Measuring viscosity holds significance in numerous industries, including food, cosmetics, paint, and pharmaceuticals. The Zahn cup viscometer is a commonly employed tool for gauging the viscosity of various viscous substances.
Image Credit: Paul N. Gardner Company, Inc.
Executing this process can be intricate, but this article outlines the correct steps for utilizing a Zahn Dip cup. Users must begin by ensuring the selection of the appropriate-sized viscosity cup for the material being measured. Subsequently, they must thoroughly cleanse the cup and the sample container with the proper solvent. Precise viscosity measurements can be consistently achieved by following the simple steps below.
Steps:
- Inspect the cup for any obstructions or damage to the orifice area, ensuring general cleanliness.
- Pour enough sample into a 500 ml container to completely submerge the cup.
- Employ a calibrated ASTM thermometer to bring the sample material to a target temperature, typically 25.0 °C ± .5 °C. Proper temperature maintenance is crucial. The temperature can be slightly elevated by wrapping a hand around the beaker and gently stirring the material.
- Submerge the cup in the material and allow 1 to 5 minutes for the cup and liquid to reach thermal equilibrium.
- Lift the cup vertically to no more than 6 inches from the liquid surface. Start timing accurately with a stopwatch to one-tenth of a second as the cup's top edge breaks the liquid surface.
- Keep the thermometer in the beaker to ensure constant temperature throughout the run.
- Halt the timer when the initial definite break in the efflux stream occurs near the cup's bottom. Minimize air bubbles, as they might cause a premature break.
- Document time and temperature, and repeat the process. Maintain temperature drift between runs at ±.2 °C. Disregard runs differing by more than 0.5 seconds.
Selecting the Correct Size
The primary step in utilizing Zahn cups is choosing the correct size. These viscosity cups are available in various sizes, each offering specific advantages. Cup size is crucial for accuracy.
Using a cup that is too small may lead to inaccurate measurements, while a cup that is too large could result in an overestimation of viscosity.
Researchers should choose the size suitable for the application to guarantee precise outcomes, and utilize the Insta Visc Viscosity Calculator app to determine the most suitable viscosity cup for a specific viscosity range.
Thoroughly Clean the Cup and Sample
Upon choosing the appropriate Zahn size cup, it becomes vital to thoroughly cleanse the cup and the sample. Any dirt or debris on either surface can lead to inaccurate viscosity readings.
Considering the materials involved, it is also important to utilize the proper solvent for cleaning the cup and sample. For instance, users can opt for an organic solvent for a plastic cup and a water-based one for a metal cup.
Similarly, a suitable solvent for the sample is based on its polarity. A water-based solvent suits a non-polar sample, while an organic solvent works for a polar one.
Users should begin by rinsing the cup with warm water to eliminate lingering solvents or particles. Then, employ a solvent like methyl ethyl ketone (MEK) to clean the cup's interior. Finally, rinse the cup with warm water to ensure no solvent remains.
For cleaning the sample, solvents such as MEK should be used to eliminate dirt or debris. Researchers should thoroughly rinse the cup with warm water to clear any leftover solvent. They should exercise caution while cleaning the orifice to prevent deposits or scratches on internal surfaces. Directly striking the orifice when removing it from the cup should be avoided.
Place a wooden dowel against the orifice and strike the dowel with a heavy object, such as a paperweight, to dislodge the orifice. Before inserting an orifice into the cup, ensure both the orifice's exterior and the cup's receiving cone are clean.
Viscosity Oils and Cups
Gardco's standard "G" Series Oils conform to ISO 9002 standards. Viscosity cups produced by the Paul N. Gardner Company are calibrated using these oils, whose centistoke viscosity is traceable to the National Institute of Standards and Technology.
These standard oils, specially prepared by the Cannon Instrument Company for the Paul N. Gardner Company, adhere to various quality standards, including ISO/IEC 17025, ISO/IEC Guide 34, ISO 9001, BS EN ISO 9002, and ANSI/ASQC Q9002.
