Simple and effective, a ball in a glass or metal tube is all it takes to indicate the rate of fluid flow
The variable area flowmeter is one of the most durable yet least expensive meters you can buy. Here's how to choose the right one for your application.
In an increasingly high-tech world, some simple devices still have a place. One of them is the variable area flowmeter. Swagelok sells two types, glass and metal. Both use the same basic design: A buoyant ball inside a vertical tapered tube.
What makes the variable area flowmeter remarkable is that it doesn't require any external power. Instead, it runs on the flow of the system fluid itself.
The fluid enters at the bottom of the tube and flows up to the top. Gravity wants to make the ball sink, while the fluid is pushing upward and the ball itself has some buoyancy.
If the tube were straight, the fluid would push the ball all the way to one end. But because the tube is tapered, the higher the ball rises, the more space there is between the ball and the tube wall. We call that the annular gap. The wider gap lets more fluid flow around the ball. When the upward and downward forces are balanced, the ball stays in one place. A scale next to the glass tube indicates the rate of flow at that position.
When we use metal tubes, we use a magnetized ball that can draw a needle up and down the outside of the tube to indicate flow rate.
Glass meters are designed for about 140 or 150 psi. Metal is for higher pressure.
Plusses and Minuses
The simple construction makes the variable area flowmeter one of the most durable and least expensive meters you can buy. It also creates very little pressure drop. Some more complex flowmeters restrict the flow of fluid in order measure it.
There are a few trade-offs, though. Variable area flowmeters aren't as accurate as some of their more complex cousins. A variable area flowmeter can measure only at a ratio of 10 to 1, called the turndown ratio. That means that if you have a meter calibrated for a maximum reading of, say, 100 gallons a minute, the smallest flow it can measure is 10 gallons a minute. If the maximum reading is 80, the minimum will be 8, and so on. If you have to measure pressure at less than 10 percent of the maximum, you'll need a different kind of meter.
Different fluids have different densities, which is the weight of the fluid at a given volume. They'll also have different viscosities or thicknesses. Density and viscosity change how the fluid pushes on the float. That's why Swagelok can custom-calibrate a variable area flowmeter for methane gas, fuel oil, or a wide variety of other fluids.
Temperature and pressure come into play as well. As temperature increases, a liquid gets thinner and won't exert as much upward force against the float. Likewise, as gas temperatures increase, volume and flow force increase.
Pressure changes aren't a big factor for liquids because they don't compress. Gases, on the other hand, get denser as pressure increases. That too changes the way the fluid reacts with the float.
So while the basic design is simple, some of the details can get more complex. For the full story on flowmeters, look at our catalogue online.