Choosing the Right Flow Meter

There are several different technologies used for flow meters that exploit different physical phenomena. Most technologies can measure a volumetric flow rate (the volume of fluid flowing through a pipe for a specified period of time), but there are also some technologies that can measure a mass flow rate (the mass of fluid flowing through a pipe for a specified period of time). You can calculate the volumetric flow rate from the mass flow rate, and vice versa, when the characteristics of the fluid (density, etc.) are known, but it is better to ask yourself from the outset what information concerning the flow will be required and to choose the appropriate technologies accordingly.

Another important factor when choosing flow meter technology is the nature of the fluid. The following table shows which fluids are compatible with each type of flow meter:

The installation conditions of the future flow meter are also an important point to consider. In a pipe network, irregularities (elbows, valves, decreases in diameter, etc.) generate disturbances in the fluid flow and some measurement technologies are sensitive to these disturbances. In order to avoid measurement errors, for each type of technology the flow meter is installed after a portion of straight piping so that the fluid flow is no longer disturbed. This length of straight pipe ahead of the flow meter is expressed as a function of the diameter of the pipe. For example, 10 to 30 D means you need a length of straight pipe 10 to 30 times the inside diameter of the pipe. For this example, if the diameter of the pipe is 10 cm, the straight pipe ahead of the flow meter will have to measure between 100 and 300 cm.

The table below gives an estimate of straight pipe lengths and common pipe diameters for the different technologies:

In addition to these parameters, the choice of a flow meter will also depend on the temperature and pressure conditions of the fluid, the measuring range required and the desired output signal (direct reading, analog, digital, etc.).

ROSEMOUNT orifice flow meter

If you need to measure the volumetric flow rate of a liquid or gaseous medium, we recommend the use of an orifice flow meter.

The principle behind an orifice flow meter is relatively simple: an orifice plate is inserted into the flow, and the constriction caused by the plate generates a pressure drop. The flow meter measures the pressure difference before and after the orifice plate and determines the volumetric flow rate.

Orifice flow meters are not adapted for particle-laden fluids because the abrasion caused by the suspended particles eventually erodes the orifice, changing its shape and size, which tends to distort the measurement.

Orifice flow meters are popular because of their accuracy, but also because of their low price.

In order to use an orifice flow meter, you will need to replace a portion of the pipe with the flow meter.

Arthur Grillo venturi flow meter

Venturi flow meters are used to measure the volumetric flow rate of a clean or loaded liquid or gaseous medium. This type of flow meter measures the pressure difference generated by the passage of the fluid through a venturi, this difference being proportional to the flow rate of the fluid.

These flow meters have a good accuracy of about 0.5%. This technology is less sensitive to particles than orifice flow meters and is therefore more suitable for loaded fluids.

In order to use a venturi flow meter, you will need to replace a portion of the pipe with the flow meter.

KRACHT gear flow meter

Gear flow meters are used to measure the volumetric flow rate of liquid media. In these flow meters, liquid flows between two oval gears. Since the volume circulating with each rotation of the gears is fixed, all you have to do is count the number of revolutions per unit of time to determine the volumetric flow rate. Thanks to their robust construction, gear flow meters are particularly suitable for highly viscous media.

LTH Electronics electromagnetic flow meter

Electromagnetic flow meters are used to measure the flow rate of electrically conductive liquids, even those that are highly loaded or abrasive. The conductive media flows through a magnetic field, a voltage proportional to the flow rate is induced in an electrode as the media passes through, allowing the flow rate to be calculated very accurately. These flow meters are particularly resistant and will allow you to obtain measurements in difficult environments.

As they have no moving parts, electromagnetic flow meters are maintenance-free, and in addition to being very accurate, they are moderately priced.

MEDENUS turbine flow meter

Turbine flow meters are used to calculate the flow rate of gases and liquids. A turbine is placed in the flow axis, the turbine is driven under the effect of the fluid’s flow and its rotation speed is proportional to the speed of the fluid.

