Air Flow Meters

Let’s start by acknowledging that the specific term “air flow meter” and other similar terms can mean vastly different things to different industries. Here is some helpful clarification regarding the items that “air flow meter” could be referring to.

In the industrial instrumentation world, the term flow meter and flow sensor are sometimes used interchangeably as a flow meter is technically a type of flow sensor.

However, in the automotive industry a “mass air flow sensor” is a car part that is integral to the fuel injection system and measures the air flow into the engine. Without proper flow, your car can experience significant performance issues and the dreaded “check engine” light. While a “mass air flow sensor” does measure flow, they are not used for any other applications besides the automotive industry.

In the HVAC field, especially for commercial and industrial use buildings, there are many systems that require measuring and monitoring the flow of air. Typically, “air flow meters” for HVAC applications measure the air velocity though the piping or duct work. The instrumentation that typically performs this task are usually referred to as anemometers.

Anemometers themselves can come in many shapes and sizes and are used in different application fields. In measuring the wind speed of outdoor weather, common anemometers are rotating vanes with cups at the end of arms that are pushed by the movement of the wind. The speed of the rotation of the main rod is converted into a wind speed measurement. Anemometers can also look like little mini fans contained in housings, which again are rotated by the movement of air passing through the blades.

This category of “air flow meters” refers to industrial flow meters that measure the flow of air through a pipe within a process or a system. Most of these industrial air flow meters can also handle certain other gas types as well.

Not all flow meter technology types are compatible with air. Some flow meter technology types are only suitable for liquids. The most common flow meter technologies used for measuring air and other gas types are:

• Rotameters/Variable Area Flow Meters
• Vortex Flow Meters
• Thermal Mass Flow Meters
• Some Differential Pressure Flow Meters
• Coriolis Flow Meters
• Oscillation Flow Meters

While there are many types of specific flow meter technologies, they can also be classified into two categories. Ones that do not utilize any moving parts in the principle of operation, and ones that do. Generally speaking (and excluding DP Meters), technologies that do not include mechanical elements will be more accurate than technologies that do. Coriolis flow meters are usually the most accurate option available for air, however they are also usually the most expensive option out there and are overkill if it is not a highly specialized application.

The typical US units of measurement for air flow are standard cubic feet per minute (SCFM) or standard cubic feet per hour (SCFH). These are both volumetric units of measurement describing how much air is moving through the pipe in reference to a unit of time, as opposed to a velocity measurement like an anemometer which states the speed at which the air is moving.

Industrial air flow meters work according to their specific technological principle of operation.

Rotameter air flow meters

Rotameters, also known as variable area flow meters, are typically a tubular flow body with a float. When air flows, the float moves up in the tube and the flow can be read by referencing the top of the float against the scale printed on the tube. These typically also provide visual flow indication through transparent flow bodies. To learn more, visit our article on rotameters/variable area flow meters.

Vortex air flow meters

Vortex flow meters operate by placing a small obstruction (known as a bluff body) in the path of flow. A series of “eddies” is created behind it. These vortices alternate side to side and the shift in pressure is processed into a flow rate. To learn more, visit our article on vortex flow meters.

Thermal dispersion air flow meters

Thermal flow meters employ a heated element, such as a probe, into the flow path. As the air passes, the heat of the probe is dissipated and the probe cools. The amount of cooling that occurs can be computed into a flow rate. To learn more, visit our article on thermal flow.

Differential pressure air flow meters

Differential pressure flow meters typically use an orifice plate, or some other change in shape, within the flow that generates a difference in pressure before and after the element. This difference is then processed into a flow rate.

Coriolis air flow meters

Coriolis flow meter operation is more difficult to summarize. To learn more about the Coriolis flow meters, visit our in-depth Coriolis flow meter article.

Not all air flow meter technologies deliver digital capabilities. Variable area flow meters do not typically provide digital displays.

If you simply need to verify that air flow is occurring in a closed pipe system, your application probably only requires a simple flow indicator and not a flow meter. If you need to know exactly how much air is flowing through the pipe at any given time, then you will need a flow meter.

Choosing the right flow meter to monitor or measure your air flow can seem like an intimidating prospect.

In general, here are some basic things to keep in mind while looking for an air flow meter.

• The type of air - Not all air is created equal. Some of it can be considered clean and some of it is dirty. Is the air dry or humid? Humid air requires special consideration. Our DOG Oscillation Flow Meter offers a unique design that drains condensate build-up from humid air.


• The total range of possible flow rates - What will the maximum flow rate be and what will the minimum flow rate be? Flow meter have minimum and maximum flow rates.


• The maximum temperature - Depending on their material of construction, flow meters can only handle a certain maximum temperature without risking damage to the instrument.


• The maximum pressure - Certain build materials are only suitable for lower pressures. High pressure applications will typically require a metallic build and specialized glass.


• The line size and connection - Not all flow meter models come in all line sizes. Also consider what connection type will most easily incorporate the meter into the application.


• Insertion or inline installation - If you prefer to not cut into your pipes, creating downtime and revenue loss, consider an insertion air flow meter.


• The accuracy - Determine what level of accuracy your application requires. High accuracy generally requires more expensive flow technology that does not include any moving parts. Many applications really do not require the level of accuracy to justify the higher cost over a mechanical flow meter like a rotameter.

An inline air flow meter is simply an air flow meter that is intalled inline with the process piping. There is an inlet and an outlet and the flow passes through the flow body that is placed into the pipe via fittings like NPT or ANSI flanges.

Industrial air flow meters may need to measure pressurized air that is higher than normal atmospheric pressure.

A simple example of compressed air is a can of pressurized air used to clean sensitive computer parts. A common compressed air application is pneumatic systems where pressure is required to create the force for movement.

Flow meters for compressed air need to be able to handle higher pressures than standard air applications. For this reason, most compressed air flow meters are built of a more rugged material like metal.

The price of an air flow meter can vary extensively and is primarily dependent on your choice of flow meter technology, your line size, and the features the air flow meter needs for your specific application.

Among the typical air flow meter technology choices (rotameters/variable area, vortex, thermal mass, differential pressure, oscillation, and Coriolis) rotameters are generally going to be the cheapest for any simple application. However, they do not typically offer functions like totalizing or batching.

Coriolis flow meters are generally the most expensive type of flow meter that can measure air. They are typically required for more specialized applications with more demanding process conditions.

Vortex flow meters, differential pressure flow meters, and thermal mass flow meters can all vary in price per model, but in general, they fall in the middle cost-wise.