There are many devices for flow measurement and the measuring of flow amount of fluids (liquids and gases). Gauges are equipped with advanced processing of measured data and the output data indicates the flow amount. Flow can be also evaluated by measuring the local or mediate speed of the medium flowing through the known cross-section, usually at given operating conditions (p, T). The current trend of flow meters is focused on the direct measurement of mass flow, i.e. measuring independent on temperature, pressure and viscosity of the fluid measured. The result of flow measurement may be presented either as a mass flow rate QM (e.g. kg·s–1), or volume QV (e.g. m3·s–1) where m is the mass, and V is the volume in the measured medium. Modern devices are equipped with electronic circuit for automated correction of temperature and pressure during the measurement. The trend of flow meter development is focused on the direct measurement of mass flow, i.e. independent on temperature, pressure and viscosity of the fluid measured.
So-called absolute methods- used for accurate measurement and verification of other types of flow meters. The measurement alone is based on the principle of admeasuring the volume of the fluid in the measured spaces.
It is used to measure the volume of gas; it is equipped with two chambers divided by membranes. It is for example used to measure the quantity of heating gases.
In a horizontal cylindrical container partly filled with liquid the measuring drum is fulcrumed which is equipped with slots for the inlet and outlet of the gas and divided into four measuring spaces by radial bulkheads. It is used for precise laboratory and verification measurements.
It belongs among the most accurate. Measuring spaces bounded by a piston and the body of the gauge are alternately filled with specific fluid and emptied. Piston gauges are suitable for the measurement of even very viscous fluids.
Determine flow based on measured values of local or average speed and knowledge of the flow profile and the clear area.
Using the dependence of dynamic pressure of the flowing medium on the speed of flow.
It is tube curved at right angle to the plane of its opening situated vertically to the direction of the flow. At the sensor’s issue the flow drops in practice to zero and all the kinetic energy transforms into potential energy. The sensor scans the total pressure pc, which is the total of static pressure ps and a dynamic pressure pd.
Measures pc and ps in one place. The mentioned sensors are used for short-term measurements and measurements of velocity profiles. The lower measured speed limit for gases is about 6 m/s and for water 0.2 m/s.
Within the pipeline a throttle organ is located, narrowing the flow cross-section. The difference of static pressures in the fluid before and after the narrowing, scanned with the differential pressure meter, depends on the value of the flow rate. The most used throttle organs are a centric circular shutter, jet and a Venturi tube. Among the special throttle organs there are square and rectangular shutters and a square Venturi tube, used with rectangular or square cross-section pipeline.
It forms a group of so-called cross-cut gauges, whose flow area changes with the flow at approximately constant pressure gradient in the narrowed cross-section. The main functional parts are a vertically placed slightly cone-shaped tube, extending up (the cone angle is less than 2°).
The speed of rotation of the rotor, paddle wheel or bolt is proportional to the mean flow velocity.
Slip is continually proportionate with the strain torque of the rotation part, i.e. the rotor, and it is influenced by the immediate value of the flow. According to the direction of flow, axial and radial flow meters are distinguished.
The turbine flow meter is a representative of axial flow meters; the rotor is made from blades attached to the hub stored in the bearings. Gauges are manufactured in a wide range of measuring ranges up to hundreds of m3/hour. Turbine flow meters are suitable for liquids and for gases. For pulse scanning of rotation speed different sensors are used from mechanical to contactless ones. Pulses are further amplified and shaped.
The rotation frequency of the turbine is proportional to the immediate flow. Contactless sensors are advantageous in terms of digital signal processing. To determine the immediate flow, the constant period is added to pulses and the A/D converter is not necessary. The relative measurement error can be <0.5%, pressure to 30 MPa and temperature of –200 ... +200 °C.
They are based on the use of Faraday’s law of electromagnetic induction during the movement of a conductor in a magnetic field. With the electromagnetic inductive flow sensor, the moving conductor is represented by an electrically conductive liquid. A permanent magnet or electromagnet creates a magnetic field within the pipeline and liquid.
The section of the pipeline between the poles of the magnet mustn’t be made of a ferromagnetic or conducting material. The internal diameter of the flow meter’s measuring tube incorporates two electrodes for sensing induced voltage. The alignment of the electrodes is vertical to the direction of the magnetic lines of force. An inductive flow meter consists of a non-magnetic cylindrical measuring tube with two appropriately placed scanner electrodes.
These flow meters can be divided into two basic groups according to either the use of the Doppler Effect, or the measurement of the ultrasound signal transit time.
The principle of an ultrasonic flow meter measuring signal transit time.
It can be used if the flowing medium contains sound reflective particles, for example solid particles or gas bubbles. The flow meter consists of an ultrasound transmitter and receiver, installed on one side of the pipeline. Ultrasonic signal of a known frequency around 1.2 MHz is transmitted to the flowing liquid, where it is reflected back from a moving particle or bubble, and when the receiver senses the reflected signal the frequency of the received signal is evaluated. The difference between the two frequencies is commensurable to the speed of the flowing media.
Ultrasonic methods can be used for measuring small and large flows of clean, contaminated, and aggressive liquids, for the measurement of pulsating flows and the measurement of sludge and melts flow at high temperatures.
Their principle is processing energy balance during heat sharing from electrically heated heating elements of the flowing fluid, during which the distribution of temperature changes. Changes in temperature are proportional to the mass of the flowing liquid. The following basic types exist.
It is a thermal mass flow meter; whose temperature sensors intervene directly in the flowing fluid – the cooling effect of the forced convection on the heated sensor is evaluated. Two resistance thermometers are located within the pipeline.