Flow meter

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Structure compact and separate version (sensor + transmitter)

A flow meter is a measuring device in accordance with the basic standard DIN 1319. It consists of two main components, the actual measuring sensor, which serves as a flow sensor , and an evaluation and supply part, which is also referred to as a transmitter or measuring transducer .

Two distinctive constructions are the transducers spatially separated from the transducer and the compact devices spatially connected to the transducer.

Components of the flow meter

Sensor

The sensor is a technical component of the flow meter that can record certain physical or chemical properties (e.g. temperature, pressure, sound, acceleration, speed, etc.) and / or the material properties of its environment qualitatively or quantitatively as a measured variable.

Transmitter

These values ​​of the sensor are then converted into other values ​​in so-called transducers (transmitters, measuring transducers), and converted into further processable values ​​(mostly electrical signals) and output. The transducer (sensor) is usually also fed via the transmitter.

At least one analog 0-20 mA, 4-20 mA or 0-10 V output is available as standard outputs of the transmitters. In addition, voltage and frequency or pulse outputs are used as options. Most transmitters today have multiple status inputs and outputs. Transmitter with interfaces and bus systems, such as B. RS232, PROFIBUS, DeviceNet or HART communication are now standard in industrial measurement technology and process automation. Some manufacturers also offer special functions, such as the output of the volume flow or the solids concentration. Batch dosing or PID control are also possible.

Areas of application

Flow meters are used in the areas of water and wastewater, chemicals and petrochemicals, oil and gas, energy and steam generation, pharmaceuticals, paper and pulp, as well as food and beverages and cover numerous industry-specific applications.

Breakdown of the flow meter

Classification scheme for flow meters

After temperature, pressure and force, flow measurement is the most important variable in industrial measurement technology and one of the fundamentals of process automation.

Principle measurement methods

The measuring methods of flow measurement for the measurement and automation of industrial processes (FCI) differ in:

  • 1. acoustic procedure
  • 2. Gyroscopic procedures
  • 3. Magneto-inductive processes
  • 4. mechanical-volumetric methods
  • 5. optical processes
  • 6. thermal process
  • 7. Differential pressure / damming method

Differentiation between meter and flow measurement

The terms meter (counting) and flow meter (measurement) do not refer to the same thing despite their similarity. Counting is the recording of a quantity within any period of time, but measuring is the recording of the quantity in the unit of time (such as: volume flow; volume flow).

Counters are therefore only calibrated to a quantity such as liters. The pointer of a counter or a roller counter (such as an odometer) always continues in the same direction. The pointer of a flow meter, however, oscillates back and forth depending on the flow rate on the scale, which can be output in l / s, l / min or m³ / h.

There are counters and flow meters in numerous applications. Counters are used to count or record, for example, the amount of water pumped in pumping stations or supplied to consumers. The meter display thus forms the basis for calculating the water delivery. The difference between a previous and a new reading of the meter represents the consumption, which is multiplied by the price of the unit of measure in order to determine the invoice amount for the water customer.

Flow meters are installed where the instantaneous output is to be recorded in the pipe network or in systems of industrial measurement technology and process automation, the flow of which is to be controlled and further processed. Finally, combinations of flow meters with counters are possible. In this case, the combined apparatus shows both the current flow rate and the amount that has flowed through. Example: Woltmann meters or also vane meters and vane flow meters

How variable area flow meters and flow meters work

The flow meter with float is an extremely precise tool for measuring volume flows in liquid media (or air, gas or water). The measuring device is installed vertically in the pipeline system so that the medium flows from bottom to top. The medium flows through the conical body of the measuring device. Inside there is a float that can move in the vertical axis. The float has a certain dead weight, which basically causes this float to sink onto the opening of the measuring device. If the medium in the pipeline system is now pressurized, the float begins to rise in the measuring cone as soon as its own weight (weight force) is reached. Due to the conical shape of the measuring device, which means that it becomes wider towards the top, the further the float is pushed upwards, at the same time more of the fluid can flow past the body (flow resistance). With this functional principle, the float levels off at a certain measured value when the forces are balanced. This measured value is the flow velocity and can be read off in l / h on the scale based on the upper edge of the float. If the volume flow then drops again, the body sinks again and the measured value changes.

Mechanical-volumetric flow measurement methods

Mechanical-volumetric flow measurement methods are divided into two groups: direct volume counters and indirect volume counters

Immediate volume counter

Direct volume counters have known volumes and are continuously filled and emptied with the measuring medium. Measurement recording by counting the fillings and / or emptying.

Leak counter

In flow meters, the material to be measured runs out of a measuring chamber with solid chamber walls without pressure (not suitable for measuring gases).

