Definition
DIN viscosity cup is a flow-out viscosity measurement device designed according to the German Industrial Standard (DIN), mainly used for the rapid determination of the kinematic viscosity of Newtonian or near-Newtonian fluids in the laboratory or production site. Its core component is a precision-machined cup-shaped vessel with a standard-sized outlet hole at the bottom. By measuring the time it takes for a volume of fluid to completely flow out under the action of gravity, the viscosity value of the fluid can be indirectly deduced.
Principle
The measurement of DIN viscosity cups is based on the simplified application of the Hagen-Poissouer law. When a fluid flows out through a short tube at the bottom of the cup driven by gravity, its flow behavior is closely related to the viscosity of the fluid. For a given viscosity cup model, its geometry (e.g., outflow hole diameter, cup volume) is standardized. Under ideal conditions, the kinematic viscosity ν of the fluid is approximately proportional to the outflow time t and can be correlated by the following empirical formula: ν = k(t - c), where k is the instrumental constant and c is the modifier related to the flow end effect. This formula has good reliability within the flow time frame specified by the standard.
Measurement method
Ensure that the viscosity cup and the sample to be tested are at a standard test temperature (usually 23°C or 25°C) before measurement. Dip the viscosity cup into the sample and lift it slowly to fill it completely. The cup body is then placed vertically above the sample, and the finger or baffle is quickly removed to open the outflow hole while the timer is activated. When the continuous flow is interrupted for the first time, the timing is stopped and the outflow time is recorded. Measurements are usually repeated three times, with the average as the final result. Finally, the outflow time is converted to the kinematic viscosity value according to the conversion table or formula corresponding to the viscosity cup model used.
Influencing factors
The accuracy of the measurement results is influenced by several factors. Temperature fluctuations can significantly alter fluid viscosity, so strict thermostatic control is a necessary prerequisite. The cleanliness of the viscosity cup is critical, and residue can change the effective diameter of the outflow hole. During operation, the cup body must be completely vertical, and tilting will cause the static pressure head to change. The processing accuracy and wear state of the outflow hole directly affect the instrument constant. In addition, the non-Newtonian properties of fluids (e.g., thixotropy, shear thinning) can deviate from the standard formula for outflow time versus viscosity, making this method more suitable for Newtonian fluids.
Application:
DIN viscosity cups are widely used in quality control and production process monitoring in coatings, inks, adhesives, varnishes, food and chemical industries. In the coatings industry, it is used to detect brush viscosity to ensure construction performance; In ink production, it is used to evaluate printability; In the food industry, it can be used to determine the consistency of syrups, sauces and other products. Its easy operation, low cost, and portability make it a common tool for rapid screening in the field. However, it should be noted that its measurement results are conditional viscosity, which is usually used for relative comparison between products of the same type or for trend monitoring in the production process.
Selection
The expected viscosity range of the fluid to be measured should be considered when selecting. The DIN standard defines a variety of models (e.g. DIN 4, DIN 6, DIN 8, etc.), and the numerical number roughly corresponds to the outflow hole diameter (mm). Smaller numbered models have finer outflow holes and are suitable for lower viscosity fluids; Higher numbers are suitable for higher viscosity fluids. The user should choose a model that allows the outflow time to fall within the standard recommended range (typically 30 to 100 seconds) based on the conventional viscosity values of the sample to be tested. At the same time, it is necessary to confirm the chemical compatibility of the instrument material (e.g. stainless steel) with the sample, and to give preference to products that comply with the current DIN EN ISO 2431 standard to ensure the reliability of value transfer.