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Overview: Testing with Viscosity Cups

The original patent for Zahn cups was issued in 1938. These early cups, though uncalibrated and unstandardized, were worth their weight in gold in the hands of a skilled formulator. Gradual improvements in design and quality followed, but it was not until quite recently that standardization among viscosity cups became the norm.

Now, thanks to rigorous quality standards, the drain time of a liquid through a known cup can be compared from lab to lab, with a known margin of error. The EZ® and S90 "Signature" series viscosity cups are manufactured to the strictest tolerances, and, when appropriately calibrated with the standard oils, can be used with assurance for such lab-to-lab comparisons.

This method measures viscosity at only one level of shear; it cannot monitor thixotropy or other second order rheological properties, nor can its results be directly compared to readings taken in-situ from a dynamic process. Nevertheless, the viscosity cup is still – and likely will long remain – a core tool for formulators and users of inks, paints, coatings, adhesives, and many other liquids. A brief test procedure follows.

Test Procedure

1. Select the viscosity cup which is sized appropriately for the liquid to be tested. Be sure that the cup is absolutely clean, and free of any burrs or other defects.

2. Adjust, as necessary, the temperature of the test liquid.

3. Use your index finger to lift the cup by its ring. Avoiding all bubbles, froth, foam, etc., immerse the cup fully into the test liquid. Make sure the cup is vertical and free of air pockets.

4. Measure and record the temperature of the liquid inside the cup.

5. Smoothly and rapidly remove the viscosity cup. Start the timing device exactly as the top of the filled cup breaks the surface of the liquid vessel you are drawing the sample from.

6. Stop the timing device at the first obvious break in the efflux stream, as observed about one to two inches below the base of the cup.

7. Record the exact drain time (to 0.1 sec), temperature, cup number, and all necessary product data. This can be converted to centistokes by using the conversion table furnished with the viscosity cup.

8. For improved assurance and precision, perform three tests, record the results from all three, and use the average as a best estimate of actual viscosity.

9. Be sure to clean the cup thoroughly, immediately after each use (nylon fishing line can be used to clean the orifice).

The following table (Molecular Weight, Density, Surface Tension, and Viscosity of Selected Liquids) is hidden from smaller viewports but is still viewable on these devices. Click here to view.

Molecular Weight, Density, Surface Tension, and Viscosity of Selected Liquids

Name Molecular Formula Mol.Wt. Specific
Surface Tension
Centipoise Centistokes
Acetic acid C2H4O2 60.05 1.043 27.5 1.06 1.02
Acetone C3H6O 58.08 0.786 23.5 0.31 0.39
Butyl acetate C6H12O 116.16 0.876 24.3 0.69 0.78
Cyclohexane C6H12 84.16 0.773 25.3 0.89 1.16
Cyclohexanol C6H12O 100.16 0.960 33.4 57.5 59.9
Ethanol C2H6O 46.07 0.787 22.5 1.08 1.37
Ethanolamine C2H7NO 61.08 1.014 48.9 21.1 20.8
2-Ethoxyethanol C4H10O2 90.12 0.931 28.9 1.85 1.99
Ethyl acetate C4H8O2 88.11 0.894 23.9 0.42 0.47
Ethylene glycol C2H6O2 62.07 1.110 48.3 16.1 14.5
Formamide CH3NO 45.04 1.130 58.1 3.34 2.96
Formic acid CH2O2 46.03 1.220 37.1 1.61 1.32
Methanol CH4O 32.04 0.787 22.4 0.54 0.69
Methyl acetate C3H6O2 74.08 0.927 24.7 0.36 0.39
Methyl ethyl ketone C3H8O 72.11 0.799 24.6 0.41 0.51
1-Propanol O4H8O 60.10 0.802 23.7 1.95 2.43
Toluene C7H8 92.13 0.865 28.5 0.56 0.65
Water H2O 18.02 0.998 72.8 0.89 0.89

(1} g/ml @ 20°C  (2} dynes/cm @ 20°C  (3} Viscosity as measured @ 25°C. Centistoke is a measure used in viscosity cup tests; it equals centipoise divided by specific density.