Ultrasonic Flow Meter

Ultrasonic flow meter in the application sound describes mechanical or also acoustic waves, which expand exclusively in elastic media. Sound waves therefore require gases, liquids or solids to be able to propagate. Sound waves differ significantly from electromagnetic waves, which can also pass through vacuum unhindered. The propagation of the sound waves can be transverse or longitudinal, depending on the medium and the exposure, which in turn plays a role in various test procedures. Basically sound is divided into four different classes. The classification is frequency-dependent, the most obvious frequency range being between 16 Hz and 20 kHz. In this frequency band lies the audible sound, i.e. those sound waves which can be perceived by the human ear. Vibrations below 16 Hz are referred to as infrasound, vibrations above 20 kHz as ultrasound. Infrasound and ultrasound are not audible to humans. The same applies to hypersonic sound, which propagates at frequencies above 1 GHz.

What is an ultrasonic flow meter?
An ultrasonic flow meter uses ultrasonic waves to determine the flow velocity of liquid media without contact. If a medium flows through a pipe system, it is possible to install an inline flow meter, i.e. to separate the pipe system and integrate a flow meter into the pipe system to measure the flow velocity. Alternatively, the flow velocity in the pipe system can also be measured from the outside by using an ultrasonic flow meter to send sound waves from outside through the medium. In this method, two ultrasonic sensors, which act as transmitter and receiver, are fixed at a defined distance from each other on a pipe carrying the medium.

The ultrasound is emitted into the medium at an oblique angle and accelerated or decelerated depending on the speed of the medium. In this way, the speed of the medium in the pipe system can be determined by means of the transit time difference method. In combination with the cross-sectional area of the pipe, the volume flow or flow rate can thus also be determined. An ultrasonic flow meter thus consists of two essential components: the ultrasonic sensor system, which is mounted on the pipe system, and the evaluation unit, which translates the signals into real measured values.

Which measuring principle does an ultrasonic flow meter use?
An ultrasonic flow meter determines the flow velocity in a medium by means of the transit time difference measurement. Two ultrasonic sensors are mounted on a pipe at a previously calculated distance and act as transmitter and receiver. The sensors are acoustically connected to the pipe surface by means of a coupling gel and sound the sound waves into the medium at a defined angle. The sound waves are accelerated in the direction of flow of the medium and decelerated against the direction of flow. By measuring these time differences, the ultrasonic flow meter determines the flow velocity if the sound velocity specific to the medium is known.

Which media can be measured with an ultrasonic flow meter?
With an ultrasonic flow meter, all liquid, homogeneous media with a low solid content can be measured. These include water, oils, alcohols, etc. The media temperatures can be between -30° C and + 160 °C depending on the selected sensor technology. Gases as well as liquids with solid content, viscous media or pastes cannot be measured with an ultrasonic flow meter. Due to the inhomogeneity of these media, the ultrasonic waves are scattered and refracted by the particles and solid components to such an extent that the signal strengths decrease considerably and precise measurement results cannot be achieved.

What are the advantages of an ultrasonic flow meter?
With an ultrasonic flow meter you can measure mobile, fast and flexible. Mobile, as it can be used independently of location due to its handy size and the self-sufficient power supply via rechargeable battery. Fast, because after the configuration of the flow meter and the sensor installation on the pipe system, the measurement can be started immediately. Flexible, because the ultrasonic flow meter can be installed with different sensors on different pipe diameters and pipe materials and at the same time it can measure a wide range of media within a wide temperature window. Also, some media characteristics.

The calibration of an ultrasonic flowmeter is usually done by comparing the measured values with those of another reliable device. In a new method patented only in 2020, the ultrasonic flowmeters using a transit time difference method are calibrated by changing the distance between the sensors as planned. This change in the position of the sensors, like the movement of a flowing medium, leads to evaluable transit time differences in the sound signals generated by the ultrasonic flowmeter. Since only time and distance are measured in this method, the ultrasonic flowmeter can be calibrated with high accuracy for the transit time difference method completely independently of the flow medium and pipe geometry.

In its calibration laboratory, PCE Instruments uses a flow calibration stand, in which the water is pumped through a closed system of copper pipes during calibration. The flow rate can be controlled by valves and by changing the pump speed on the pump's control panel. Up to now, however, only one-point calibrations have been carried out at one flow rate and the system remains set fixed at a pressure of about 4 bar. The four measuring sections are arranged one above the other as long horizontal sections. The reference meter is installed in the uppermost pipe. The sensors from the ultrasonic flowmeter to be calibrated are fixed to one of the three lower pipes with Velcro strips. The dimensions of the copper pipes are 28.0 x 1.0 mm for the two upper pipes, 54.0 x 1.5 mm for the third pipe from the top and 88.9 x 2.0 mm for the lowest one.

