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.
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
|Methyl ethyl ketone||C3H8O||72.11||0.799||24.6||0.41||0.51|
(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.