A flow cup, sometimes called an efflux cup or viscosity cup, is a simple gravity device that permits the timed flow of 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 (expressed in stokes and centistokes) that is dependent upon the specific gravity of the draining liquid. However, the conditions in a simple flow cup are seldom ideal for making true measurements of viscosity.

For many applications it is not necessary to know the absolute viscosity of a paint or varnish. The efflux time, measured in seconds, is often sufficient for a relative classification.

At least 50 types of flow cups have been developed and used over the years, mainly for production control and field inspection purposes. Most of these simple 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 principles that should be recognized:

• 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.
• The diameter of the orifice should be selected and maintained so as to provide flow times falling within prescribed

Ford Viscosity Cups
The Ford Viscosity Cups are guaranteed to be within 23% (drain time of calibration oil) throughout the recommended be use range.
For low viscosity liquids

• Body made of solid bar aluminum
• Stainless steel orifice
• Calibrated against standard oils referenced to certified NIST oils (National Institute of Standards and Technology of United States)
• Certified cups available on request
   

DIN Flow Cup
This cup holds 100 ml ± 1 ml, and has an integrated orifice with a diameter of 4 mm ± 0.02 mm.

• For low viscosity liquids
• Body made of anodized aluminum
• Stainless steel orifice, interior polished
• Calibrated against standard oils referenced to certified PTB oils (Federal Institute of Physics and Metrology of Germany) to be within 3% (drain time of calibration oil)

 
DIN EN ISO Flow Cup
This cup has a longer orifice, less tapered body and slightly different inner dimensions than the DIN 53211* flow cup and thus provides different efflux times. The extended measurement range makes the DIN EN ISO cup a useful supplement of the DIN cup.

• Recommended for international use
• Body made of anodized aluminum
• Stainless steel orifice, interior polished
• Calibrated against standard oils referenced to certified PTB oils to be within 3% (drain time of calibration oil)