THE PRESSURE SENSORWith the steam age came the demand for pressure measuring
instruments. Bourdon tubes or bellows, where mechanical
displacements were transferred to an indicating pointer were
the first pressure instruments, and are still inuse today.Pressure metrology is the technology of transducing pressure
into an electrical quantity. Normally, a diaphragm construction
is used with strain gauges either bonded to , or diffused into it,
acting as resistive elements. Under the pressure-induced strain,
the resistive values change.In capacitive technology, the pressure diaphragm is one plate
of a capacitor that changes its value under pressure-induced
displacement.
Pressure sensing using diaphragm technology measures the
difference in pressure of the two sides of the diaphragm.
Depending upon the relevant pressure, we use the terms
ABSOLUTE, where the reference is vacuum (1st picture),
GAUGE, where the reference is atmospheric pressure
(2nd picture), or DIFFERENTIAL, where the sensor has two
ports for the measure of two different pressure.
The piezoresistive pressure sensor, or silicon cell.
This type of pressure sensor consists of a micro-machined
silicon diaphragm with piezoresistive strain gauges diffused
into it, fused to a silicon or glass backplate.The resistors have a value of approx. 3.5 kOhm. Pressure
induced strain increases the value of the radial resistors (r),
and decreases the value of the resistors (t) transverse to
the radius. This resistance change can be high as 30%.
The resistors are connected as a Wheatstone Bridge, the
output of which is directly proportional to the pressure.
Whetstone Bridge Circuit
Leadouts from the bridge.
1). Gold or aluminium wires are welded to the aluminium contacts
on the chip and to the glass feed-through, pins of the header.2). TAB (Tape Automated Bonding). The contacts on the chip
have a gold dot.A pretinned felxible printed circuit is directly soldered to these
gold dots and the other end to a PC-board, or the header.In the first method, the sensor must be fixed on the header. The
TAB printed circuit, however, holds the sensor in place itself.
Low cost sensors.
Low cost sensors are devices where the they are exposed to
the media without protection.The glass feed-through and the silicon cell is mounted in a
plastic housing with pressure ports for positive and negative
pressure. (1st picture) The silicon sensor with the TAB print
is fixed between two plastic mouldings with pressure ports.
(2nd picture)The silicon sensor is bonded to a brass pressure port. The
contacts are made either by gold wires to soldering pins, or
by TAB flexible printed circuit. (3rd picture)Piezoresistive OEM Pressure Transducer.
The silicon sensor on the glass feed-through is mounted in
a stainless steel housing, isolated by a thin stainless steel
diaphragm and filled with silcone oil. The pressure acts on
the diaphragm and is transfered through the oil onto the
sensor. These transducers are fully tested for temperature
and linearity and the compensation resistor values given on
the individual test sheets.
Low cost Pressure Transducers.
Nickel diaphragms in brass housings brazed under high
temperature or brazed steel diaphragms in steel housings
allow the fabrication of isolated pressure sensors with low
production costs, without substantially limiting the area
of application.Pressure Transducers.
Pressure transducers are pressure measuring instruments,
ready to use. It is an OEM transducer with pressure port,
integrated compensation resistors and a cable or connector.Transducers give an unamplified signal into a separate
instrumentation amplifier or indicator. They can be
considered as passive bridges, being interchangeable
between different manufacturers.
Pressure Transmitters.
In pressure transmitters, the full signal conditioning circuitry
is integrated in the housing. The sensor signal is conditioned
into standard output signals of 0...100mV, 0...10V, 0.5...4.5V,
and 4-20mA.
Normally, the signal is independent from the excitation
(i.e. 8...28V), but in ratiometric transmitters, the signal is
proportional to the excitation.
The accuracy of a transmitter is best described by an error band.
This band covers all errors over the full pressure and temperature
range. Typical errors are also given. The typical error describes
the accuracy which can normally be expected in a measurement.
Above information kindly supplied by Keller UK Ltd.
Visit their website at www.keller-druck.comTo learn of THE HISTORY OF PRESSURE MEASUREMENT, Click here...
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