Flow Meter

A flow meter is used to determine the velocity and, under certain conditions, also the quantity of the flowing medium. Among other things, flow meters can be differentiated according to the type of the medium to be measured and the measuring principle. Often the flow meter is designed for a certain state of the flowing material, i.e. it measures either gases or liquids. With specially equipped devices it is also possible to measure the bulk materials able to flow during transport in manufacturing or storage processes.

Even those who have little to do with the flow meter, for sure have already observed the flow meter at the fuel dispenser while refueling and possibly read the water meter or gas meter before paying the bills for the water and gas quantities used. These flow meters are used to record the quantity and must be calibrated because of the volume-dependent prices. These devices and those used for continuous monitoring and control of processes are called stationary flow meters, because they are permanently used at the same place. Mobile devices are suitable for occasional testing, measurement at different locations and for troubleshooting in existing plants and systems.

Selection criteria for a flowmeter
Flow meters are used in many different areas. In order to narrow down the choice for your own requirements, it is advisable to first clarify the measuring conditions.
- Should the flowmeter be permanently installed at one point or should it be applied for the measurements at different points?
- Is the material gaseous, liquid or bulk?
- Does the material flow through a completely filled pipe cross section?
- If so, may the flowmeter protrude into the pipe cross-section?
- Which flow velocities should be measured?
- What accuracy is expected from the flowmeter?
- Should the measurement be carried out during a running process and should the process be influenced?
- Should the measured values be stored or transmitted?
- Should the flow meter be calibrated and adjustable?
Below, some flow meters are briefly described according to their operating principle.

Measuring Principles of the Flowmeters
Various measuring principles can be used to measure the flow velocity and the volume flow. Flow meters with Pitot tube, impeller and hot wire are among those devices where the sensor is positioned directly in the flow and influences it to a greater or lesser extent. The flow meters with Pitot tube are well suited for strong flows and for gases with high particle content. Liquid volumes and average flow velocities for gases in the range of 5 m/s to 20 m/s can be measured well with the impellers. Hot-wire anemometers with their thermal sensors are particularly suitable as flow meters for low velocities of the gaseous media. Both the ultrasonic flowmeter and the magnetic inductive flowmeter function as measuring devices without obstructing the flow.

Pitot tube manometer
The flow meter that determines the flow velocity by means of the pressure difference is the Pitot tube anemometer. The pressure measurement works via a pipe with specially arranged openings, which detects and records the pressure of the flowing medium. If the static pressure of the not moving medium, which is also measured, is subtracted from this dynamic pressure, the differential pressure is obtained. From this the flow velocity can be determined. The volume flowing through the pipe can be determined from the flow velocity and the pipe cross-section.

Impeller meter
The flowmeter that determines the velocity of the medium from how fast a specially shaped impeller is moved through the flow is the impeller anemometer. These devices are often used for outdoor wind measurements. They are permanently installed on buildings or on wind turbines and can be rotated into the flow by coupling them to a wind direction meter or wind direction indicator. Mobile devices are used in the fields, on the construction sites or airfields or in buildings to check the ventilation ducts. Some of these devices can also determine the volume flow in the case of the fixed flow cross-section. The propeller-like impeller is held directly in the flow, whereby for many devices it is essential to pay attention to the direction of the flow. A special form which is independent of the horizontal flow direction is the cup cross anemometer. In this case, three or four half-shells are arranged around the axis of rotation, which is perpendicular to the flow, and are set in motion by the flow. Impellers are also used for continuous volume measurement of liquids. Many water meters and heat meters work with the impellers in the flow direction.

Hot-wire anemometer
A hot-wire anemometer is the flowmeter that works with heated electrically conductive materials. The sensor cools down faster at higher flow rates. The evaluation can be done by two different methods. The first method, Constant Current Anemometry (CCA), keeps the heating current constant. Due to the temperature change, the electrical resistance of the heated sensor element changes and a changed voltage can be measured. The second method, the Constant Temperature Anemometry, works with the constant temperature. In this method, the flow velocity is determined by how much heating current must be used to maintain the temperature.

Ultrasonic flowmeter
Ultrasonic flowmeters can be used to measure the flow velocity and volume of liquids flowing through the pipelines without direct contact. For this purpose, the ultrasonic flowmeter has two sensors, which are placed on the pipeline as transmitter and receiver and are fixed there. The measurement is possible on both metallic and plastic pipes. The sensors can also be used for many different pipe cross-sections. For this reason and because no changes are necessary for existing pipelines, the ultrasonic flow meters are well suited for mobile measurements. When interpreting the values, it must be taken into account that the measurement result can be falsified because of the inhomogeneity of the liquid, for example caused by high viscosity, entrained gas bubbles or solids.

Magnetic inductive flowmeters
Magnetic inductive flow measurement (EMF) works without moving parts and without direct contact with the flowing medium. However, it requires that the medium to be measured is electrically conductive and is guided through a special measuring section. For this purpose, a specially prepared pipe cross section is integrated into the flowmeter. This ensures that the resulting magnetic field is not dissipated via the pipe material and that the voltage can be tapped off via two electrodes which are in galvanic contact with the flowing medium. The lower limit value of the conductivity depends on the device and can, for example, be 50 microsiemens per centimeter. Before you make a decision about such a magnetic inductive flowmeter, check whether your medium is sufficiently conductive. Magnetic inductive sensors react very quickly and require only a short measuring distance. The magnetic inductive flowmeter is independent of temperature, density, viscosity and delivers accurate results even with gas bubbles and solids carried along.