Accompanying graphs with the cup display the cup's viscosity number and the standard “G” Series oil used for calibration. Typically, cup calibration occurs at 25 °C. It is important to note that the viscosity of most liquids, including standard oils, varies with temperature.
The graph displays the efflux time in seconds for the given cup-oil pairing at temperatures between 20 °C to 27 °C. This cup is suitable for checking with the designated “G” oil within this range.
For optimal accuracy, it is recommended to use the standard oil labeled viscosity at 25 °C. Converting centistoke viscosity to efflux time can be done using the provided cup's formula or table.
Using silicone fluids to calibrate viscosity cups should be avoided as they alter the cup's surface interface and affect calibration.
Referring to ASTM D445, viscometers designated for silicone fluids should only be used for such fluids exclusively. Moreover, solvent washings from these viscometers should not be utilized for cleaning other viscometers.
Guide for Removing the Calibrated Oil from Gardco Viscosity Cups
All remaining material in the cup should be flushed out using an appropriate solvent. Suitable solvents include light naphtha, heptane, octane, highly aromatic solvents, or any other petroleum-derived hydrocarbon solvent. Varsol® from Exxon Company is a recommended commercial solvent for this purpose.
The viscosity cup should be thoroughly dried with a lint-free cloth. A highly volatile solvent for a secondary cleaning can be used to ensure rapid evaporation of any remaining hydrocarbon solvents post-flushing.
Different solvents are recommended for aluminum and stainless-steel cups. For aluminum cups, Hypersolve, MEK, and Alcohol can be used, while Hypersolve and Alcohol are suitable for stainless-steel cups.
Acetone, due to its high volatility and ability to dissolve traces of petroleum solvents and water, is commonly employed as the second solvent. In the third step, a low-velocity stream of clean air should suffice to evaporate any remaining traces of volatile solvent.
Take care to avoid rapid evaporation of these solvents, as it may significantly cool the surface, potentially causing a film of water to form on the cup due to humid air being brought below the dew point.
Observe and Report the Flow Rate
There is a direct relationship between a substance's viscosity and the time it takes for the liquid to discharge from the cup. The viscosity of the material can be precisely calculated by measuring the time it takes for the sample to drain.
The flow characteristics of all Gardco-produced viscosity cups are precisely delineated by a mathematical formula that correlates their efflux time with 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 of a second within the standard cup range. The resulting table is provided with each cup.
The POISE is the basic unit of viscosity, representing a defined mechanical measurement of a liquid's resistance to flow when gravity is not a factor. 100 CENTIPOISE is equivalent to 1 POISE. However, gravity acts as the force propelling the liquid through the orifice in a viscosity cup.
A material with high density will empty from a cup in less time than a material with low density but with 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 is equivalent to 1 STOKE. The CENTISTOKE serves as the reference unit in all viscosity cup measurements.
The graphs accompanying the cups establish a relationship between efflux time in SECONDS and viscosity in CENTISTOKES. While these graphs provide a general understanding of this connection, the table, which furnishes values to the nearest tenth of a second, is typically consulted.
If there is a need to determine the relationship beyond the table's range, the mathematical formulas depicted on the graphs can be utilized.
There are a few things to keep in mind when performing this measurement
- Lift the cup vertically, keeping it within 6 inches of the liquid surface
- Start the stopwatch as soon as the top edge of the cup breaks the liquid surface
- Stop the stopwatch when the first distinct break in the efflux stream occurs near the cup's bottom
After noting the flow time, the Insta Visc Viscosity Calculator app can be employed to determine the viscosity value of the material.
The flow time is directly linked to the liquid's viscosity; a longer flow time indicates higher viscosity. Using a viscosity chart allows the accurate measurement of the material's viscosity.
Summary
Zahn cup viscometers offer a reliable, cost-effective, and user-friendly method for measuring material viscosity. Following the correct procedure and utilizing a viscosity chart or the Insta Visc Viscosity Calculator app ensures accurate results, aiding in the viscosity measurement efforts.
This information has been sourced, reviewed and adapted from materials provided by Paul N. Gardner Company, Inc.
For more information on this source, please visit Paul N. Gardner Company, Inc.