Selecting the Right Flow Meter

1/27/2019 | 5 min read

Highly versatile instruments, flow meters are used to measure the speed and volume of gas or liquid flowing through a pipe. Flow meters are available which make use of different measuring principles. Examples include devices that can be used for mass flow measurement based on the Coriolis or thermal principle, volumetric measurement based on the vortex or ultrasonic principle, electromagnetic measurement of volume flow, and so on. The volumetric flow rate signifies the volume of fluid that passes a fixed point per unit time, while mass flow rate represents the total mass of a substance that passes a given point per unit of time.

Two major types of flow can be measured: open-channel flow and closed-conduit flow. Open-channel flow occurs when the fluid is flowing freely, with part of the surface area exposed to the atmosphere. Rivers, streams, and creeks, for example, have open-channel flow, as do man-made constructions such as culverts and aqueducts.

In contrast to open-channel flow, closed-conduit flow — also known as pipe flow — involves the flow of liquids through pipes, channels, and other containers used in industry. Flow is more easily controlled and manipulated in closed conduits, allowing gaseous materials to pass, as well as liquids. Open-channel flow is dictated by gravity, while closed-conduit flow is the result of differences in pressure.

Types of Flow Meters

For both types of flow, flow meters serve as crucial tools for measuring and studying the movement of liquids, gases, and steam. There are several different kinds of flow meters available.

Differential pressure flow meters function by calculating the pressure drop against an object placed into the pipe. The construction of these meters is based on Bernoulli’s equation, in which the change in pressure is a function of the square of the fluid velocity. Below are a few of the most popular types of differential pressure flow meters.

Orifice plates measure the pressure on both sides of a partially constricted pipe. The difference between the upstream and downstream measurements provides an accurate measurement of the overall flow. The orifice plate design is known for its versatile and straightforward construction but is prone to degradation over time.

In contrast to the orifice plate tube, a Venturi tube setup is more expensive and requires more specialized equipment, but can measure smaller changes in low-pressure systems. In a Venturi tube, fluid flows through a length of pipe with a varying diameter in order to record and compare the pressure differences. The pressure differential is then used to derive the flow rate.

Finally, flow nozzles are a simple, popular tool for measuring air and gas flow. Gas is accelerated through a nozzle, and the increase in velocity and the corresponding decrease in gas density are recorded.

Key Considerations for Flow Measurement

There are many factors to consider when choosing a device for measuring flow in a specific application. To ensure that you select the ideal meter for your unique job, give consideration to the following questions:

What kind of substance is being measured — gas or liquid?
How accurate of a measurement is required?
What types of meters are compatible with the temperature and velocity of the flow being measured?
Will the fluid be corrosive to the meter being used?
What kind of measurements are desirable, and in what form should they be delivered?

Keeping these engineering considerations in mind will help ensure optimal performance and reliability. In general, flow meters built to provide greater accuracy and greater durability will cost more and may also require more maintenance, so it may be worth contacting an engineer skilled in fluid flow measurement to help guide you through the selection process.

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