As a reference measuring device, a magnetic inductive flow meter of the type VMM induQ from the company SIKA, which is easy to operate and read, was integrated as a fixed component in the system. During measurement, a magnetic field of constant strength is generated in the measuring tube of the reference meter. The magnetic field induces voltage in the flowing conductive water, the level of which is proportional to the flow rate. This voltage is measured, from which the flow velocity is determined and displayed in metres per second.

The measured value displayed by the ultrasonic flow meter to be calibrated is compared with this value. Of course, the direct comparison of the measured values only works if both measuring points have the same pipe cross-section and the same flow conditions. Thus, at the flow calibration stand at PCE Instruments, if the ultrasonic flowmeter is calibrated on the 28.0 x 1.0 mm cross-section pipe, it can be compared directly with the value of the magnetic inductive reference meter. For the other two cross-sections, the corrected nominal set points of the flow velocity can be read out on a separate evaluation device.

When determining the flow velocity with the ultrasonic sensors, the sonic velocity of the flowable medium plays a decisive role. However, the sonic velocity is not only dependent on the composition of the material, but also on its temperature. Therefore, the temperature of the medium must always be taken into account when determining the flow velocity with the ultrasonic flowmeter. The evaluation unit located on the flow calibration stand above the test pipes shows not only the flow velocity but also the water temperature recorded with an inline sensor and, in addition, the current values for the air temperature and humidity.

In the factory calibration certificate for the ultrasonic flowmeter, the ambient conditions, the calibration device, the calibration procedure, the measurement conditions and the target/actual comparison result(s) are recorded. If the actual value displayed by the ultrasonic flowmeter deviates from the nominal value (setpoint) of the reference device more than specified in the device .

Ultrasonic flowmeters can also be used to determine heat quantities if additional sensors are used to determine the temperature of the heat transfer medium before and after heat emission. For time-limited control measurements, mobile device sets, which can be quickly and in a leak-proof manner mounted on the pipeline and dismantled again with little effort, are particularly suitable. Ultrasonic flow meters can measure the flow rate without interference in the pipe system and without direct contact with the heat transfer medium or coolant. Especially practical are sets for heat quantity measurement with handy devices in which both the ultrasonic sensors and the temperature sensors are attached with Velcro connectors.

Such a mobile ultrasonic heat meter set usually includes three components, the ultrasonic flow meter, a two-channel temperature data logger and the software for evaluating the data. The ultrasonic flow meter determines the flow velocity from the transit time of the ultrasound in the flowing medium. From this flow velocity and the previously entered pipe cross-section, it determines the volume flowing through the system. The temperature data logger records the temperatures at the flow and return of the pipe system. For this purpose, one temperature contact sensor is fixed on the outside of the flow pipe and one on the return pipe. The measurement and calculation data from the two devices can be stored or transferred to a computer during the measurement.

The evaluation software, which is specially adapted to such unit sets, calculates the heat output in kW or kJ/h and the heat quantity in kJ or kWh during the measurement period from the transferred data for the volume flow and the temperature difference between the flow and return pipes. It can immediately display the recorded and calculated data in tabular and graphic form, thus enabling direct evaluation of the measurements on site with a laptop. If immediate evaluation at the measuring point is not possible, the measurement data stored in the ultrasonic flow meter and in the temperature data logger can be transferred and evaluated later in the office.

Ultrasonic flowmeters for heat quantity determination enable fast and uncomplicated measurements in the air-conditioning technology and in industry, whether for time-limited control measurements at various locations or for the permanent monitoring of heating or cooling systems. Due to the fact that the sensors do not come into contact with the heat transfer medium during the measurement, the ultrasonic flow meters can be used without any problems with aggressive media, but also in the areas with high hygiene requirements, such as in the pharmaceutical industry and food production. The measuring process from the outside is usually also preferred because no processes have to be stopped for the measurement and because the flow behaviour of the fluid is not influenced by the contact sensors at the measuring points.

The measuring accuracy of the ultrasonic flowmeters and thermometers can be regularly checked in particularly suitable for this task calibration laboratory and confirmed by an ISO calibration certificate. For this purpose, the calibration laboratory must be equipped with a suitable flow calibration stand.

