This article will walk you through how to measure paint viscosity for spraying with an HVLP spray gun. These instructions pertain to the Zahn viscosity cup.
Viscosity refers to the thickness or internal friction of a coating. This measurement determines which size turbine and atomizing set (needle, nozzle and air cap) to use with your HVLP paint spray gun.
The easiest and most affordable way to measure viscosity is with a Zahn Viscosimeter. The Zahn cup is a stainless steel cup with a very small hole in the bottom. There is a long handle with a ring at the top that is attached to the side of the cup.
Zahn cups are available in five models numbered #1 - #5. The difference is size is based on the varying size of the holes in the bottom of the cup. The #2 is best for measuring thin liquids including spray coatings.
You will need a stopwatch in order to get an accurate measurement. The liquid temperature should be 77°F or 25°C. Ensure that the cup is clean and dry with no residue.
Here’s how to measure viscosity:
Step 1: Immerse the cup in the liquid. Stir it around for a few seconds to allow any air bubbles to dissipate.
Step 2: With a finger in the ring, quickly lift the cup vertically out of the liquid.
Step 3: Start the timer as soon as the top edge of the cup breaks the surface of the liquid.
Step 4: Stop the timer when you see the first break in the liquid stream at the bottom of the cup.
Step 5: Write down the number of seconds on the stopwatch.
Step 6: Repeat steps 3 – 5 until you have obtained a consistent reading.
Step 7: Thoroughly clean the cup with spray gun cleaner and dry it with a soft, lint-free cloth.
Measurements are provided in centistokes, and are expressed in seconds.
The thicker the coating, the longer it will take to flow through the hole, producing a higher reading. In general, liquids with higher viscosities will require a larger turbine as well as larger size needles, nozzles and air caps.
Check out the conversion chart below for recommendations on atomizing set sizes based on viscosity measurements.
Article last updated on May 30, 2021.