Flow sensor

Flow sensor (also flow meter ) is a collective term for all sensors that measure the flow of a gas or liquid through a pipe.

Next will Flowmeter called a device in medical technology, which measures the flow of blood or the flow of breathing gases, and can be derived from different values. ( Flowmetry ).

A flow measurement is made for three main reasons. On the one hand, these are commercial reasons. The flow rate is then part of a contract or basis for taxation. Then the flow is integrated over time and the total amount is obtained. Secondly, there are recipes in which several quantities of substance have to be mixed into one batch in batch processes . Thirdly, material flows can also be mixed directly in a specified ratio without a batch process.

Display types

A distinction is essentially made between two types of output signals:

1. Volume flow. To do this, the flow rate of the medium is measured and then the flow is determined using the formula volume flow = pipe cross-sectional area * speed. The result is usually given in m³ / h or l / min, the SI unit is m³ / s.
2. Mass flow is important when measuring fuel quantities, for example . An example of this is an air mass meter as it is found in automobiles. Here the measured value is mainly given in kilograms per hour (kg / h) air. In addition, temperature and humidity are measured in the air mass sensor and thus the proportion of air that flows into the engine is determined. The engine management system uses this to regulate the corresponding amount of fuel. The measurement of the volume flow would not be sufficient for this application, since the density of the air is highly pressure-dependent and temperature-dependent. If only the air volume flow were measured, the engine control would not be able to guarantee the exact mixing ratio that is necessary for complete combustion of the fuel .

Important parameters

Sensors used in industry mostly use standard signals : Either electrical current from 0 to 20 mA or 4 to 20 mA or electrical voltage from 0 to 10 V.

The measuring range can be specified by:

• lower measurement limit, e.g. B. 1 l / min
• upper measuring limit, e.g. B. 100 l / min
• Measuring span or dynamic 1: 100

The conventional measuring principles described here allow reliable measurements down to flow rates of a few ml / min. Below this, the reliability and measurement accuracy of these methods decrease sharply. For smaller flow rates, down to the range of nanoliters per minute, sensors based on microsystem technology are used. These usually work with thermal measuring methods.

The measurement deviation is usually limited by specifying the relative error limit, e.g. B. 1% of the current measured value.

The pressure loss is an important parameter because it always means an energy loss in the pipe system. Different types differ characteristically in their pressure loss:

• Ultrasonic flow sensor based on the transit time principle without flow straightener: almost no pressure loss
• Differential pressure orifice measurement: high pressure loss that increases sharply with the flow velocity
• Thermal air mass sensor in automobiles: low pressure loss as small measuring elements protrude into the flow channel
• Magneto-inductive flow sensor (MID): almost no pressure loss
• Vortex flow meter or vortex flow sensor: low pressure loss

Measurement accuracy and spans of some sensors

Ultrasonic flow sensor

Change in travel time of the sound due to the flow

The non-contact ultrasonic flow meter by determining the transit time is a non-invasive flow meter, as no objects disturb or change the flow in the pipe. There are two formats: transit time and doppler. The relative error limit is approx. 0.1 to 2%. The measuring span, which is the ratio of the smallest to the highest measurable speed, is approx. 1: 100.

A basic distinction is made between clamp-on devices (i.e. the sensors are strapped onto the pipeline from the outside) and in-line devices.

Differential pressure method

Scheme of an orifice measuring section with corner pressure extraction according to DIN EN ISO 5167-2

There are a variety of different flow sensors that work according to the differential pressure method, e.g. B. Pitot tube , Prandtl probe or measuring orifice . All work according to the law of conservation of energy ( flow according to Bernoulli and Venturi ) with the result that the volume or mass flow is proportional to the square root of the differential pressure between two measuring points. In order to make the final formula manageable for the user, all constant values ​​of the measuring device (pipe and throttle cross-sections) and the measuring medium (density ) are often summarized  as a constant : ${\displaystyle Q}$ ${\displaystyle \rho }$ ${\displaystyle c}$

${\displaystyle Q=c\cdot {\sqrt {\Delta p}}}$

The equation shown above applies when the density of the fluid equals the density at calibration. However, since the density of gases is strongly dependent on temperature and pressure , the following equation can compensate for this influence within certain limits.

${\displaystyle Q=c\cdot {\sqrt {\Delta p}}\cdot {\sqrt {\frac {\rho _{\text{Kalibrierung}}}{\rho _{\text{Messung}}}}}}$

In air flow meters, calibration is usually used for density  . ${\displaystyle \rho _{\text{Kalibrierung}}=1{,}20\,\mathrm {kg/m} ^{3}}$

In addition to the classic pitot tubes, there are other manufacturer-specific designs that can have various advantages under certain conditions, such as: B. lower requirements on the inflow conditions, lower pressure losses or easier installation. However, in contrast to the primary elements, these devices require calibration for the respective application.

