The Proper Process for Measuring with a Viscosity Cup

Viscosity is an important property to measure when it comes to many industries, such as food, cosmetics, paint, pharma and many more. Zahn, Ford, DIN, ISO, Fisher, Parlin and other types of 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 many types of viscosity cups.

Check out our demonstration videos for a visual step by step process of how to use a Standard Viscosity Cup and a Dip Viscosity Cup.

Selecting the Correct Size

The first step in using viscosity 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 GARDCO Viscosity Calculator App to determine which viscosity cup would be best for your specific viscosity range.

After use remove any remaining material in the cup 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® (registered trademark of the Exxon Company) is a commercial solvent that works very well for this purpose. Completely dry the viscosity cup with a lint free cloth. Then use a highly volatile solvent for a second cleaning 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.

It is recommended that calibration of the cup be confirmed periodically, or if dropped or otherwise subjected to damage, with the appropriate standard oil selected from the specification table.

Viscosity Oils

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.

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. 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. A point of caution, even the standard oils change in viscosity in the range of 3% - 8% per degree Celsius at the 25° normal measuring range. Temperature of products being measured should be adjusted to within 0.1° of specified temperature if accurate results are to be obtained.

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.

Calibration Procedures

Procedure

1. Examine the cup for obvious obstruction or damage to the orifice area and general cleanliness of cup.
2. Place cup in stand and level the top edge. The bottom of the cup should be approximately 6” above the container bottom.
3. Select the recommended Certified Viscosity Standard for the cup to be calibrated. Make sure the expiration date for the standard has not passed.
4. Pour a sufficient amount of oil into a 250 ml container to exceed the capacity of the cup.
5. Bring oil to target temperature of 25.0°C ± .5°C. Proper temperature is very important. Use a certified thermometer to monitor this process. By wrapping a hand around the container and slowly stirring oil, the temperature can be slightly raised.
6. Fill cup to the top edge (slightly overfill cup and remove excess with a straight edge or glass plate), while holding a finger over the orifice. Pour slowly to minimize air bubble formation.
7. Simultaneously release finger from the orifice and start timing with a stopwatch with accuracy of 1/10th second or better.
8. Hold the thermometer in the efflux stream to insure the temperature remains constant throughout the run.
9. 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.
10. Record time and temperature and repeat. Keep temperature drift between runs at ±.2°C. Disregard any runs that are .5 seconds apart.
11. Adjust times for temperature deviation and average three valid runs.
12. Divide the design time listed on Viscosity Standard by the average and determine the correction factor.
13. Determine if the cup is in or out of tolerance according to manufacturer's specification.

Standard Cup Procedure meets the requirements of ASTM D1200 & D5125

Procedure

1. Examine the cup for obvious obstruction or damage to the orifice area and general cleanliness of cup.
2. Select the recommended Certified Viscosity Standard for the cup to be calibrated. Make sure the expiration date for the standard has not passed.
3. Pour a sufficient amount of oil into a 600 ml container to totally submerge the cup.
4. Using a calibrated ASTM mercury-filled thermometer bring oil to target temperature, 25.0°C ± .5°C. Proper temperature is very important. By wrapping a hand around the beaker and slowly stirring oil, the temperature can be slightly raised.
5. Place cup in oil and allow 1 to 5 minutes for cup and oil to reach thermal equilibrium.
6. Raise the cup vertically to a distance of no more than 6” from the surface of the oil. Start timing with a stop watch with accuracy of one tenth second or better as the top edge of the cup breaks the surface of the oil.
7. Leave the thermometer in the beaker to insure the temperature remains constant throughout the run.
8. 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.
9. Record time and temperature and repeat. Keep temperature drift between runs at ±.2°C. Disregard any runs that are more than .5 seconds apart.
10. Adjust times for temperature deviation and average three valid runs.
11. Divide the design time listed on Viscosity Standard by the average and determine the correction factor.
12. Determine if the cup is in or out of tolerance according to manufacturer's specification.

Dip Cup Procedure meets the requirements of ASTM D4212

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.

Equivalent Charts cover GARDCO Calibrated Viscosity Cups and are based on conversion formulas of a type & with parameters as referenced in The Encyclopedia of Polymer Science & Engineering (Vol. 4, Second Edition, John Wiley & Sons, Inc.). Standard oils traceable to the National Institute of Standards & Technology were used in experimental development of these formulas.

Varsol® is a registered trademark of the Exxon Company

Caution

A point of caution: Even the standard oils change in viscosity in the range of 3% - 8% per degree Celsius at the 25° normal measuring range. Temperature of products being measured should be adjusted to within 0.1° of specified temperature if accurate results are to be obtained.

Things to keep in mind when performing this measurement:

When using a dip cup raise the cup vertically to no more than 6” from the surface of the liquid. The stopwatch should be started as soon as the flow begins in a Standard Cup or when the top edge of the Dip 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 GARDCO Viscosity Cup Equivalent Excel download or our GARDCO Viscosity Calculator for Desktop or Mobile 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.

GARDCO produced viscosity cups are calibrated with standard “G” series oils. These standard calibration oils prepared expressly by the Cannon Instrument Company for the Paul N. Gardner Company are produced in accordance with ISO/IEC 17025, ISO Guide 34, ISO 9001. Centistokes viscosity of these oils is traceable to the National Institute of Standards and Technology.

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. The cup may be checked with the indicated “G” oil with reasonable accuracy within these limits. For best accuracy, the temperature of the standard oil should be 25°C. The standard oil value in centistokes is printed on the standard oil bottle label. Conversion from viscosity to efflux time in seconds is by formula which defines the characteristics of the cup.

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