Pressure transmitter

A pressure transmitter (engl. Pressure transducer ) is an electrical transducer for measuring pressure (gauge pressure, absolute pressure or differential pressure). For this purpose, the pressure to be measured is converted into the mechanical deflection of a measuring membrane, which can then be recorded and processed electrically. Depending on the manufacturer, the following physical effects are mainly used as the measuring principle:

Change in resistance
- Strain gauges
- Piezoresistive effect
Voltage change
- Piezoelectric effect
Change in capacitance
Change in inductance
Hall effect

If pressure transmitters are used to measure aggressive and / or explosive process media in liquid or gaseous form, suitable measures are required to protect the transmitter against the operating conditions. It must also be ensured that the transmitter does not pose a risk to the process environment. A large number of product approvals are therefore available (e.g. certificates for explosion protection or use in oxygen applications).

Executions of

Basically, depending on the scope of services, a distinction can be made between several transmitter classes:

Pressure transmitter with analog output

Single range transmitter
Transmitter with adjustable measuring range
communication-capable transmitter with adjustable measuring range (SMART transmitter)
- Special case: HART transmitter

Fieldbus transmitter

These deliver a digital output signal on a fieldbus .

Print types

Relative pressure measurement

The relative pressure measurement refers to the atmospheric pressure. An example of this is the pressure measurement on a steam boiler. The pressure gauge indicates 0 bar for the cooled and vented boiler. If the boiler is heated, an overpressure builds up, which can be measured by a relative pressure transducer. In the past, the unit atü ( overpressure atmosphere ) was also used. Today the relative pressure is usually given in kPa, MPa or bar. Hybrid devices use a normal Bourdon manometer, whereby the measuring element deflection or the rotary movement of the pointer axis is converted into an analog ( 4-20 mA signal) or digital measuring signal by means of a Hall sensor .

Differential pressure measurement

If the difference between two pressure levels is to be determined in one measurement, differential pressure measurement is used. The differential pressure transmitter carries two pressures on the front and back of the sensor via two measuring diaphragms. The inputs are usually referred to as PLUS and MINUS or HI and LO. The output signal of the transmitter thus represents the difference between the pressures. A differential pressure transmitter can therefore always be used as a relative pressure transmitter if the negative side remains open to the atmosphere. SMART and fieldbus transmitters also usually offer the option of symmetrical measurement (for example from −20 kPa .. 20 kPa).

Absolute pressure measurement

In the case of absolute pressure measurement, the measured value relates to the absolute vacuum. The barometer, which shows the air pressure, serves as an example. If a technical measurement is carried out as an absolute pressure measurement, it should be noted that the measured value also changes when the air pressure changes. This must be taken into account when evaluating the measured value. Therefore, absolute pressure measurements are mostly used when the measured value is to describe the quality of a vacuum. Technically, an absolute pressure transmitter can be designed with a special sensor in a similar design to a relative pressure transmitter. Alternatively, the minus side of a differential pressure transmitter can be evacuated and closed.

Pressure transmitter applications

In addition to pure pressure measurement, pressure transmitters are also used to measure other physical quantities. Pressure transmitters are used particularly frequently to measure level, density and flow.

Level measurement

The level measurement takes advantage of the fact that a column of liquid exerts a hydrostatic pressure that is proportional to its height. This means that the measured value of a relative pressure transmitter mounted on the bottom of the container can be used as a measure for the fill level. The pressure p is calculated from the height h using the following formula:

       $p=\rho *g*h\,$

In addition to the acceleration due to gravity g, a prerequisite for correct level measurement is knowledge of the density ρ of the medium.

If the container is closed, it must be taken into account that the internal pressure in the container will falsify the measured values. This is noticeable through the additional static pressure : ${\ displaystyle p_ {i} \,}$

       $p=\rho *g*h+p_{i}\,$

In order to be able to carry out a correct level measurement in this case too, a differential pressure transmitter is used. The minus side is connected to the top of the container. now acts on both the plus and the minus side and falls out of the differential measurement. ${\ displaystyle p_ {i} \,}$

Despite its disadvantages, hydrostatic level measurement is the most widely used measurement method because it is inexpensive and reliable. Alternatives are, for example, level measurement using ultrasonic or radar signals.

Density measurement

Density measurement is a variant of level measurement using a differential pressure transmitter. The formula is solved for ρ. h must now be specified as a constant.

       $\rho ={\frac {p}{g*h}}\,$

This is done by attaching the process connections plus and minus at a defined height difference on the container. Assuming that the minimum filling level of the tank is always above the upper connection, the output signal of the differential pressure transmitter is now proportional to the change in density of the process medium.

literature

Günther Pfeifer, Roland Werthschützky: Pressure sensors. VEB Verlag Technik, Berlin 1989, ISBN 3-341-00660-5