In industry, the measurement of flow rates - whether of liquids or gases - plays an essential role in controlling, regulating and monitoring processes. The right choice of measurement method is crucial for the efficiency and safety of operating processes. In this article, we provide an overview of the most common measurement methods.
The choice of flow measurement method depends on a variety of factors, including the type of medium, the required precision and the field of application.
Mechanical flow measurement
mechanical flow measurement is one of the longest established measurement methods. It includes classic methods such as impeller, paddle wheel and oval wheel meters:
- Impeller meter: Here, the flow of the fluid drives an impeller. The speed of the wheel is proportional to the flow velocity and therefore to the volume flow.
- Paddle wheel meter: Similar to the paddle wheel meter, but with an arrangement of paddles that measure the flow rate.
- Oval gear meter: Two oval-shaped gear wheels that roll against each other and whose rotation enables direct measurement of the volume flow.
These devices use physical components that move in the flow medium. The rotating part is proportional to the flow velocity and therefore to the volume flow rate.
While mechanical flowmeters are often robust and simply built, making them inexpensive and versatile, mechanical wear over time leads to measurement inaccuracies, with solids flowing in the medium in particular making regular cleaning and inspections necessary.
Calorimetric flow measurement
Calorimetric flow measurement is a versatile technology that is particularly suitable for measuring flow rates and recording flows in pipes and air ducts.
The process uses two temperature sensors: one sensor that measures the temperature of the medium (reference sensor) and a second sensor that heats the medium (heating element). The faster the medium flows, the faster the heat supplied is dissipated. The cooling rate or the temperature difference between the two sensors is a measure of the flow rate of the medium and is used to calculate the flow rate.
The advantages of this technology are particularly noteworthy:
- No moving parts: This means less wear and tear and high reliability over long periods of time.
- Insensitive to dirt: As there are no moving parts in the medium, calorimetric sensors are extremely resistant to contamination.
- Easy installation and maintenance: Calorimetric sensors can be easily integrated and maintained in a wide variety of systems.
- High flexibility: They can be used for gases and liquids, over wide temperature and pressure ranges and in a wide variety of pipe sizes.
Limitations of the calorimetric measurement result from external temperature fluctuations or heat sources in the vicinity of the sensor, as the calorimetric flow measurement is based on temperature changes. Measuring media with large particles, abrasive substances or high solid content can be difficult as these can damage the sensor surface or impair its heat transfer.
Further information on calorimetric flow measurement can be found here.
Ultrasonic flow measurement
Ultrasonic flow measurement has established itself as a reliable method for measuring the flow rates of liquids and gases. With its ability to measure without direct contact with the medium, it offers a versatile solution for a wide range of applications.
Ultrasonic flow measurement is based on the use of high-frequency sound waves. There are two main types of this measurement technology:
- Transit time difference method: In this method, two ultrasonic sensors are mounted on the outside of the pipe, between which ultrasonic signals are sent in both directions. The flow rate is determined based on the time difference required for the signal to travel upstream or downstream. The higher the flow velocity, the greater the time difference.
- Doppler method: This method is used when the fluid contains particles or bubbles. The sensors send ultrasonic signals into the medium, which are reflected by the particles or bubbles and received again. The frequency shift (Doppler effect) that occurs when the sound waves are reflected by moving particles is a measure of the velocity of the fluid.
These technologies are non-invasive and can therefore reduce the risk of contamination or leaks in pipes. The devices also require little maintenance and can be used flexibly. Despite the basic ease of use, the correct alignment of the sensors is often difficult and requires precise knowledge of the local flow conditions.
Magnetic-inductive flow measurement
The magnetic-inductive flow measurement only works with electrically conductive fluids. It is based on Faraday's law of electromagnetic induction, which states that a voltage is induced in a conductor passing through a magnetic field. In flow measurement, the conductive fluid is passed through a magnetic field. The flowing fluid acts as a conductor. Perpendicular to the direction of flow and the magnetic field, a voltage is generated which is measured by the measuring device. This voltage is proportional to the speed of the fluid and therefore directly to the volume flow.
Due to the contactless measurement, there is no impairment of the flow and no contamination of the medium. The devices offer high measuring accuracy and are insensitive to pressure and temperature fluctuations in the medium. On the other hand, it must be noted that the flow of distilled water, oil, gases or non-conductive liquids cannot be measured using this method. In addition, electromagnetic measuring devices are often expensive to purchase.
Coriolis mass flow measurement
Coriolis mass flow measurement is one of the most accurate methods for determining the mass flow of gases and liquids. Coriolis mass flow meters contain one or more vibrating tubes through which the medium flows. If no medium flows through the tubes, they vibrate at a natural resonance frequency. When the medium starts to flow, an additional force is generated due to the Coriolis effect, which causes a change in the oscillation shape. This change is proportional to the mass of the flowing medium. Sensors and electronics in the measuring device record these changes and convert them into a signal that represents the mass flow rate.
Coriolis mass flow measurement is a highly accurate technology that is particularly valued in applications where precision and reliability are essential. At the same time, the measuring principle is cost-intensive and can be of limited use with corrosive or highly viscous media.
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