Measuring Viscosity with a Zahn Dip Cup: The Proper Process

Viscosity is an important property to measure when it comes to many industries, such as food, cosmetics, paint, pharma and many more. Zahn cup viscometers are a common tool used to determine the viscosity of a wide variety of viscous materials.

It can be a tricky process, but this article will outline the proper procedure for using a Zahn Dip cup. First, make sure that you have selected the correct size viscosity cup for the material you are measuring. Next, thoroughly clean both the cup and your sample container with the proper solvent. With these simple steps below, you can measure accurate viscosity every time!

1. Examine the cup for obvious obstruction or damage to the orifice area and general cleanliness of cup.
2. Pour enough of your sample into a 500 ml container to totally submerge the cup.
3. Using a calibrated ASTM thermometer bring your sample material to a target temperature (usually 25.0°C ± .5°C). Proper temperature is very important. By wrapping a hand around the beaker and slowly stirring your material, the temperature can be slightly raised.
4. Place cup in the material and allow 1 to 5 minutes for cup and liquid to reach thermal equilibrium.
5. Raise the cup vertically to no more than 6” from the surface of the liquid. Start timing with a stopwatch with accuracy of one tenth second or better as the top edge of the cup breaks the surface of the liquid.
6. Leave the thermometer in the beaker to ensure the temperature remains constant throughout the run.
7. Stop the timer when the first definite break in the efflux stream occurs near the bottom of the cup. Be sure to keep air bubbles at a minimum; they will sometimes cause an apparent break prematurely.
8. Record time and temperature and repeat. Keep temperature drift between runs at ±.2°C. Disregard any runs that are more than .5 seconds apart.
    

Selecting the Correct Size

The first step in using Zahn cups is to select the correct size. These viscosity cups come in a variety of sizes, each with its benefits. The size of the cup you use is important for accuracy. If you use a cup that is too small, you will not be able to measure the sample accurately. Conversely, if you use a cup that is too large, you may overestimate the viscosity of the sample. Make sure to select the appropriate size for your application to ensure accurate results. You can use the Insta Visc Viscosity Calculator app to determine which viscosity cup would be best for your specific viscosity range.

Thoroughly Clean the Cup and Sample

Once you have selected the correct Zahn size cup, it is important to clean both the cup and your sample. Any dirt or debris on either can lead to inaccurate viscosity measurements.

It is important to use the correct solvent when cleaning the cup and sample. Different solvents can be used to clean different types of materials. For example, an organic solvent can be used to clean a plastic cup, while a water-based solvent can be used to clean a metal cup. The same is true for the sample. A water-based solvent can be used to clean a non-polar sample, while an organic solvent can be used to clean a polar sample.

To clean the cup, first, rinse it with warm water to remove any residual solvents or debris. Next, use a solvent such as a methyl ethyl ketone (MEK) to clean the inside of the cup. Be sure to rinse the cup again with warm water to remove any remaining solvent.

To clean the sample, use a solvent such as MEK to remove any dirt or debris. Be sure to rinse the sample with warm water to remove any remaining solvent.

Particular care should be used in cleaning the orifice to avoid leaving deposits or scratches on internal surfaces. Never strike the orifice directly when removing it from the cup. Place a wood dowel rod against the orifice and strike the dowel with a heavy object such as a paper weight and catch the orifice. Prior to inserting an orifice into the cup, ensure that the exterior of the orifice and the receiving cone of the cup are clean.

Viscosity Oils and Cups

Standard "G" Series Oils Meet ISO 9002 - Gardco produced viscosity cups are calibrated with standard “G” Series oils. Centistoke viscosity of these oils is traceable to the National Institute of Standards and Technology. These standard oils, prepared expressly by the Cannon Instrument Company for the Paul N. Gardner Company, are produced in accordance with ISO/IEC 17025, ISO/IEC Guide 34, ISO 9001; BS EN ISO 9002; ANSI/ASQC Q9002.

