Intro to flow measurement and control - part 2 Mechanical Flow Meters
The most common and oldest flow meters are mechanical. That is they rely on moving parts to "count" how much fluid or gas has flowed through a conduit or channel. There are advantages as well as disadvantages to using a mechanical flow meter. One of the advantages is they are usually simple devices and easy to set up. To offset this advantage is the downside that they often have a negative effect on the downstream flow as they have to restrict flow or siphon off a portion of flow. Other advantages of mechanical flow meters are they are easier to manufacture and therefore cost less and they have a broad range of uses.
There are many types of mechanical flow meters and each has specific uses they are chosen for. One type of flow meter uses an inline chamber that has an impeller. The chamber has a known volume that does not change so that each rotation of the impeller gives a number that is used in calculations to figure out flow rate. The do impede the flow to some extent and are susceptible to wear. This type of flow meter can be purely mechanical or they can use electronics to display a readout and microchips to do the math.
Venturi meters, orifice plates and vortex meters are other types of mechanical flow meters. Each of these can cause a pressure drop downstream of their installation and may not be ideal when you need a steady pressure downstream of the installation of your flow meter. These types of flow meters work on Bernoulli's principle. Bernoulli's principle asserts that an increase in velocity will result in a decrease of pressure. So for instance the venturi flow meter measures pressure changes that can be used to calculate flow rate.
The vortex flow meter works similar to the orifice plate. They measure the pressure changes of vortexes created as liquids pass an item placed in the flow. Vortex meters have less of an impact on the downstream pressure and should be considered with downstream pressure is important.
There are many variations of the basic mechanical flow meters. For instance some are intended as stand alone meters that have a readout that is human readable but do not interact with other components. Some offer no read out at all but report directly to other devices such as flow control devices and a flow computer.