Pressure and level measurement
ure 6) are particularly suitable, either with a conventional pressure
port (Figure 4) or a flush diaphragm design (Figure 5). Conventional
pressure transmitters are the most commonly employed, due to their
wide use throughout various industries, mostly without the need
for special requirements on the measurement technology (such as
scalability of the measuring range or integrated tank linearisation).
Figure 4: Conventional pressure transmitter with pressure port.
Figure 5: Conventional pressure transmitter with flush diaphragm.
Figure 6: Process pressure transmitter.
Conventional sensors stand out through their excellent price and
performance ratio. They are robust, simple to install and operate,
fast, and available with a range of accuracies of up to <0,1 %. Process
pressure transmitters, however, are primarily used in applications
with special demands on the measurement technology (such as bus
signals, scalability of the measurement range, integrated tank lin-
earisation, etc) and are mainly found in applications in the chemical
and petrochemical industries. The extensive adjustability and high
intelligence of these programmable process pressure transmitters is
also reflected in their price level in relation to conventional industrial
transmitters, easily five to ten times as expensive as a conventional
pressure sensor. Specifically in the water and wastewater industry,
submersible pressure transmitters (level probes or submersible
probes) are frequently used to measure the level in reservoirs, wells
or other open bodies of water (Figure 7).
Figure 7: Submersible pressure transmitter.
Submersible pressure transmitters are specifically designed to oper-
ate while continuously submerged in liquids. They mainly differ from
conventional pressure sensors in their media resistance, pressure
tightness, cable quality and ingress protection. The design of differ-
ential pressure transmitters (Figure 8) is considered as state-of-the-art
technology within the chemical and petrochemical industry.
Figure 8: Differential pressure transmitter.
Differential pressure transmitters offer the ability to measure and
eliminate the effect of the gas phase of in a gas-tight sealed tank, and
correctly display the level of the liquid phase within the tank. The op-
erator therefore has a hydrostatic level measurement which displays
the correct liquid height without the need of any additional compensa-
tion or additional sensors. The sophistication of this measurement
technique is however reflected in the costs of both the instrument
itself as well as the corresponding installation.
Electricity+Control
June ‘13
52