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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).

Molecular Weight, Density, Surface Tension,
and Viscosity of Selected Liquids
Name
Molecular Formula
Mol.Wt.
Specific
Density
(1)
Surface Tension
(2)
Viscosity
         
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.

Related Links:
S-90 Signature Cups
EZ® Cups
Calibration Oils
Stopwatches and Timers
Tensiometers

Thermometers
Weight Per Gallon Cups

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Phone 1-800-833-4644 (U.S.) or (603) 543-0038; fax (603) 543-1334
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