Displacement counter

With the displacement meter, the material to be measured is displaced by movable measuring chamber walls (suitable for liquids and gases).

Indirect volume counter

Indirect volume counters have no measuring chambers, indirect volume measurement for example: by measuring the path or the speed of the volume flow.

Classification of flow and volume measuring devices (applications)

There are two main application groups for the measurement and automation of industrial processes (FCI):

  • 1. Flow and volume measuring devices in closed pipelines
  • 2. Flow and volume measuring devices, open channels and gravity lines

1. Flow and volume measurement in closed pipelines

Flow and volume measurement in closed pipelines (pressure lines) encompasses a wide range of different processes and physical methods and effects that can be used for the respective measurement and are divided into the following subgroups: flow meters and flow meters / volume meters .

Flow meter

are divided into the following subgroups: volume flow and mass flow.

Volume flow

Differential pressure measurement method

Mass flow

Volume meter volume counter

This group is divided into indirect and direct:

Indirect

Indirect volume counter

no measuring chambers, indirect volume measurement e.g. B. by measuring the path or the speed of the current

Vortex and swirl flow meters

The following function applies to both flow meters: if a flowing medium encounters an obstacle, pressure fluctuations develop in the medium, which lead to vortex shedding at this obstacle. Vortex and swirl flow meters take advantage of this phenomenon. Vortex or swirl bodies are generated in the flow meter via geometrically defined obstacles and their frequency is recorded by a sensor. This is used to precisely and reliably determine the flow rate in liquids, gases and steam.

Immediate

Immediate leak counter

contain solid chamber walls, the material to be measured leaks out of the measuring chamber without pressure (not suitable for measuring gases) volume meters with solid chamber walls (discharge meters) are for example the drum counter

  • greatest importance among leak counters
  • also allows measurement of heavily soiled liquids
  • low pressure loss
  • The viscosity of the medium should not be too high so that the material to be measured can be emptied quickly enough
  • Accuracies of up to 0.1% can be achieved (error curve depends on the surface tension of the liquid)
Immediate displacement meters

The material to be measured is displaced by the movable measuring chamber walls (suitable for liquids and gases). Volume meters with movable chamber walls (displacement meters) are

2. Flow and volume measuring devices for open channels and gravity lines

Venturi duct knife

In this article, the term Venturi channel is intended to serve as a collective term for all hydraulic measuring channels, such as Venturi channels, Parshall channels , Palmer-Bowlus channels, etc. The Venturi channels in wastewater treatment plants almost always have a rectangular cross-section.

Rectangular and triangular weirs

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Electromagnetic flow meters

Electromagnetic flow meters, which work according to the principle of speed measurement in a known flow cross-section (whereby the full filling of the pipe ensures that the flow cross-section is given and constant), were not suitable for direct use in partially filled pipes until some time ago . New developments in magnetic-inductive flow meters for partial filling through built-in partial quantity detection, empty pipe detection and their measuring accuracy enable them to be used in such applications today. There is thus no need for the user to make complex transfers from gravity flow to pressure pipe flow and back.

Standards and guidelines

European Measuring Instruments Directive MI-001

The first Measuring Instruments Directives with the number 001 refers to the EU-wide Directive 2004/22 / EC for so-called cold water meters for measurement in commercial transactions (previously named as legal for trade). It was adopted in Brussels in April 2004 and has to be ratified in the member states by the end of October 2006. However, national exceptions are possible, but prohibit alternative regulations.

MI-001 is essentially based on EN 14154 and ISO 4064, which are still the standard regulations for the water industry and all responsible authorities. There have been various amendments with different classifications for the measurement accuracy and requirements with regard to self-control.

The new editions from 2005 are almost identical to MI-001. The old regulations will be valid for a transitional period of 10 years parallel to MI-001.

swell

  • Rüdiger Settelmeyer: Process automation - from field device to automation solution. Christiani, Konstanz 2007, ISBN 978-3-86522-305-0 .
  • DIN 19 559 parts 1 and 2, flow measurement of waste water in open channels and gravity lines.
  • H.-B. Horlacher, H.-J. Lüdecke : Flow calculation for pipe systems. expert Verlag, Renningen 2006, ISBN 3-8169-2448-4 .
  • Calorimetric measurements. TU Braunschweig (PDF file)

literature

Individual evidence

  1. ↑ DIN 1319-1: 1995-01, Fundamentals of measurement technology - Part 1: Basic concepts, Beuth Verlag, Berlin.
  2. ↑ = https://www.pkmsa.ch/index.php/de/durchflussmesser.html