Electronic Pressure Measurement – Measuring Principles and Pressure Measuring Instruments
Instrument types at a glance
Common types of pressure measurement instruments include pressure transmitters, level transmitters,
pressure switches and process transmitters. These electronic pressure measuring instruments consist of
a pressure connection, a pressure sensor, electronics, an electrical connection and the case.
In addition, there are also instruments referred to as pressure sensor modules, often consisting of a bare
pressure sensor and simple mechanical and electrical interfaces. These modules are particularly suitable
for integration into users’ systems for custom applications.
Pressure transmitter
A pressure transmitter has standardized connections on the process and electrical output signal side,
and converts the applied physical pressure to a standard industrial signal. The pressure connection is
used to direct the pressure to the sensor. It usually has an industry standard thread and an integrated
sealing system to enable easy connection of the pressure transmitter by installing it at the required
measurement point. A case protects the sensor and the electronics against environmental influences.
The electronics transform a low level sensor signal into a standardized and temperature-compensated
signal; for example the common industrial output signal of 4 … 20 mA. The output signal is transmitted
by way of a standard electrical connector or cable for subsequent signal evaluation.
Level transmitter
The level transmitter, also referred to as a submersible transmitter, is a special type of pressure
transmitter used for level measurements in tanks, wells, shafts, and bore holes. For this application the
level transmitter measures the hydrostatic pressure at the bottom of the vessel or well. The choice of
material for the case, cable and the seals at connection points are extremely important due to their
complete and permanent submersion into the medium. Venting of the sensor system is required for
gauge pressure measurement and is achieved through a hollow ventilation tube integral to the cable.
Pressure switch
In many applications electronic pressure switches replace mechanical pressure switches because they
offer additional functions such as digital display, adjustable switch points and considerably higher
reliability. They are frequently used in machine building.
Electronic pressure switches are based on electronic pressure transmitters and offer the same
functionality as a pressure transmitter. The integrated electronic switches can perform simple control
tasks by closing or opening an electrical circuit. The switch points and the reset points can be
programmed individually.
By default, a pressure switch only outputs binary signals such as switch point or reset point “reached” or
“not reached”, but it does not output how far the measured pressure is from the switch or reset point.
That is why many pressure switches have a display and an analog output signal. The programming
parameters and applied pressure can be read from the display. In addition, the measured pressure can
be transmitted by the analog output signal to a downstream control unit. This widely adopted type of
electronic pressure switch includes switches, a pressure transmitter and a digital indicator − all in one
instrument.
Process transmitter
The process transmitter is a pressure transmitter with a pressure range that can be set within a
predefined pressure range (turndown). It is normally used in process engineering, since in this
application it is necessary to adjust every single measuring point to many specific requirements that
must be individually set by the operator on site. Process transmitters have high measurement accuracy
within the entire pressure range. In addition, the pressure range, the zero point and additional
parameters can usually be set individually. For this purpose many process transmitters have both
digital display and additional operating elements and extensive operating software built into the
instrument.
Pressure transducer
Suppliers of pressure transducers usually offer a large number of sensor module configurations that can
be matched to specific user requirements. They have, for example, user-specific pressure connections
and/or a user-specific electric interface. Very few manufacturers of electronic pressure measurement
technology even offer the so-called “bare” pressure sensor as a module. For these, the users must
develop their own design solutions in order to port the pressure to the sensor and evaluate the sensor
signal.
The correct function of pressure transducers must be proven by the user design. Therefore, this option
is usually only suitable for engineered mass-produced equipment.