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Beschreibung:
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Flow Meters
A flow meter (or flow sensor) is an instrument used to measure the linear, nonlinear, mass, or volumetric flow rate of a liquid or a gas. When choosing flow meters, one should consider such intangible factors as familiarity of plant personnel, their experience with calibration and maintenance, spare parts availability, and mean time between failure history, etc., at the particular plant site. It is also recommended that the cost of the installation be computed only after taking these steps.
First Steps to Choose the Right Flow Meter
The first step in flow sensor selection is to determine if the flow rate information should be continuous or totalized and whether this information is needed locally or remotely. If remotely, should the transmission be analog, digital, or shared? And, if shared, what is the required (minimum) data-update frequency? Once these questions are answered, an evaluation of the properties and flow characteristics of the process fluid, and of the piping that will accommodate the flowmeter, should take place. In order to approach this task in a systematic manner, forms have been developed, requiring that the following types of data be filled in for each application: Download the Flowmeter Evaluation Form.
Fluid and flow characteristics
The fluid and its given and its pressure, temperature, allowable pressure drop, density (or specific gravity), conductivity, viscosity (Newtonian or not?), and vapor pressure at maximum operating temperature are listed, together with an indication of how these properties might vary or interact. In addition, all safety or toxicity information should be provided, together with detailed data on the fluid's composition, presence of bubbles, solids (abrasive or soft, size of particles, fibers), tendency to coat, and light transmission qualities (opaque, translucent or transparent).
Electromagnetic Flowmeter (EMF)
Electromagnetic flow meters (EMF) have been used for continuous flow measurement in industrial applications for many years. They are ideally suitable for liquids with an electrical conductivity greater than 5 μS/cm. In addition to water and wastewater, other contaminated liquids are typical areas of application.
The underlying measuring principle of the EMFs is based on Faraday's law of induction. The coils of the flowmeter generate a magnetic field orthogonal to the flow direction. According to Faraday's law of induction, a conductive liquid passing through the magnetic field induces a current between the two electrodes, which can be used to measure the voltage. The higher the flow velocity, the higher the induced voltage. This voltage signal is converted into a standard signal (e.g. 4 - 20 mA or P) by the integrated electronics.
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