Pressure transducer

A pressure transducer ( engl. pressure transducer ) is an electrical transducer for measuring pressure ( gauge pressure, absolute pressure or differential pressure). For this, the pressure to be measured in the mechanical deflection of a diaphragm is formed, which can then be electrically detected and processed. The measurement principle come depending on the manufacturer, especially following physical effects are used:

  • Change in resistance Strain gauge
  • Piezoresistive effect
  • Piezoelectric effect

If pressure transmitter for measurement of aggressive and / or explosive process media in either liquid or gaseous form are used, appropriate measures are required to protect the transmitter against the conditions of use. Just need to ensure that the transmitter is not a threat to the process environment. Therefore, a large number of product approvals available (for example, certificates for explosion protection or use in oxygen applications).

  • 2.1 relative pressure measurement
  • 2.2 Differential Pressure Measurement
  • 2.3 Absolute pressure measurement
  • 3.1 Level Measurement
  • 3.2 density measurement

Versions of

Basically can be divided into several classes of transmitters depending on the scope of services:

Pressure transmitter with analog output

  • The single-
  • Transmitter with adjustable range
  • Communication-capable transmitter with adjustable range (SMART transmitter) Special case: HART transmitters

Fieldbus transmitters

These provide a digital output signal on a field bus.

Print Types

Relative pressure measurement

The relative pressure measurement refers to the atmospheric pressure. An example is the pressure measurement to a steam boiler. For the cooled and vented boiler shows the pressure gauge 0 bar. If the boiler is heated, a pressure builds up which can be measured by a Relativdruckmessumformer. Previously, hence the unit atm (atmospheric pressure ) was common. Today, the relative pressure is usually given in kPa, MPa or bar. Hybrid units use a normal Bourdon pressure gauge, wherein the Messgliedauslenkung or rotational movement of the link shaft by means of a Hall sensor in an analog (4-20 mA ) signal or a digital measurement signal is converted.

Differential pressure measurement

Is a measurement of the difference between two pressure levels can be determined, then the differential pressure measurement is used. The differential via two measuring diaphragms two pressures on the front and back of the sensor. The inputs usually referred to with PLUS and MINUS or HI and LO. The output signal of the transmitter thus represents the difference of the pressures. A differential can therefore also be used as always Relativdruckmessumformer when the minus side to the atmosphere remains open. SMART and field bus transmitter usually also provide the possibility of symmetric measurement ( for example, -20 kPa .. 20 kPa).

Absolute pressure measurement

When the absolute pressure measurement, the measured value refers to the absolute vacuum. An example is the barometer that indicates the air pressure. If a technical measure designed as absolute pressure measurement, it should be noted that the measured value with change in air pressure also changed. This is taken into account in the evaluation of the measured value. Therefore, absolute pressure measurements are usually used when the measured value is to describe the quality of a vacuum. Technically, an Absolute pressure transmitter with a special sensor in a similar design as a Relativdruckmessumformer be executed. Alternatively, one can at a differential evacuated the minus side and sealed.

Applications of pressure transmitters

Pressure transmitter can find the pure pressure measurement, even use in the measurement of other physical quantities. Particularly common pressure transducer for the measurement of level, density and flow are used.

Level measurement

For level measurement is utilized that a column of liquid exerts a hydrostatic pressure which is proportional to its height. So that the measured value of the bottom of the container mounted Relativdruckmessumformers can be used as a measure for the filling level. The pressure p is calculated using the following formula from the height h:

Requirement for correct level measurement is therefore next to the acceleration due to gravity g, the knowledge of the density ρ of the medium.

If it is a closed container which has to be considered that the tank internal pressure acts messwertverfälschend. This manifests itself by the additional static pressure felt:

In order to perform a correct level measurement in this case, one uses a differential. The minus side is connected to the tank top. now acts on both the positive and on the negative side and will fall out in the differential measurement.

Despite its drawbacks, the hydrostatic level measurement is the measurement most commonly used method because it is inexpensive and reliable. Alternatives include level measurement using ultrasonic or radar signals.

Density measurement

A variety of level measurement using differential pressure is the density measurement. The formula is solved for ρ. h must then be specified as a constant.

This is done by attaching the process connections plus and minus in a defined height difference on the container. With the proviso that the minimum filling level of the container is always above said upper connection piece, the output signal of the differential pressure is then proportional to the change in density of the process fluid.

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