Intro to Flowmeters - Part 2

The first flowmeters we will look at are the mechanical and differential pressure types. Both of these flowmeters will cause a downstream pressure drop. These two types of flowmeters either constrict or siphon off a portion of your flow in order to be able to take a flow measurement reading. A good example is the mechanical types of flowmeters. They take a portion of your flow (in some cases the entire flow) and pass them through a chamber with a propeller attached to a shaft. This action causes a drop in pressure downstream of its installation.

The differential pressure flowmeters operate by taking a reading of pressure loss at a constriction that is placed across the flow. Most often a portion or sampling of the flow is measured instead of the entire flow and the portion measured is returned to the flow downstream of the flowmeters installation.

With flowmeters that affect the flow rate and pressure you may need to compensate for the pressure drop by increasing the upstream or delivery of the fluid so that with the pressure drop the downstream pressure is within tolerance for your needs. Another option is to select a flowmeter type that has no effect on the downstream pressure.

One of the reasons that people still use differential pressure flowmeters is that they are low cost to set up, have a wide range of uses and they are simple but sturdy devices. These are commonly used in industrial applications but come with a medium to high-pressure drop depending on which type/model is used.

Mechanical flowmeters is another basic type of flowmeter with a wide variety of models to choose from. Each model will have advantages and disadvantages that will fit a number of various applications. The basic operation of a mechanical flowmeter is that liquid is passed through a chamber of known volume and that fluid turns some type of paddle wheel that turns a shaft connected to some device to count revolutions. Since the volume passing over the paddle is known the revolutions can be added up to indicate flow rate.

Most of the flowmeters that fall into the type "differential pressure" or "mechanical" usually siphon off a portion of the flow into a tube or secondary channel to be measured. This affects downstream pressure and may require you increase the upstream pressure to compensate. These flowmeters do not actually read the pressure of the liquid, they only read the volume of liquid that passes their installation.

In spite of their shortcomings mechanical and differential plate flowmeters are still very popular. Contributing to their popularity is that they are easier to manufacture so they cost less. These are used widely in industrial plants, automotive, chemical plants as well as other applications. One other caution is that these types of flowmeter may be damaged with "dirty" liquids or liquids with particulates that may be abrasive in nature or clean to parts of the channel or flowmeter themselves.