How to Calculate the Rate With an Area Flow Meter

Area Flow Meter | Flowmetrics

A variable area flow meter operates on mechanical principles. Their construction is simple yet durable, making them suitable for a variety of applications. The basic operational principle is that changes in flow rate can be observed and measured as the flow displaces a piston, float or vane until it reaches a steady state. As the liquid or gas passes thorough the housing tube, the movement of the measuring part indicates rate. This type of meter can be placed almost anywhere along the pipe and does not require complicated maintenance routines. It is also highly versatile and can be used to measure flow rates of liquids, gases and steam.

Generally, flow rate meters measure the volume or mass of gas or liquid that is displaced during a specific period of time, for example, liters per second. Density is an important factor in determining rate.


Area Flow Meter Mechanism

One very common type of variable area flow meter is a rotameter. This meter allows the flow to pass through a vertical tube with a float in it. The weight of the float balances on the upward motion of the flow. At its most basic, a rotameter has the float inside a glass tube with flow unit markings so that the position of the float relative to the unit markings can be readily observed. Others may be magnetically coupled to an external indicator. Additionally, variable area flow meters may have a rotating vane or an oscillating piston in place of the float. The principle is the same: the force of the flow displaces the inner mechanism.


What You Need for Accurate Measurements

In order to accurately calculate flow rate based on readings from an area flow meter, it is necessary to know the three main factors affecting the float’s motion.

  1. Buoyancy – Which depends on the density of the measuring medium and the float’s volume.
  2. Weight of the Float – Which will depend on the type of material it is made of.
  3. Force of the Flow – When this changes, an area flow meter will indicate this immediately, thus acting as a warning system for potential problems such as blockages.


Other Factors

This kind of meter can be used to measure flow with widely varying viscosity. Typically, an area meter will specify a wide range of viscosity within which the float’s design allows it to operate without affecting rate measurements. For gases, density can be a significant factor that affects measurements. It is important to calibrate the meter based on the type of flow as well as the operating environment. Rate calculations allow for the incorporation of correction factors for pressure, density and temperature changes.



Today, many area flow meters are connected to converters that convert the rate values to an output signal. This enables remote flow monitoring and recording. Many area flow meters are also equipped with alarms that go off when changes outside the normal range are detected.


Float Design

Varying float designs have been developed for use in particular applications. Ball floats come in several different materials and are suitable for measuring small flow rates. The key to accurate measurement is to have a float that is denser than the measuring liquid; it should not be able to float on top of the medium if there is no flow. Floats with tail guides can improve accuracy but should not be used in higher viscosities. Low pressure drop floats are designed specifically for low-pressure gases.  Available also are pole guided floats, BL floats, tapered floats and floats with guide rods. Floats that are based on springs allow for mounting of the flow meter in horizontal pipes, as the spring eliminates the effect of gravity on the float’s position.


Calibration and Maintenance

There are several steps you should take to ensure maximum accuracy from your area flow meter. Many floats have a painted indicator to make it easy to tell if the float is stuck or has reached equilibrium. If you are relying on the markings placed directly on the glass housing, you need your flow to be relatively clear; otherwise, the indicator may not be visible. Other options include using a metal tube with magnetically coupled indicator.

There is a wide variety of variable area flow meters on the market today. If this type of flow meter is suitable for your intended application, narrow down your selection further by checking out the specifications. To do this effectively, you should know your work environment as well as your flow profile. Most flow meters will have a Reynolds number range indicated, so calculating your flow’s Reynolds number will help you find the best one for you. The Reynolds number of a flow describes its flow profile and represents the ratio of momentum versus viscosity. You can calculate it using the viscosity numbers of the medium together with the maximum and minimum flow. Since flow does not stay the same, you want your meter to include a range that covers foreseeable fluctuations.

Temperature and pressure are also significant factors. High temperatures can expand meter parts as well as affect flow. High pressures can damage the housing and other components. For particularly extreme environments, you may want to select highly durable materials. These will usually connect to a remote indicator.

While variable area flow meters are constructed simply and do not require a lot of maintenance, blockages or debris can prevent proper operation. Smaller particles typically get cleaned out of the housing by the pressure of the flow. If you meter indicates a significant drop in flow, it is likely a sign of a problem you should deal with right away. Variable area flow meters are also highly versatile and can be used with many types of flow. However, it is important to calibrate your meter properly depending on the kind of flow.

Proper calibration is crucial to receiving accurate and precise rate measurements from your variable area flow meter. Working with an experienced manufacturer will help you get the most out of your meter. Your manufacturer can help you select the best flow meter for your type of application and advise you on proper installation and maintenance.

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