Calorimetric flow meter

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Calorimetric measuring principle
Measuring principle differential method

A calorimetric flow meter (also known as a thermal flow meter) is an electronic flow meter that works on the calorimetric (thermal) principle. This measuring principle of the calorimetric method is used both for flow monitoring, for flow meters or as mass flow meters.

Measuring principle

Flow measurement on a thermal basis can be classified into two methods:

Hot wire method

An electrically heated fine metal wire (e.g. platinum, tungsten or their alloys) with a small diameter (1 to 20 μm) is wrapped around a measuring tube and then brought into the gas flow. King's law describes the effect that the amount of heat H withdrawn from an electrically heated wire per unit of time as a function of the flow velocity v of the gas with density p in the event that the wire is flown perpendicular to the longitudinal axis and there is an incompressible flow.

Heating process

The fluid to be measured is heated by the supply of heat flow through a heating coil (see schematic diagram). This calorimetric principle is based on temperature sensors. One of the sensors is constantly heated so that a constant temperature difference is set between the two temperature sensors when the medium is stationary. The temperature is measured with the help of temperature sensors. Establishing a thermal contact requires sufficient heat conduction, convection or a radiation equilibrium between the measurement object (solid, liquid, gas) and the sensor. The measurement accuracy can be impaired, for example, by an unbalanced thermal radiation balance, the movement of the material to be measured or by heat dissipation along the sensor. Theoretically, the measurement accuracy is limited by the random Brownian molecular motion .

If the medium flows, the heat generated in the sensor is dissipated through the medium and the sensor is thus cooled. The cooling process is a measure of the flow rate.

Properties of the heating process:

  • high heating power required
  • depending on specific heat (see sketch application limits measuring range)
  • linear measurement effect
  • Can be used for gases and liquids regardless of the density of the medium.

Types

This measuring principle (heating method) of calorimetric (thermal) measurement is used in process measurement technology as a limit switch (flow monitor), flow meter or as a mass flow meter.

A measuring device consists of two parts: the actual measuring transducer and an evaluation and supply part, the transmitter (measuring transducer) in accordance with the basic DIN standard 1319. Two distinctive types are the transducers spatially separated from the transducer and the compact devices spatially connected to the transducer.

Flow monitor

It is used for flow and temperature monitoring of liquids and gases, as both the flow velocity and the temperature are recorded by the sensor. The sensor signal is compared with the reference data stored in a microcontroller, and if there are deviations between the desired and the existing flow rate, an output signal is output for further evaluation. This electronic flow monitor continuously monitors liquid media and the respective preselected measured values ​​are output as limit values ​​(switching function). It is used wherever flows with minimal pressure loss are to be safely monitored.

Flow meter

Flow profile in a pipe

With the calorimetric flow meter, a volume flow is output as a measured value. A volume flow is understood to be the volume of a medium that moves through a cross-section within a unit of time. With this formula, if the cross-sectional area is known, the volume flow can be calculated if the flow velocity is measured at one location.

: Volume flow in [m³ / s], [l / min], [m³ / h] : Volume in [cm³], [dm³], [m³] : Time in [s], [min], [h], Da the flow velocity in a cross section is not constant (flow profile), the mean flow velocity c is determined by integration :


c  : Velocity at one point of the cross-section (a function of the location ⇒ f (x, y), if flow direction = z)

Mass flow meter

When used as a calorimetric mass flow meter , the mass flow is required, for example, in [kg / s], so for incompressible flow:

If the cross-section changes, the following applies to the volume flow at constant density because of the conservation of mass:

These values ​​are processed in an evaluation unit assigned to the measuring sensor and provided accordingly as output signals.

Advantage: Changes in the operating conditions such as pressure and temperature do not need to be compensated because the mass flow is measured here. This principle is mainly used in gas mass flow control systems: Many devices contain control electronics and a proportional valve at the same time and are therefore called mass flow controllers (MFC).

Control loops and evaluation

Measurement control circuit and signal processing

Special electronic control and amplification is required for the operation of calorimetric flow meters.

Constant temperature operation (CTA)

With the so-called CTA method, the sensor is kept at an average constant temperature by means of a fast control loop. Since the sensor temperature can be determined, a theoretical correction of the temperature influence is possible. This operating mode also has a wide frequency range.

Formula for the conversion

The first fundamental work was carried out by LV King in 1914. The formula named after him applies to the required electrical power:

  • Current through the sensor
  • Ohmic resistance of the sensor
  • Sensor temperature
  • Temperature of the fluid
  • Constant that depends on the physical boundary conditions
  • Flow velocity in the vertical plane

The required power is therefore dependent on the temperature difference between sensor and fluid as well as the flow velocity and the physical boundary conditions of the respective construction. There are different approaches for the compensation function. The following approach has proven itself in practical application:

  • a, b, e constants that depend on the physical boundary conditions and are determined during calibration
  • Output voltage of the measuring bridge in the transmitter

The sensor temperature cannot be determined directly, but is calculated using the bridge settings. This corrected sensor temperature is determined via a corresponding calibration at different temperatures.

Applications

  • Steel industry: coolant circuits, coolant and lubricant monitoring
  • Chemical industry: dry running protection for pumps
  • Beverage industry: Monitoring of cleaning processes, filling systems, etc.
  • Flow measurement in compressed air technology

advantages

Advantages:

  • no moving parts in the flow, such as turbines or floats, so that this sensor can also be used with solid-laden flows
  • the medium to be monitored only touches one material
  • compact design
  • a sensor (probe) for different nominal sizes
  • low pressure drop
  • high operating pressures, up to 200 bar

disadvantage

  • can only be used in the lower temperature range max. 70 ° C

Individual evidence

  1. DIN 1319-1: 1995-01, Fundamentals of measurement technology - Part 1: Basic concepts, Beuth Verlag, Berlin.

literature

  • Heinz Bernard, Frank Grunert, Frank Dornauf, Armin Brucker, Friedrich Hofmann: Flow and quantity measurement technology (= atp practical knowledge compact. Volume 5). ISBN 3-486-63074-1 .

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