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A viscosity cup is an invaluable tool if you want to get the best results from your stain, paint, varnish, or lacquer project. If you don’t own a viscosity cup and are itching to get started on your project, use the information in this article to make your own in less time than it would take to run to the store.
You’ll also learn what viscosity is, why it’s important, and how to use your new cup.
What Is Viscosity?
Viscosity, simply put, is a measurement of how much or how little a given fluid ‘flows’. It describes how much internal friction there is at a molecular level. Water molecules easily slip past each other, making it easy to pour or spread. Water has low internal friction, and therefore, low viscosity.
Household paint, on the other hand, can be highly viscous. High internal friction makes the molecules harder to separate, resulting in slower pour times than water, and more difficulty spreading the material.
To find out exactly how viscous a substance is, use a viscosity cup.
Why Do I Need a Viscosity Cup?
Knowing the viscosity of your paint, stain, or wood finish is helpful anytime you’re operating outside of the ideal temperature conditions. It is essential when applying these materials with a spraying device.
In order to apply one of these products with a sprayer, for instance, you must thin it to a viscosity that will easily travel through the hose, nozzle, and spray tip, without clogging.
Unfortunately, you can’t just find a ratio of product to thinner that works once, and stick with it for the rest of your woodworking life. What works on a cool, dry day is different from what works in warm, wet weather.
Viscosity cups solve this problem by giving you a way to measure the actual viscosity of your substance in real time. The viscosity will change based on storage methods, ingredients, age, and environmental conditions.
Paint from cans stored on a concrete floor, for instance, will be more viscous than the same paint from a can stored on the shelf. When they get up to room temperature or are left in the sun, the viscosity will lower.
When you purchase a paint sprayer, the label will likely list a desired range of viscosity, as measured by a specific type of viscosity cup. Popular models are Zahn and Ford, but many, many others exist, making things even more complicated.
Conversion charts are used to help you understand how the measurement on the label will correspond to the measurements of other viscosity cups. Learn more about what an HVLP sprayer is, and why viscosity is an important measurement.
Making a Viscosity Cup
More than 20 different versions of viscosity cups are available on the market today. Highly precise viscosity cups used for scientific purposes can cost close to $300. This level of precision is not necessary for working with paint, stain, and wood finishes.
While cheaper versions of viscosity cups are available, you can also make your own. Here’s how:
- Get to know the most common viscosity cups. The two biggest names in the game are Zahn and Ford. ISO, AFNOR, and ASTM cups are other well-known types of cup. Viscosity measurements are listed as following: number of seconds that the fluid flows, the type of cup and the size of the bottom opening, measured in millimeters. Thus, if your paint sprayer asks for paint with a viscosity of 15-30, Zahn #4, this would refer to paint that takes between 15 and 30 seconds to pass through a Zahn-type viscosity cup with a four millimeter opening at the bottom.
- Decide what kinds of viscosity cups you need. Take a look at the labels of any finish you are currently storing, and note which type of viscosity cup is mentioned. Pay particular attention to the last number, which will tell you how many millimeters the opening should be.
If you own a sprayer already, consult the manufacturer’s details and note the desired viscosity range. Maybe you’re planning to purchase a sprayer for the first time, or have already purchased one and didn’t realize you needed to measure the viscosity. Consult your specific product manual for guidance.
- Gather your materials. To complete this project, you will need: a permanent marker, an empty plastic bottle with the label removed, at least one tightly fitting plastic screw cap for the bottle, and a drill with spur-point bits. These bits, which are sometimes referred to as dowel bits, use a pointed tip and carefully aligned spurs to make clean holes through many materials, including plastic.
- Measure your volume. Most viscosity cups hold about one-tenth of a quart of liquid. This is roughly equivalent to six and a half tablespoons or 100 milliliters. Measure out this amount of water using measuring cups or spoons, and pour it into the plastic bottle. Tightly screw on the lid, and turn the bottle upside down, so that the liquid pools near the bottle opening. Use the permanent marker to draw a line around the outside of the bottle at the surface of the liquid. This will be your volume reference. Unscrew the cap and dump out the water.
- Mark your handle. Viscosity cups have handles so you can submerge the entire cup in the liquid of your choice. This will give you a more accurate measurement of viscosity than pouring the liquid into the cup, as the added force of pouring will change the internal friction of the liquid. Use the permanent marker to draw a vertical line from the bottom of the bottle to the volume reference line you made in the previous step. About one inch from the first line, draw a second, parallel line. This is the outline of your handle.
- Cut away the excess plastic. Use a utility knife or scissors to remove the bottom from the bottle. Then, cut along the volume reference and handle lines. You should be left with a small cup, connected to a strip of plastic. Be careful not to cut off the handle.
- Drill the bottom opening. A Ford 4 viscosity cup covers the widest range of viscosities, so if you’re only drilling one bottle cap, make it a four millimeter opening. If you have several bottle caps that fit the same bottle, you can drill different sized holes in each one, creating a versatile and customizable set of viscosity cups out of just one bottle. Use a four millimeter bit if your drill will accept it, or a 5/32 bit if not. Spur-point bits should drill a clean hole, avoiding the need to remove burrs after drilling.
How to Use a Viscosity Cup
A viscosity cup is a small container with a large opening at the top and a smaller opening at the bottom.
To use a viscosity cup:
- Hold the handle and dip the cup into your liquid.
- Remove the cup, and start a stopwatch when the bottom opening clears the liquid’s surface.
- Count off the seconds that the stream of liquid flows steadily out of the bottom of the cup.
- When the stream skips, stop the stopwatch. The number of seconds that pass from removing the cup to the break in the stream gives you a measurement of the viscosity of the liquid.
Once you understand what viscosity cups are and how to use them, it is easy to make your own. Use a spur-point bit to drill a hole through a plastic bottle cap. Cut away excess plastic from the bottle, leaving a 100 milliliter cup with a handle. Use the resulting viscosity cup to measure the flow rate of paint, stain, or wood finish.