Ultrasonic flowmeters are conquering more and more technical fields of application, as they measure without affecting the flow behaviour and can be optimised for many different measuring conditions. The user can choose from a wide variety of models for stationary or mobile use. The highest hygienic requirements can be met both in the case of the fixed inline devices and the device models where the sensor has no direct contact with the flowing medium. The possibility of measuring from the outside means that the flow velocities in existing pipe systems can be recorded without any problems and without having to cut open the pipe. There are also approved ultrasonic flowmeters for custody transfer applications, such as measurements for consumption billing of water, other liquids, gases, steam or compressed air or even hydrogen.

Medical and laboratory technology
In medicine and in the analysis of liquids and gases, the ultrasonic flowmeters are used to precisely monitor the low flow rates. Whether in intensive care, during operations, in dialysis or in the analysis of respiratory gases, special sensors have been developed to monitor the flow rates. They enclose the usual tubing and measure the flow rate from the outside with high accuracy.

Pharmaceutical industry and biotechnology
In the production of medicines and in biotechnology, too, high purity of the substances and exact adherence to the intended flow ratios are essential. The ultrasonic flowmeters can be used both for dosing liquid active ingredient components as required and for monitoring specified cleaning processes.

Semiconductor production
In the production of semiconductors, special demands are made on the manufacturing environment, as the smallest foreign bodies can render the components unusable. Various high-purity abrasive, adhesive or corrosive liquids are used in the manufacturing process. To ensure that the individual processes run safely and effectively, the flow rate of the substances can be monitored with the help of the ultrasonic flow meters continuously and non-intrusively during the processes.

Chemical and petrochemical industry
In the chemical and petrochemical industry, the ultrasonic flowmeters enable the safe measurement of aggressive or toxic substances. Whether for process monitoring, dosing of additives or continuous line monitoring – ultrasonic flowmeters are suitable for a wide variety of tasks. The device models with ATEX approval for potentially explosive areas are available.

Oil, gas, district heating, cooling
In the field of the thermal energy supply, the ultrasonic flowmeters are not only suitable for custody transfer or internal continuous consumption measurement. Mobile device types can also be used without problems for short-term control measurements without interventions in existing systems. They are suitable, for example, for energy audits, for coordination of the system components and for process optimisation. But the inspection of the permanently installed consumption meters can also be carried out with the suitable mobile ultrasonic flow meter.

Water and wastewater management
For consumption measurements in drinking water supply networks, calibrated ultrasonic flowmeters are available for inline measurement, which do not require an additional inlet or outlet section. The accumulated consumption value is not only displayed, but can also be read out wirelessly. It is therefore no longer necessary to go to the basement for reading. In addition, the measurement data from these water meters can be used to detect leaks in the domestic installation or to limit the position and extent of leaks in the event of failures in the drinking water network.

These digital meters no longer have to be replaced every 6 years like the old mechanical meters, but only after 15 years. The ultrasonic flowmeters therefore offer considerable advantages as water meters for both the water supplier and the end customer. There are also special ultrasonic flow meters for the wastewater and for level and flow monitoring of the rivers, e.g. during heavy rain events.

Ultrasonic flowmeters are also suitable for monitoring and controlling very small flow rates in pharmaceutical production, biotechnology, semiconductor manufacturing and medical technology. A distinction must be made between clamp-on systems and inline systems. While the sensors of the ultrasonic flowmeters in the clamp-on systems are placed on the outside of the pipe or hose and have no direct contact with the flowing medium, in the inline systems they are directly wetted by the fluid.

High-precision ultrasonic flowmeters specially designed for small flow volumes are used in medicine, for example in

- ventilators,
- cardiac support systems,
- heart-lung machines,
- dialysis machines and
- transfusions and
- in the analysis of samples in gas chromatographs.

In industry, these devices can be used, among other things, to monitor and control the process-dependent admixing of active ingredient components and the dosing of cleaning solutions and process cooling water according to the specified criteria. The ultrasonic flowmeters for small flow rates are increasingly being integrated into the process flows in many branches, including

- in the pharmaceutical industry
- in laboratory technology
- in biotechnology and
- in semiconductor production.

Since it is very important in the medical field, but also in many industrial applications, to precisely monitor the small flow rates, the ultrasonic flow meters used there are equipped with interfaces. Via these interfaces, the measured values are passed on directly to the process monitoring and, if necessary, pumps or valves can be controlled automatically. Some device models are additionally combined with sensors for air bubble detection so that the unwanted bubbles can be automatically detected and reported.

Air bubbles are absolutely undesirable not only in most liquid transfers in medical applications, but also in many applications of low-dose liquids in industry. For example, an air bubble in photoresist can make the coated components unusable and air pockets in cooling water can lead to the process interruptions.