The kinetic energy of a fluid (e.g. flowing water in a pipe) is converted into potential energy (pressure). The best-known primary element, the orifice plate, is nothing more than a perforated disc to narrow the pipeline, which forces the fluid to increase its speed (increase in kinetic energy). This reduces the pressure after the diaphragm (reduction of the potential energy). The measuring range is between 1: 3 and 1:20, depending on the primary element and the quality of the sensors and evaluation used.

requirements

For normal measuring points, exact flow values ​​can also be achieved without calibration on site in the operating state, if the following conditions are met:

1. In the vicinity of the throttle device, the flowing medium must completely fill all cross-sections of the pipeline.
2. The flow must be stationary or at least quasi-stationary, i.e. H. the flow rate may only change slowly at the measuring point. Vibrations in the flow affect the measurement accuracy and should be avoided if possible.
3. The substance must be in the pure phase; Solid bodies in gases and liquids, coarse moisture in gases and steam, etc. complicate the measurement and make special measures necessary. Measurements of a substance whose state is close to a transition point also require special attention, as the change in pressure at the throttle device can cause disturbances due to the transition to another phase (liquids close to the boiling point, saturated steam, etc.).
4. The density of the medium and its composition, as well as pressure, temperature and humidity must be known when calculating the flow control device.

The venturi represents the basic function of the throttle elements described in ISO 5167

Primary elements

See also : Orifice plate

There are a number of different primary elements. The classic primary elements (measuring orifice, venturi, nozzles ...) are described in detail in the standards of the DIN EN ISO 5167 series. In addition to the exact designs, information on flow calculation and accuracy can also be found here . With the throttle elements described in ISO 5167, a very high measurement accuracy can be achieved, so that they are mostly used for calibrating other flow measuring devices.

Magnetic-inductive flow sensor (MID)

→ Main article : Electromagnetic flow meter

• Relative error limit approx. 0.1%
• Measuring span up to 1: 1000

Measuring principle: Moving charge carriers (e.g. ions in liquids = electrically conductive media) are separated from each other in a magnetic field.

Vortex flow sensor

See also : Kármán vortex street

This article or the following section is not adequately provided with supporting documents ( e.g. individual evidence ). Information without sufficient evidence could be removed soon. Please help Wikipedia by researching the information and including good evidence.

A disruptive body in the flow creates eddies (Latin vortex ). The frequency of the vortex shedding is characterized by the Strouhal number . The flow rate can be deduced from the frequency.

Types of flow sensors

Flow meter with impeller

Oval gear meter: With every half revolution, a "volume quantum" flows through above and below.

A distinction is made between the following flow sensors:

• Immediate volume counter
  • Counters with constant measuring chamber volume (e.g. drum knives)
  • Meter with variable measuring chamber volume (e.g. gas meter)
  • Oval gear meters, (rolling) piston meters
• Indirect volume counter
  • z. B. vane anemometer , Woltmann counter
  • Turbine flow meter
• Variable area flow meter
• Magnetic inductive flow meter (MID)
• Balometer
  • Inductive flow meters (IDMs)
• Ultrasonic flow sensor (USD)
  • Ultrasonic Doppler Profile Flowmeter
• Coriolis mass flow meter
  • Mass flow measurement method based on the Coriolis principle
• Vortex flow meter
  • Measurement method that determines the flow velocity based on the frequency of the Kármán vortex street
• Correlation flow meter
  • Variations carried along with the flow are measured by means of any two suitable sensors at a certain distance. (e.g. density, permeability) The flow rate and thus the flow rate can be determined from the transit time and the distance between the sensors
• Laminar flow meter
  • According to Hagen / Poiseuille's law, the volume flow in a pipe is proportional to the pressure drop over a pipe length . If the viscosity, pressure drop and temperature are the same, the volume flow can be calculated ${\displaystyle l}$
• Flow meter with flow measuring probes
  • Determination of the flow rate by determining the flow profile (see flow measurement)
• Flow measurement with throttle devices (details in ISO 5167)
  • Orifice plate
  • Venturi nozzle
  • Venturi
• Measurement method for open systems
  • Weir measurement in which the volume flow is determined by means of the overflow height and the weir width b.
• Thermal mass flow measurement (mass flow controller, mass flow controller, MFC)
• Air mass sensor
• Laser Doppler (due to its characteristics, is used almost exclusively in research and development)

Not all measurement methods are common or are used regularly in industry. Compared to other sensors that are used in automation , a flow sensor is relatively expensive. Flow sensors can easily cost twenty times as much as a temperature sensor.

See also

• Flow measurement technology

literature

• DIN EN ISO 5167 1-4, flow measurement of fluids with throttle devices, 2004, Beuth-Verlag
• G. Strohrmann, Measurement technology in chemical companies, Munich 2004, Oldenbourg Industrieverlag
• O. Fiedler, flow and flow measurement technology, Munich 1992, Oldenbourg Industrieverlag
• G. Strohrmann, Measurement technology in chemical companies, Munich 2004
• O. Fiedler, Flow and Flow Measurement Technology, Munich 1992

Web links

• Freeware for the flow calculation of throttle devices according to DIN EN ISO 5167