Which flow measurement principle should you choose?

Magnetic Inductive Flow Meters

There are several reasons why magnetic inductive flow meters are today considered the optimal choice within a sea of industries and utility companies. The measurement type, which has more than 50 years of history, has several obvious advantages. It can especially be highlighted that magnetic inductive flow meters are easy to install and almost maintenance-free, as they have no moving parts. The meter can be used in almost all liquids including water, wastewater, and chemicals, as long as the medium is conductive.

The Measurement Principle of Magnetic Inductive Flow Meters

The measurement principle of magnetic inductive flow meters is based on Michael Faraday’s law of induction. The law provides the relationship between an electric voltage and a changing magnetic field. In practice, the law states that if you move an electrical conductor through a magnetic field, a voltage will arise perpendicular to the conductor’s direction of movement, which is directly proportional to the conductor’s speed of movement. In the principle of magnetic inductive flow measurement, it is thus the medium that is the conductor. It is also worth noting that magnetic inductive flow meters measure with an accuracy down to 0.25% of the measured value.

Variable Area Flow Meters (VA)

Another old acquaintance within flow measurement technology is variable area flow meters or VA flow meters. The advantages of VA meters are many, but most importantly, the meter is mechanical and does not require a power supply to deliver accurate and valid measurements in gases and liquids. Additionally, the cost of VA meters is very small compared to other measuring units.

The Measurement Principle of VA Meters

The measurement principle of VA meters is quite simple. The meter itself consists of a single conical measuring tube with an engraved scale. Inside the meter is a float that indicates the medium’s flow. As the flow increases, the float will position itself either high or low in the tube. If the float is at the bottom of the tube, it indicates that there is no flow in the medium. The accuracy of a VA measurement is usually within the range of 1.5-3% of full scale.

Vortex Flow Meters

Vortex flow meters are a versatile and good alternative to magnetic inductive flow meters. The vortex principle places no requirements on the medium’s electrical conductivity. In liquid measurement, it is thus possible to measure, for example, oil or solvents as well as liquids at very extreme temperatures. Vortex meters also function impeccably as gas and steam meters.

The Measurement Principle of Vortex Meters

The measurement principle of vortex flow measurement is universal and is based on physicist Theodore von Kármán’s observations. If a body is placed in the product stream, a vortex is formed behind the body. The frequency of the vortex is proportional to the flow speed. The disadvantage of this form of measurement is that vortices cannot be formed at very low flows. This means that the vortex principle has a lower limit for where the principle can be used for measurement.

Ultrasonic Flow Meters

There are two different principles for ultrasonic flow measurement. One is called the Doppler principle, which is used when dealing with liquids that contain particles or air bubbles. Here, measurements are typically made with an accuracy of between 3 and 5%. The other principle is called “transit time” and can be used for pure and homogeneous liquids. Here, the accuracy is somewhat better – typically around 0.5-3%. Ultrasonic meters are especially useful for large pipes and aggressive media.

Measurement Principles for Ultrasonic Measurement

There are two different principles for ultrasonic flow measurement. One is called the Doppler principle, which is used when dealing with liquids that contain particles or air bubbles. Here, measurements are typically made with an accuracy of between 3 and 5%. The other principle is called “transit time” and can be used for pure and homogeneous liquids. Here, the accuracy is somewhat better, typically around 0.5-3%. Ultrasonic meters are especially useful for large pipes and aggressive media.

Mass Flow Meters

Mass flow meters come in two different versions or types: thermal and Coriolis meters. Common to both types is that they are used in applications where a direct measurement of the flow’s mass is desired, e.g., in kg/h. This can be an advantage if you are dealing with media where the density is temperature-dependent.

The typical areas of application for thermal mass flow meters are gas measurement, but the measurement principle can advantageously be used elsewhere, e.g., for process control, consumption and supply monitoring, leak detection, or monitoring of distribution networks.

For Coriolis meters, it applies that they are very precise, and the measurement principle is particularly useful in life sciences, the food industry, and chemical manufacturing. The meters can measure almost all liquids, including cleaning agents, solvents, liquids, crude oil, vegetable fat, animal fat, latex, silicone oils, alcohol, fruit solutions, toothpaste, vinegar, ketchup, mayonnaise, gases, or liquid gases.

Measurement Principles for Mass Flow

As described, there are two types of mass flow measurement. For thermal meters, the thermal dispersion principle applies, which determines how a body cools when exposed to a moving medium.

In flow measurement, the principle is used by placing a sensor in the medium. The sensor is then subjected to controlled heating. The power used to heat the sensor is proportional to the medium’s mass flow. The advantage of this principle is that it provides a very precise and quick indication of fluctuations in the flow.

The other form of mass flow meters is based on the so-called Coriolis effect. The effect was first described by the Frenchman Gaspard-Gustave Coriolis in 1835. The effect describes the phenomenon that when the Earth rotates on its axis, a point at the Equator moves much faster than a point at the poles. This fact affects, among other things, winds and ocean currents.

In flow measurement, the principle can be used by passing the medium, whose mass flow you are interested in measuring, through a tube that is set into oscillation. Two sensors placed on the inlet and outlet sides register the oscillations. The oscillations are directly proportional to the mass flow.