the outer cylinder, which is also the measuring container, be at least a certain size. For example, the NDJ-1 rotational viscometer produced by Shanghai Tianping Instrument Factory requires that the diameter of the measuring beaker or straight-tube container be no less than 70 mm. Experiments have shown that, especially when using rotor number 1, a too small inner diameter of the container leads to significant measurement errors. IV. Select the rotor appropriately or adjust the speed so that the reading falls within the range of 20 to 90 marks. These instruments use a dial and pointer to display readings; the combined stability error and reading deviation amount to 0.5 divisions. If the reading is low, around 5 divisions for example, the resulting relative error exceeds 10%. However, by selecting an appropriate rotor or rotation speed to keep the reading at 50 divisions, the relative error can be reduced to 1%. If the indicated value is above 90 marks, the torque generated by the hairspring becomes too high, which can lead to creep and damage to the hairspring; therefore, it is essential to choose the rotor and rotation speed correctly. V. Frequency correction. For domestic instruments, the nominal frequency is 50Hz, and the power supply frequency in China is also 50Hz. Tests using a frequency meter show a variation of less than 0.5%, so frequency correction is generally not required for measurements. However, for some instruments in Japan and Europe and the United States, whose nominal frequency is 60Hz, frequency correction is necessary; otherwise, an error of 20% will occur. The correction formula is: actual viscosity = indicated viscosity × nominal frequency ÷ actual frequency. VI. The depth to which the rotor is submerged in the liquid and the effect of bubbles. Rotational viscometers have strict requirements regarding the depth to which the rotor is immersed in the liquid; it is necessary to follow the instructions provided (some dual-cylinder instruments have strict requirements as regards the amount of liquid used for testing, and this amount must be measured using a measuring cylinder). Bubbles often form as the rotor is immersed in the liquid; most of these bubbles rise and disappear after the rotor has been rotating for some time. However, bubbles remaining attached to the lower part of the rotor cannot always be removed, and their presence can cause significant deviations in the measurement results. Therefore, immersing the rotor slowly at an inclined angle is an effective approach. VII. Cleaning of the rotor. The rotor used for measurement (including the outer cylinder) must be clean and free of contaminants; it should generally be cleaned promptly after measurement, especially after testing paints and adhesives. Pay attention to the cleaning method; it is possible to soak it in suitable organic solvents. Under no circumstances should hard tools such as metal knives be used for scraping, as severe scratches on the rotor surface can lead to inaccuracies in the measurement results. VIII. Other issues that require attention. 1. Most instruments require level adjustment; pay attention to level issues after replacing the rotor and adjusting its height, as well as throughout the measurement process, otherwise it may cause reading errors or even prevent readings from being obtained. 2. Some instruments require protective frames, which must be installed in accordance with the instructions; otherwise, it will cause reading errors. 3. Determine whether it is an approximately Newtonian fluid; for non-Newtonian fluids, the rotor, rotation speed, and rotation time must be selected carefully to avoid misinterpreting the result as an indication of instrument inaccuracy. In summary, although rotary viscometers have a simple structure and are easy to use, if not used properly, even an instrument that has passed calibration may fail to provide accurate measurement results, thereby affecting product quality.