This technology works very well for large flow volumes, but for lower flow volumes the turbine turns too slowly to obtain an accurate measurement.

These flow meters are accurate and offer a low response time to flow variations, they are independent of the pressure and temperature of the medium and can operate at high pressures and temperatures.

The turbine is a moving part subject to wear and tear however, so these flow meters require regular maintenance.

BLUE-WHITE ultrasonic flow meter

Ultrasonic flow meters are used to measure the flow rate of clean, loaded, highly loaded or even viscous liquids, but are not recommended for highly viscous liquids.

The operating principle of these flow meters is relatively simple: two transducers alternately emit and receive ultrasound through the fluid. The difference in transit time is proportional to the speed of the fluid and therefore to the flow rate.

The main advantage to ultrasonic flow meters is that the transducers do not need to be inserted in the flow itself, so they can be installed on tapping clips or on the outside of the pipe, without any drilling. This technology can be used for spot measurements without needing to stop the circuit to set up the transducers.

However, ultrasonic flow meters are expensive and not suitable for low flow rates.

KOFLOC variable-area flow meter

Variable-area flow meters are used to measure the volumetric flow rate of clean, loaded and viscous liquids as well as clean gases. However, we do not recommend using this type of technology for highly viscous liquids or highly loaded liquids.

This type of flow meter must be installed vertically, as the fluid flows from the bottom to the top by pushing a shaped float. The height at which the float stabilizes corresponds with the flow rate and allows direct reading on a graduated scale printed on the transparent side of the tube.

These very inexpensive flow meters only provide a visual reading of the flow measurement (the flow meter has a graduated scale).

Bronkhorst vortex flow meter

This type of sensor uses the phenomenon of vortex creation downstream to an obstacle, similar to vortices formed by the wind blowing over the pillars of a bridge. An obstacle is placed in the flow, vortices then form and detach from the obstacle, dragged by the flow. A pressure sensor is used to detect the pulse created by the detachment of a vortex. The number of vortices and therefore of pulses is proportional to the flow rate of the fluid.

Vortex flow meters are used to measure the flow of liquids, gases and vapors. It is possible to work with loaded fluids, but regular maintenance is then essential.

This type of flow meter is suitable for high flow rates, if the flow rates are too low, the formation of vortices will be too limited, and the flow reading will be either impossible or inaccurate.

Vögtlin thermal mass flow meters

Thermal mass flow meters allow you to measure the mass flow rate of clean liquids and clean or loaded gases directly. The principle of thermal flow meters is relatively simple: a heated temperature sensor is cooled by the circulation of the fluid, this cooling being proportional to the mass flow of the fluid.

These flow meters have several advantages: they directly give a mass flow rate independent of the temperature and pressure of the fluid, they have no moving parts (low maintenance), they cause little disturbance to the flow (negligible pressure losses) and, given the small size of the sensor, they can be found in a “plug-in” configuration on a pipe connection.

YOKOGAWA Coriolis flow meter

Coriolis flow meters are used to measure the mass flow of a fluid. They can be used for almost all media: clean, loaded and highly loaded liquids and gases. However, they are not recommended for viscous, conductive or multiphase fluids.

A Coriolis flow meter consists of a straight or U-shaped tube equipped with vibration sensors at the inlet and outlet of the tube. When a fluid flows through the tube, the tube is vibrated perpendicularly in the direction of flow, which generates Coriolis forces that cause a phase shift of frequency vibrations. This phase shift, measured by the two sensors, is proportional to the mass flow rate of the fluid flowing through the tube.

Coriolis flow meters have the advantage of simultaneously providing three pieces of information about the flowing fluid: its mass flow rate, density and temperature. However, this technology is still very expensive and the device is larger than average.

Coriolis flow meters are generally used in the pharmaceutical industry, for water treatment plants, nuclear facilities and in the measurement and transfer of natural gas.

A Coriolis flowmeter is installed replacing a portion of the pipe.