Calibration for flowmeters
In order to ensure that the flowmeter outputs correct values, it should be calibrated regularly. When selecting the device, it should be considered how and where the flowmeter can be calibrated. For devices that continuously monitor and control processes, it is advantageous if the device does not have to be sent for external calibration. It is also advantageous for many production processes to be able to test directly with the respective medium and under local conditions. In the case of calibrated water meters that are to be permanently ready for use, the meter is replaced with a tested one after the calibration period has expired. By this procedure, the measuring point is blocked only for the period of replacement and can be used again immediately afterwards. The removed water meter is then tested externally and adjusted or disposed of in the case of deviations. For a mobile flow meter that has to be calibrated externally, the time period should be planned in advance. The time intervals between the checks should be determined depending on the consequences of inaccurate measured values.

The device got its name due to its ability to operate in the ultrasonic frequency range and to register what is going on when the ultrasonic waves and acoustic oscillations go through the flow of liquid / gas. The faster the material moves / flows in the pipe, the more the frequency of the wave changes. In fact, by measuring the acoustic effects, the device determines the volume and consumption / flow of a certain material (water, oil, gas, various flowing products). The medium is flowing in the pipe at different speed, and the measurement carried out both, along and against the flow (upstream and downstream) is meant to determine, in the end, the flow rate and volumetric flow. There is no big difference in measurements if there is no flow, but with the increase of the flow, it is much more difficult for the wave to move against the whole flow than down the flow, and that is why the quicker the flow in the pipe becomes, the bigger the difference in up- and downstream measurements will be (different time required for passing a certain distance at certain velocity). Introduction of the acoustic transducer reacting to the density of the medium, provides that even the mass flow can be measured. A lot depends on the liquid, the flow of which should be measured. If the material is clean, the high frequencies are applied, and there is guaranteed a very reliable and accurate measurement to expect. If there are some solid particles in the liquid, spread unevenly, or the material is not clean enough, the frequencies should be lowered, to avoid their scattering and absorption of oscillations.

An ultrasonic flow meter is easy to operate and handle, it is a stable instrument, that has no moving parts and is quite robust with a long-term service life, nevertheless before its application, it is necessary to carry out the check of the liquid flowing so that to avoid uncertainties and failures during the measuring process. When applying flow meters, it should be taken into account that if some materials / liquids are very resistant to ultrasonic waves and reflect them rather than let them pass through, the measurement will be very inaccurate and errors may occur. The particles that may be present in the material have their own speed, different from the whole flow, in which they are in, and theoretically, this also may bring to big fluctuations and errors in the measurements of the flow rate. In spite of the possible difficulties, it is fair to say that modern ultrasonic devices have reached such a level that they cope even with such obstacles as very obscure and inhomogeneous liquids.
The range of application possibilities of an ultrasonic flow meter is high: almost all kinds of industry where the material is flowing in the pipes, most spread in water / wastewater, food and beverages producing industries, pharmaceutical, chemical and others.
Ultrasonic flow meters have got evident advantages which are very valuable and make it an irreplaceable tool when it goes about maintenance and service checks, as well as for measurement which should be preferably carried out without intrusion into the pipes. Depending on the material and kind of enterprise the measurement is carried out at, intrusion into the pipes may be quite undesirable (power or chemical plant), that is why application of non-contact meter that measures the flow of sometimes hazardous substance is very convenient. The flow meter with the clamp-on transducer may be even mounted on the pipe, and carry out the measurement externally, from outside, but there are a few things to pay attention to: it should be mounted and applied on the pipe correctly to avoid the weakening or loss of the ultrasound signal so that the latter was be able to fully reach the liquid. Application of the clamp-on transducer makes it possible to apply the device anytime, not depending on the kinds of liquid and its properties, since the device remains dry and not damaged. The measurement is carried out continuously and the results can be received in real time.

A mobile / portable ultrasonic flow meter facilitates the measurements to such an extent that an operator does not require any additional instruments for carrying out immediate checks wherever it is necessary (from water to oils and chemicals). It is ideal for the situations when quick measurement should be carried out for detecting a problem or just checking the general condition of the ongoing process. Due to the rechargeable battery pack the portable device ensures a few hours of regulative work and the internal data logger stores the measurements for further analysis and processing. Mobile flow meters can always be used as a reference for fixed installed flow meters, since, in spite of their compactness, they provide good measurement results.

One of new application fields is the application of a flow meter for HVAC control. Since nowadays there are strict requirements and regulations as for heating, air conditioning and ventilation, especially in closed spaces, rooms, offices and similar, the necessity to carry out accurate measurement of the air flow is quite high. For this type of measurements, temperature becomes an important parameter and the measurement is carried out taking a thermal flow principle into account. The best solution, when it goes about measurements in heaters, heating systems, fire extinguishing systems is a clamp-on heat meter. Without extra invasion into the pipe with hot / cold substance inside, the meter is fixed on the pipe (the size of the pipe does not matter) and allows carrying out the monitoring in the most difficult to reach and hazardous parts.
Application of the flow meter helps not only to find out the problems and carry out troubleshooting in the pipes, it provides possibilities for monitoring and quality control (for example, in beverages production, or analysis of the heating equipment and installations). If the device has got dual or multichannel option it makes it possible to carry out simultaneous measurements not only at a few points of one pipe, but also in a few pipes, thus providing better accuracy and possibilities to compare the values. Either clamp-on and whetted transducers can be used to make the measuring process more convenient and they are suitable for both, low and high operating temperatures, which ensures the application for monitoring of the cooling as well as for the heating systems.