Flow Cup Viscosity Testing

Flow cup viscosity testing is a simple method for checking the consistency of a liquid coating by timing how long it takes a known volume to drain through a calibrated hole in the bottom of a cup. The result, called the efflux time, is reported in seconds with the cup type and size, for example “22 seconds, Zahn #2.” Flow cups are used in paint shops, spray booths and field applications to control thinning and keep spray results consistent from batch to batch.

Why viscosity matters

Viscosity affects how a coating atomizes, flows, levels and sags. If a material is too thick, spray patterns may be coarse and the film may show orange peel or dry spray. If it is too thin, it may run, sag, or deposit too little film per pass (see sags and runs). Temperature has a large effect on viscosity, so controlling and recording it is part of good practice. For the underlying science, see rheology and viscosity.

Types of flow cups

Dip cups

Dip cups have a handle or ring and are dipped into the liquid, lifted out, and timed as the material drains. They are popular in the field and at spray guns.

• Zahn cups are the best-known dip cups in North America. They come in sizes numbered 1 to 5, with larger numbers having larger orifices for thicker materials. ASTM D4212 covers dip-type cups including Zahn and similar designs.
• Shell cups are similar dip cups used in some industries.

Orifice (stand) cups

Stand-mounted cups are filled to a level, then the orifice is uncovered and the flow is timed. These are typically used in labs and quality control.

• Ford cups are common in North America and are covered by ASTM D1200.
• ISO cups are specified by ISO 2431 and are widely used in Europe and internationally.
• DIN cups follow the older German DIN 53211 standard, now largely replaced by ISO 2431.

Each cup family has different geometry, so results from one cup type cannot be converted reliably to another without a correlation for that material.

How to perform a dip cup test

1. Condition the sample to the specified temperature. Many procedures use 25 °C (77 °F), but follow the product data sheet or quality plan.
2. Mix the coating thoroughly, including any thinner, and allow entrapped air to escape.
3. Choose the right cup. Pick a size that gives an efflux time in the cup’s recommended range, typically between about 20 and 80 seconds for most cups.
4. Immerse the cup fully and hold it in the liquid for a short time so it reaches the liquid’s temperature.
5. Lift the cup vertically and start the timer when the top of the cup clears the surface.
6. Stop the timer at the first definite break in the stream, as defined by the method.
7. Repeat the measurement and record the average along with the temperature and cup type.

How to perform an orifice cup test

1. Level the cup on its stand.
2. Close the orifice with a finger or plug and fill the cup until it overflows, then strike off the excess with a flat glass plate or straight edge.
3. Release the orifice and start timing.
4. Stop at the first break in the stream.
5. Record temperature, cup type and efflux time.

Limitations

Flow cups are designed for Newtonian or nearly Newtonian liquids, whose viscosity does not change with shear rate. Many modern coatings are shear thinning or thixotropic, which means they thin under stirring or spraying and thicken at rest. For these materials, a flow cup result may vary with handling and may not predict application behavior well. Rotational viscometers, such as Brookfield or Krebs-Stormer instruments, give better information for structured coatings.

Other limitations include:

• Sensitivity to temperature; a few degrees can change the result substantially.
• Wear or damage to the orifice, which changes calibration.
• Dried coating residue in the cup.
• Operator variation in timing the end point.

Flow cups are generally not suitable for very high-solids, plural-component materials such as polyurea, which are controlled by heating and pressure at the proportioner rather than by thinning.

Calibration and care

• Clean cups immediately after use with a suitable solvent and soft cloth. Never use wire or abrasive tools in the orifice.
• Check cups periodically against a certified viscosity standard oil at a controlled temperature.
• Replace cups whose results drift outside tolerance.
• Store cups so the orifice is protected.

Using flow cups in practice

Thinning control

A shop may set a target efflux time for a primer or topcoat, such as the value listed on the data sheet. Operators add thinner in small increments until the target is reached, while staying within the maximum thinning allowed for VOC compliance (see thinning coatings and VOC regulations).

Batch acceptance

Incoming coating batches can be checked against a specification range as part of quality control plans.

Records

Viscosity checks are often recorded with batch numbers and ambient conditions in daily inspection reports.

Frequently asked questions

Can I convert Zahn seconds to centipoise?

Approximate conversion charts exist, but they assume Newtonian behavior and specific conditions. Treat them as rough guidance only.

Which cup should I use?

Use the cup named on the product data sheet or specification. If none is named, pick a cup size that gives an efflux time within its valid range.

Why does my reading change during the day?

Temperature changes are the most common reason. Solvent evaporation from an open container also thickens the material over time.

Related entries

• Rheology and viscosity
• Thinning coatings
• Density of liquid coatings
• ASTM standards for coatings
• Flash point of coatings (new)
• Pot life and gel time (new)