Graphs included with the cup will show the viscosity cup number and the standard “G” Series oil used for its calibration. Normally, cup calibration is at 25.0° Celsius. Viscosity of most liquids, including the standard oils, are dependent on temperature. Efflux time in seconds for the indicated cup-oil combination from twenty (20) to twenty-seven (27) degrees Celsius is shown in the graph. The cup may be checked with the indicated “G” oil with reasonable accuracy within these limits. For best accuracy, the standard oil label viscosity with temperature at 25.0° Celsius should be used. Conversion from centistoke viscosity to efflux time in seconds is by the formula or table furnished with the cup.

Warning - Silicone fluids should not be used to calibrate viscosity cups. These materials change the interface between the cup surface and the test material and therefore change the cup calibration. The following is taken from ASTM D445: Viscometers used for silicone fluids should be reserved for the exclusive use of such fluids. Solvent washings from these viscometers should not be used for cleaning other viscometers.

Guide for removing the calibrated oil from Gardco viscosity cups. 

Any remaining material in the cup must be removed by flushing with a suitable solvent. Light naphtha, heptane, octane, highly aromatic solvents, and or any other petroleum-derived hydrocarbon solvent can be used. Varsol® is a commercial solvent that works very well for this purpose. Varsol is a registered trademark of the Exxon Company

Completely dry the viscosity cup with a lint free cloth. Use a highly volatile solvent for a second cleaning as since any remaining hydrocarbon solvents from the first process will evaporate quickly after the sample has been flushed from the cup. Hypersolve, MEK and Alcohol can be used in aluminum cups and Hypersolve and Alcohol for the stainless-steel cups. Acetone is commonly used as the second solvent because of its high volatility and its ability to dissolve traces of petroleum solvents and water.

In the third process a low velocity stream of clean air will be sufficient to evaporate remaining traces of any volatile solvent. Be aware, avoid rapid evaporation of these solvents as this can cool the surface to such an extent that humid air may be brought below the dew point, causing a film of water to form on the cup.

Observe and Report the Flow Rate

The viscosity of a material is directly related to how long it takes for the liquid to flow out of the cup. By timing how long it takes for the sample to empty, you can accurately calculate the viscosity of your material.

The flow characteristics of all Gardco produced viscosity cups are accurately defined by mathematical formula relating their Efflux time to the viscosity of standard oils which are traceable to the National Institute of Standards and Technology. For convenience, the formula has been solved for each tenth second within the normal cup range. The resulting table is furnished with each cup.

The POISE is the fundamental unit of viscosity. It is a defined mechanical measurement of the resistance of a liquid to flow where gravity is not a factor. 100 CENTIPOISE = 1 POISE. However, gravity is the driving force causing liquid in a viscosity cup to flow through the orifice. A high-density material will flow from a cup in a shorter time than a low-density material of the same viscosity. The STOKE is defined as the POISE divided by specific gravity (or weight per gallon in pounds times 0.120). 100 CENTISTOKES = 1 STOKE. The CENTISTOKE is the unit of reference in all viscosity cup measurements.

The graphs included with the cups relate efflux time in SECONDS to viscosity in CENTISTOKES. The graphs may be used for determining the rough relationship between these factors, but usually reference will be made to the table that is furnished with each cup which provides values to the nearest tenth of a second. If there is a necessity to determine the relationship beyond the range of the table, the mathematical formulas shown on the graphs may be used.

There are a few things to keep in mind when performing this measurement:

• Raise the cup vertically to no more than 6” from the surface of the liquid
• The stopwatch should be started as soon as the top edge of the cup breaks the
  surface of the liquid
• The stopwatch should be stopped when the first definitive break in the efflux stream occurs near the bottom of the cup

Once you have recorded the flow time, you can use the Insta Visc Viscosity Calculator app to find the viscosity value of your material. The flow time will be directly correlated to the viscosity of the liquid; the longer it takes for the liquid to flow, the higher its viscosity will be. By using a viscosity chart, you can determine the viscosity of your material with a high degree of accuracy.

Summary

Zahn cup viscometers are a reliable, economical, and easy-to-use way to measure the viscosity of a material. By following the proper procedure and using a viscosity chart or the Insta Visc Viscosity Calculator app, you can obtain accurate results that will be helpful in your viscosity measurement efforts.

Also, visit our 30 Seconds in a Zahn Cup article and the Viscosity Cup FAQs Viscosity Cup FAQs for additional information.