A variable area flow meter is a specialized form of flow meter designed to determine the pressure of a gas or liquid in a piping system. They are commonly used in scientific and industrial settings. There are two popular designs that are in use today, though they function in a very similar way.
The oldest form of variable area flow meter is a cone-shaped device made of glass or clear plastic called a direct reading flow meter. The narrow end of the cone is put into the pipe system with the wide, sealed end face up. When the pipes fill with a liquid or a gas it enters the meter. A small bead inside the glass meter rises or falls with the strength of the pressure, and markings on the side of the glass allow a person to get an accurate reading.
The direct reading version of a variable area flow meter is still commonly used. This meter has some drawbacks, though. It is somewhat large, and it works using gravity, so the wide end must be facing up for an accurate reading. This is not always practical in a small or enclosed piping system. Also, if the fluid being measured leaves residue on the glass, it will have to be replaced or cleaned.
If any of these issues is a concern, there are other more recently-invented versions of variable area flow meters. One type that can work in tight spaces and doesn’t require gravity for readings is a correlated flow meter. These use a magnetic bead and a spring to measure the amount of pressure in a tube. As the pressure increases, the bead is pressed against the spring. Then, the spring conveys the amount of pressure it is receiving to a digital screen.
The drawback to the correlated version of the variable area flow meter is that it is more difficult to read than the cone shaped variety. It also doesn't use common measuring units on the display screen. After the correlated flow meter displays a number, the reader must then use a chart to convert this number to a metric or imperial measurement. Also, this system does not give accurate readings if the fluid’s temperature is too high or too low.
If the fluid to be measured may damage either of the previously mentioned meters, another type of variable area flow meter called a vane meter can be used. These meters put a disk in the flow of a pipe. This disk is connected to a movable stem. The motions of the disk and stem are sensed by a motion detector which measures them and determines the pressure. A drawback is that the vane meter is more complicated than the other types and requires more maintenance.
Although every variable area flow meter has drawbacks, they are also relatively simple, affordable, and commonly used. More complicated meters can be purchased for higher prices. The basis for all of them is the principle of providing slight resistance to a flow in order to determine its strength.