Overview
The viscosity cup method is an empirical method for evaluating the kinematic viscosity of a certain volume of fluid by measuring the time it takes to flow out of a small hole at the bottom of a particular container under gravity. Based on the basic principle of Poischer's law, the outflow time and the kinematic viscosity of the fluid have an approximately linear relationship within a certain range. For varnishes such as Newtonian fluids or near-Newtonian fluids, the longer the outflow time, the higher the viscosity under the same test conditions. This method is easy to operate, low equipment cost, and is widely used in on-site rapid inspection and production process control in coatings, inks and related industries.
Preparation before the test
Preparation is the basis for ensuring accurate and reliable test results. First of all, according to the estimated viscosity range of the varnish to be tested, it is necessary to choose the appropriate standard viscosity cup model, common models such as ISO cups, DIN cups, Ford cups, etc., with different pore sizes. The test environment should be kept clean, free of strong drafts and vibrations, and the temperature should be controlled within the standard range, usually (23±0.5)°C, because the temperature has a significant effect on the viscosity of the varnish. All testing instruments, including viscosity cups, thermometers, stopwatches, and containers, must be thoroughly cleaned with a suitable solvent and completely dried before use to prevent residue from affecting flow. The varnish sample to be tested should be placed in a constant temperature environment in advance and stirred well to eliminate bubbles and temperature gradients.
Assay steps
The measurement process should be carried out strictly according to the steps to reduce the operating error. First, place the viscosity cup on a solid horizontal stand with a receiving container underneath. Use your finger or a suitable plate to block the bottom flow outlet. Slowly pour a well-to-mix and balanced varnish sample into the cup until the liquid level rises above the top edge of the cup to avoid bubbles. Use a scraper or ruler to scrape off excess samples horizontally along the edge of the top of the cup at one time so that the liquid level is flush with the top of the cup. Quickly remove the finger or plate blocking the outlet and start the stopwatch at the same time. When the sample flow is interrupted for the first time, stop the stopwatch immediately and record the total time of outflow in seconds (s). The same sample should be repeated at least three times, and the difference between the measurement values should be within the allowable error range of the average value.
Results report
Data processing should follow the principle of scientific rigor. Calculate the arithmetic mean of the three valid measurements of the outflow time. If you are using a specific model of viscosity cup, you can convert the average outflow time (t) to kinematic viscosity (ν) according to the conversion formula or conversion table provided. The conversion relationship is usually given in the form of an empirical formula, for example: ν = K (t - C), where K and C are the instrumental constants of the viscosity cup of this model. The final test report should contain the following information: the identification of the varnish sample, the standard model of the viscosity cup used, the test temperature, the measured value of the triple outflow time, the average outflow time, and whether it is converted to kinematic viscosity value according to the standard requirements.
Influencing factors
The test results are affected by a variety of factors and require special attention during operation. Temperature fluctuations are a major source of error, and it is important to ensure that the sample, instrument, and ambient temperature are consistent and consistent. The cleanliness, smoothness of the inner wall and whether the small holes are regular and undamaged directly affect the flow state. The operation techniques, such as the speed of the scraping action and the instantaneous action of opening the outlet, should strive to be consistent and fast. The presence of undissipated bubbles or impurities in the varnish can interfere with normal flow. In addition, this method is suitable for Newtonian fluids in a certain viscosity range, and for non-Newtonian fluids with strong thixotropy, the relationship between outflow time and viscosity may not conform to the standard conversion relationship, and the results can only be used as a reference.
Safety and maintenance
During the experiment, it is necessary to pay attention to the routine safety of the laboratory. When using volatile solvents to clean the instrument, it should be done in a well-ventilated place. Discarded varnish samples and cleaning solvents should be disposed of in accordance with relevant regulations. The viscosity cup is a precision instrument, and should be cleaned immediately after use, and do not scratch the inner wall and flow hole with hard objects. When not used for a long time, it should be properly stored after drying, and its measurement performance should be checked regularly to meet the standard requirements.
Reference Standards
GB/T 1723-1993 Coating viscosity determination method
ISO 2431:2019 Paints and varnishes — Determination of flow time by use of flow cups
ASTM D4212-2016 Standard Test Method for Viscosity by Dip-Type Cups