Ask the Expert Question-and-Answer Archive

by Ron Joseph

December, 2002

Paint and Coating Viscosity Measurements Using a Zahn Cup

Q. I have heard about a stainless steel paint drip measuring utensil cup for measuring the viscosity (thickness) of the paint. Apparently, you time how long it takes the amount of paint to drip out of the cup from the time it was first poured (between 17 to 21 seconds). Can you tell me where I can purchase this?

A.You are correct. You can purchase a stainless steel cup that has a small orifice of very precise dimensions drilled into the bottom. Using the long handle that is brazed onto the cup, you immerse the cup into your paint or organic coating. As you lift the cup vertically out of the coating you start a stop watch and measure the time it takes for the coating to drain. You stop the watch immediately the paint stream breaks. You DO NOT wait until all of the paint or coating has drained. Because viscosity is affected by the temperature of the coating you might want to measure the temperature at the same time you perform the viscosity test. On a cold day the same coating will take longer to efflux from the cup than on a warm day. If you intend to set a viscosity standard for the coating, you should perform the measurement at a constant temperature.

If you intend to measure the viscosity of organic coatings, you will want to purchase a Zahn #2 and/or a Zahn #3 viscosity cup. The Zahn #2 cup is intended for coatings that have a relatively low viscosity, generally in the range of 18 - 35 seconds. For high solids coatings you might find that the Zahn #2 cup is too insensitive and that viscosity measurements go well above 35 seconds. I have measured coatings with a Zahn #2 measurement well in excess of 1 minute. For these higher viscosity coatings I suggest that you use a Zahn #3 cup so that with the larger orifice at the bottom of the cup, you can bring the measurements back down to 18 - 35 seconds. The most important consideration in selecting between Zahn #2 and Zahn #3 is that you get a clear break in the paint stream as it effluxes from the cup. If you find that as the cup empties the paint stream stops and starts, and stops and starts again, then you are probably using a cup for which the orifice is too small, and you should probably go to the next higher cup number.

Someone might recommend that you use a Ford #4 cup instead of a Zahn cup. Before you purchase one or the other, here are some differences. The Ford #4 viscosity cup has a very much larger orifice than the Zahn cups, and for the most part they are used in paint or coating laboratories where more accurate measurements are required. They are well suited to laboratories, but probably are less convenient for paint production shops. If you have a mixing table in your paint shop and you want accurate viscosity measurements, then the Ford #4 cup is fine. But if your operation is not so critical of viscosity and if the painters are likely to forget to properly clean the cup after using it, the Zahn cups are probably your preferred choice because they are less expensive.

You can purchase Zahn and Ford #4 cups by going to www.byk-gardner.com or www.gardco.com and look in their catalogs for Viscosity Measurements.

Please be aware, that if you are subject to an EPA or State regulation on VOC you should not add thinners or solvents to the coating to bring the viscosity into a desired range, By arbitrarily adding thinners you might exceed the VOC regulatory limit for your coating.