A flow cup, sometimes called an efflux or viscosity cup, is a simple gravity device that measures the timed flow for a known volume of liquid passing through an orifice located at the bottom of the shaped cup. Under ideal conditions this rate of flow would be proportional to the kinematic viscosity, which is dependent upon the specific gravity of the liquid and typically measured in stokes, centistokes, or mm2/s. For many applications, it is not necessary to know the absolute viscosity. The efflux time is often sufficient for a relative classification of the viscosity. Conditions such as the temperature of the fluid can affect the measurement, so consult the instruction manual and standard method for each cup.
At least fifty types of flow cups have been developed and used over the years, mainly for production control and field inspection purposes. Most of these cups are of two main types – mounted on a stand for filling and draining, or dipped directly into the liquid container before draining back into the same container. No matter which type of cup is used there are several fundamental considerations that should be recognized:
The diameter of the orifice should be selected and maintained so as to provide flow times falling within prescribed minimum and maximum limits.
The temperature of the draining liquid should be controlled and measured in the efflux stream only after it passes through the drainage hole.
Precautions should be taken whenever thixotropic or other non-Newtonian liquids are tested for viscosity, because there is no definite rate of shear generated in a flow cup.
Certified Viscosity Cups
For a cup guaranteed to meet its specifications, consider using a certified cup. Other instruments for measuring viscosity in the laboratory that have improved precision and repeatability include the Brookfield Viscometer and the Cone & Plate Viscometer.
Checking Procedures and Calibration of Viscosity Cups
Consult the standard method listed for your cup to determine the procedures for checking and calibrating the cup.
Viscosity Cup Equations
The equations listed in this chart along with their constants K and C can be used to convert the time of flow (T) in seconds to Kinematic Viscosity (V) in centistokes.
1 stoke = 100 centistokes (cSt)
1 centistoke = 1 mm2/s
Viscosity in centistokes × Specific Gravity = Viscosity in centipoise
Tracy Hill is a Technical Writer with BYK-Gardner USA, Columbia, MD.
Viscosity Cup Conversion Chart
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BYK-Gardner Viscosity Cups
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