Home > An Introduction to Presure Sensors
An Introduction to Presure Sensors
The technologies of pressure sensors
1.1 Sensors with resistor bridge
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The deformation caused by the pressure is measured using resistive elements. The
variation of the value of the resistance is proportional to the pressure.
The supply voltage or current will be applied between ± IN; the output voltage
between ±OUT and is proportional to the applied pressure.
Without pressure, the resistors are identical and the output is 0.
Applying a pressure, the four bridge-resistors will change their
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value: the arrow upwards indicates an increasing of the resistor-value, the arrow
downwards a decreasing.
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1.1.1 Glued strain-gauges
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The resistors are realized with a metal foil applied on a plastic substrate (similar
to a flex print). These elements are glued on a metal-diaphragm. This is the oldest
pressure sensor technology and there are only few companies which produces sensors
in this way. It gives very low output signals and is not suitable for pressure below
1 bar.
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1.1.2 Thick-film sensors
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The resistors are applied on a ceramic diaphragm using the same technology as for
the hybrid circuits. They have very low output signals; for pressures lower than
1 bar the diameter of the sensor increases to 30 to 40 mm. This technology is suitable
for high volume / low cost production.
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1.1.3 Thin-film sensors
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A insulating film is applied to a metal diaphragm; after that operation, the resistors
are applied on the insulating film using the same technology. These sensors have
a low output signal and the limit of the low pressure ranges is 1 .. 2 bars. However
they are very suitable (and popular) in the high pressure ranges, especially in
hydraulic applications.
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1.1.4 Piezoresistive silicon sensors
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The resistors are placed on a silicon chip using standard semiconductor technology.
The diaphragm, also on the silicon chip , is realized by etching the back side.
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For absolute pressure sensor: the etched silicon chip is bonded to a glass plate
under vacuum; the pressure behind the diaphragm is 0 (vacuum)
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For gauge pressure sensor: the back side of the silicon diaphragm is exposed to
ambient pressure.
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Silicon sensors have high output signals and are suitable for low pressures down
to 20 mbars. This technology also allows for high volume production. In harsh environment
they need to be protected against the surrounding media.
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1.2 Capacitive pressure sensors
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The deformation caused by the pressure is measured using a capacitive element. The
pressure P will cause a variation of the distance between the two electrodes.
The variation of the distance H between the two electrodes will change the capacity
of the system.
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Capacitive pressure sensors can be realized in different technologies: ceramic,
silicon, etc.
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1.3 Other measurement principles and technologies
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There are some other principles and technologies for the pressure measurement: with
Hall element, Inductive sensors, etc.
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The encapsulated, oil filled pressure transducer
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To be compatible with a harsh industrial environment, the piezoresistive sensor
has to be protected by a stainless steel housing. The chip is glued into the housing
and bonded to the pins. The housing is closed by a stainless steel diaphragm and
oil filled. There are other housing-materials like titanium or Hastelloy.
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Gauge pressure transducer
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This assemby allows applications in all media which are compatible with the housing
material.
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Specification
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A pressure transmitter is a pressure sensor with compensation, and an amplifier
with a standard output, built in a housing with a process and an electrical connection.
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3.1 Pressure range
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The pressure range defines the pressure where the sensor fullfills its specifications.
The limits of the pressure range are fixed by the initial and the final values (for
example 0 .. 3 bar gauge, -1 .. 1 bar gauge, 800 ... 1200 mbar absolute).
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3.2 Overpressure
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The overpressure is the maximum pressure where the sensor will still operate within
specification without damage.
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3.3 Burst pressure
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The burst pressure is the maximum pressure where the sensor doesn‘t fail (where
no process-media can leak through the sensor.
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3.4 Compensated temperature range
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The compensated temperature range defines the temperature where the sensor fullfills
his specifications.
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3.5 Sensitivity
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The sensitivity is the relationship between the output signal and the pressure changes.
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3.6 Accuracy
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The accuracy of a pressure sensor is the sum of the linearity, the repeatability
and the hysteresis errors. It will be indicated in % FS (Full Scale). This indication
is valid for a constant temperature; the temperature errors are specified separately.
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Linearity error:
the maximum variation of the output signal from a reference line.
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Repeatability: Variation between output values measured at identical
conditions. |
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Hysteresis:
The maximum signal-difference between 2 measurements at the same pressure performed
at increasing and at decreasing pressure.
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3.7 Offset Thermal Shift
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The offset thermal shift is the variation of the sensor- signal in dependence of
the temperature without pressure. The value specifies the maximum signal variation
related to the temperature. The units are %FS/°C
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3.8 Sensitivity thermal shift
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The sensitivity thermal-shift is the variation of the sensitivity in dependence
of the temperature. The value specifies the maximum sensitivity variation related
to the temperature. The units are %/°C.
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3.9 Long term stability
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The long term stability is the capability of the instrument to maintain the same
behaviour over time. The parameters which can change over time are offset and sensitivity.
The units are %FS/year.
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3.10 Measurements to qualify the PTX pressure
sensors
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All sensors are measured and qualified individually. Each sensor has to be subjectetd
to a temperature cycle to characterize the following parameters:
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Offset-compensation resistor |
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Offset thermal shift compensation resistor |
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Sensitivity thermal shift compensation resistor |
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Thermal shift of the compensated sensor |
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Stability |
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Temperature hysteresis |
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Accuracy |
The full test-cycle will be, at least,about 20 hours long.
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Applications |
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Industrial plants: |
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Cooling systems (for example in nuclear reactors) |
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Thermal power-plants (steam pressure, turbine control ... )
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Steel production plants |
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Paper production plants |
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Chemical process-control |
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Gas plants (oxygen) |
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Cooling machines |
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Vacuum applications (!) |
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Pipelines |
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Ships (ballast tanks, Level control, Motor control ...) |
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Barometer |
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Bioreactor |
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Density measurement (pressure and temperature)
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Submersible level transmitter PTX22 and PTX23
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In many level measurement applications it is suitable to install a submersible transmitter.
These instruments are completely immersed in the process medium in depths ranging
from 0.5 m to some 100 m.
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These instruments are normally equipped with a cable, in some cases with a (special)
connector. These transmitters, being continously submersed, have to be perfectly
tight during their complete lifetime.
To avoid corrosion, the fluid has to be compatible to the transmitter-housing material.
The cable material and the cable seal has to be choosen following their medium compatibility.
The temperature range is limited to max. temperatures of 50°C: higher temperatures
will change the cable seal, causing the failure of the instrument. For higher temperatures,
we need special solutions.
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Applications
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Groundwater level measurement in boreholes / pump stations |
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Level measurement in lakes and rivers
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Level measurement in dams |
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Level measurement in waste water plants |
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Level measurement in vessels and tanks in industrial applications
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Level measurement in tanks with gasoil |
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Level measurement in ballast tanks (ships)
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