Definition
A four-cosity cup is a flow-out viscometer used to measure the viscosity of Newtonian or approximate Newtonian fluids in motion. Its design is usually a cylindrical cup with a short-flowing nozzle of a specific size in the center of the bottom, which determines the viscosity of the fluid by measuring the time it takes for a certain volume of fluid to completely flow out of the spout under gravity. Due to its ease of operation and low cost, this instrument is widely used in the production and quality control of coatings, inks, adhesives and other industries.
Measurement principle
The measurement of the four viscosity cups is based on the Hagen-Poissouer law and the gravity outflow principle. When the cup is filled with fluid, the fluid flows out through the short pipe spout at the bottom under gravity. For Newtonian fluids, their kinematic viscosity and outflow time are approximately linear within a certain range. In general, the kinematic viscosity ν of a fluid can be calculated by the empirical formula: ν = K (t - C). where t is the outflow time (seconds), K is the instrument constant (usually in mm²/s²), and C is the correction value related to fluid kinetic energy and end effect. This formula is suitable for a specific shear rate range where the fluid exhibits good Newtonian properties.
Measurement method
Make sure the viscosity cup is clean and dry before measurement, and adjust the temperature of the fluid sample to a standard temperature (e.g. 25°C). Use the appropriate stopper to block the spout and pour the sample into the cup until the liquid level overflows the top edge. Scrape off excess fluid so that the liquid surface is flush with the cup mouth. Quickly remove the plug and start the timer at the same time to record the time when the first break point occurs during the fluid outflow, which is the outflow time. Averaging several repeated measurements increases the reliability of the results. After the measurement is completed, the viscosity cup should be thoroughly cleaned immediately to prevent the residue from clogging the flow nozzle after drying.
Influencing factors
The accuracy of the measurement results is influenced by several factors. Fluid temperature is the primary factor, and temperature changes can significantly alter fluid viscosity, so it needs to be measured under constant temperature conditions. The dimensional accuracy and interior wall finish of the flow nozzle directly affect the flow rate, and any wear or contamination can introduce errors. Consistency in operating techniques, such as synchronization of removing the plug and starting the timing, can also affect the time reading. Non-Newtonian properties of fluids, such as thixotropy or shear thinning behavior, can cause outflow time versus viscosity to deviate from the standard formula. In addition, environmental conditions such as air flow may also interfere with the fracture judgment of the fine liquid flow.
Applications
Coated 4 viscosity cups are mainly used in industrial scenarios where fluid viscosity needs to be quickly and relatively compared. In the coating industry, it is used to control the viscosity of paints and varnishes; In the field of printing inks, it is used to ensure the printability of inks; In adhesive manufacturing, it is used to monitor the consistency of resins or colloids. It is also commonly used in laboratories and production lines in chemical, food, textile and other industries as a routine tool for process control and quality inspection. The measurements are often used for internal quality control or rapid adjustments in the production process, rather than as a precise arbiter of absolute viscosity.
Instrument selection considerations
When selecting, it is necessary to select the viscosity cup with the corresponding designation according to the expected viscosity range of the fluid to be measured. Common designations such as Tu-4 cup have a fixed nozzle diameter and length corresponding to a specific measurement range (e.g., an outflow time between 20 and 100 seconds is preferable). If the fluid viscosity is high and the outflow time is too long, a model with a larger nozzle diameter can be considered; On the other hand, for low-viscosity fluids, a small pore size model should be selected to ensure adequate measurement time resolution. In terms of material, stainless steel cups are durable and easy to clean, making them suitable for most chemical environments. Users should confirm that the instrument complies with relevant national or industry standards (e.g., GB/T 1723, ASTM D4212) and regularly calibrate it with standard viscosity oil to verify the accuracy of the instrument's constant K value.