Tutorial: Rotameters (variable area) for compressed air flow metering
Measuring flow when the fluid is compressed air or other gas presents a different set of challenges than liquid, due to the fact that volume changes with pressure. To avoid that issue, air and gas flow is typically measured in SCFM (standard cubic feet per minute) rather than ACFM (actual cubic feet minute). Flowmeters that measure the mass of gas solve this issue easily, but they are generally expensive instruments. Rotameters (aka, variable area flowmeters) provide a gas flow measuring technology that is simple, inexpensive and relatively accurate, but they can get you into trouble if you don’t know how to interpret what they say.
If you aren’t familiar with the technology, Rotameters use a float inside a tube that has a tapered bore with its largest diameter at the top. They are mounted vertically with the inlet at the bottom and the outlet at the top. (Some designs use a spring loaded float that can be mounted in any position.) Air (or other gas) flowing up the tube will lift the float until it reaches the point where the annulus around the float is large enough that the gas can move around the float without raising it any higher. The tubes are normally clear so you can see the float, and a scale is attached to determine the reading. They use a visual indication, but can be outfitted with accessories to provide electronic reading options.
Rotameters generate their reading according to velocity through the tube, which is more a function of volume, not mass. So, if the pressure of the gas going through the meter increases, the volume measured in SCFM also increases, even though the float is in the same position and gives the same reading. This is where things get tricky and you have to know what you’re dealing with.
If you want your reading in SCFM and the air is compressed to some extent, you have to keep some simple things in mind.
First, you should install pressure gages at the inlet and outlet of the Rotameter. There will be a pressure drop across the device, and it is important to know the pressure at both ends.
Second, choose your scale carefully: Some manufacturers have a complex scale that is actually a chart with curves indexed for various pressures. When reading the meter, you have to follow the curve to the correct pressure point for the true flow reading at that pressure. These scales are handy and can operate over a wide range, as long as the operator reads them correctly.
A meter that has a single generic scale will need to be manually corrected. Most flowmeters are based on atmospheric pressure, so any deviation from that will require correction. Your vendor should be able to supply you a formula that will tell you how to calculate the true SCFM reading based on the operating pressure. Usually this involves the reading at the inlet, which is why you need both gages.
If you plan on using the meter for a specific set of conditions, some vendors will make you a custom scale to give a correct reading at that pressure. E.g., if you have an application that always operates at 40 psi (measured downstream of the meter), the vendor can make the unit specifically for that pressure. As long as you stay with that pressure, you will have proper readings. If you move off that pressure, you will have to calculate a correction, andthe vendor should provide you the formula for that.
While this may sound like a lot of trouble, bear in mind that Rotameters can be reasonably accurate (