Pressure and level measurement
measurement. The influence of temperature, especially its influence
on the density of the specific medium, must always be incorporated
into the calculation of the level for a correct level measurement. An
increase in the process temperature may lead to a lower density of the
medium and a correspondingly increasing level, however, not always
in the same proportion as the increase in the hydrostatic pressure.
This leads to an inaccuracy in the calculation, eg to a lower reading of
the level. Therefore, hydrostatic level measurement is primarily used
in applications which operate within known process limits or with a
known medium density. Should the process have a strongly chang-
ing or an unknown density, this would normally be compensated for
through additional sensors. Therefore, various pressure sensors, with
additional integrated temperature sensors, are available which enable
the measurement of the medium temperature for density compensa-
tion. The medium and its characteristics, in particular its viscosity and
solids content, will decide between using a pressure sensor with a
traditional design with pressure port or one with a flush diaphragm.
A pressure sensor with a pressure port (Figure 4) should always
be used when the medium has a low viscosity and is free from
coarse-grained contaminants. However, if a medium has a tendency
to build-up, is highly viscous or contains many particulates, then
one should select a sensor with a flush diaphragm (Figure 5). In
contrast to a flush diaphragm sensor, a sensor with a pressure port
can become blocked or the media in it can harden or crystallise. Any
blockage in the pressure port slows down the measurement, or, in the
most extreme case, completely prevents a correct pressure measure-
ment. If one pays attention to the characteristics of the medium to be
measured when choosing a conventional pressure transmitter, then
hydrostatic pressure measurement can be used reliably even under
the harshest of conditions. Submersible pressure transmitters (Figure
7), as a specific design variant of a pressure transmitter, are used in
contaminated media, such as wastewater, as well as in clean media,
such as fuel or groundwater. For this, both flush product variants as
well as variants with large, widened pressure ports are used, in order
to ensure a high reliability of level measurement in the submersed
application.
In the differential pressure measurement with process transmit-
ters (Figure 9) the mounting position is a common source of inaccura-
cies in the level measurement. The measuring points of the medium
and the gas phase are typically connected to the differential measuring
cell of the transmitter by oil-filled capillaries (in Figure 9, indicated in
blue and red respectively). The height difference of these measuring
points to the differential pressure transmitter leads to an additional
hydrostatic pressure within the capillaries (Figure 10) themselves.
This effect generates an additional over- or under-pressure in the
hydrostatic pressuremeasurement, ultimately distorting themeasure-
ment. The resulting inaccuracy of themeasurement must be corrected
within installation by a position correction and proper configuration
of the differential pressure transmitter, so that a fully automatic
compensation of this disturbance factor can be made. It is further on
strongly recommended to position the transmitter below the height
of the level measurement point in order to eliminate any negative
hydrostatic pressure or under-pressure from the level measurement.
Figure 9: Hydrostatic level measurement using differential pressure sensors.
What is the future of hydrostatics?
Through the wide spread utilisation of industrial pressure sensors and
due to the annual manufacturing of millions of hydrostatic sensors,
hydrostatic pressure sensors have achieved a significant price advan-
tage over many other level measurement methods. The utilisation of
pressure sensors for level measurement will continue to grow above
all in general applications without any special requirements on the
measurement technology. Consequently, in the future, hydrostatic
level measurement will continue to show a growing market share
against many other level measurement principles, and will enable
cost-effective level measurement in many new applications.
Trends
Alternative materials
In recent years one has been able to observe the trend for hydrostatic
level measurement technology to replace alternative measurement
principles, and to be used more and more in many process meas-
urement applications. Thus, a clear trend has been recognisable
towards the measurement of aggressive media (eg acids and bases)
which go beyond the usual application conditions for the operation
of conventional pressure sensors in machine building. These media,
often present in process industries, are being met by pressure sen-
sor manufacturers with a clear adjustment in their product design.
Thus, more and more, one finds the use of alternative materials and
coatings for conventional pressure sensors and their wetted parts.
Titanium, gold, ceramic, Teflon and many other materials are already
available on themarket and are commonmaterial options for pressure
sensors - and their importance will increase in the future.
Hygienic design
The key phrase ‘hygienic design’, originally derived from the phar-
maceutical and food and beverage industries, is also finding ever-
Electricity+Control
June ‘13
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