Page 152 - Instrumentation Reference Book 3E

136 Measurement of pressure

diaphragm chamber to isolate the force-summing The use of digital pressure transducers has
diaphragm from the mounting and other environ- grown dramatically with the trend towards digital
mental stresses. data-acquisition and control systems. Inherently
The transducer is modular in construction to digital sensors such as frequency output devices
allow the use of alternative diaphragm configur- have been combined with microprocessor based
ations and materials. For most applications the systems to provide unparalleled accuracy and
diaphragm is stainless steel and the thickness is performance, even under extreme environmental
selected according to the required measurement conditions. The design and performance require-
range. For some applications, enhanced corro- ments of these advanced transducers include:
sion resistance is required, in which case Inconel
625 or other similar alloys may be used as the Digital-type output
diaphragm material, but to retain the same mar- Accuracy comparable to primary standards
gin of safety a thicker member is usually Highly reliable and simple design
required and this in turn reduces the sensitivity. Insensitivity to environmental factors
The sensor is a sputtered thin-film strain gauge Minimum size, weight, and power consump-
in which the strain-gauge pattern is bonded into tion
the structure of the sensor assembly on a mole- Ease and utility of readout.
cular basis and the sensor assembly itself is Over the last three decades, Paroscientific Inc.
welded into the remaining structure of the trans- has developed and produced digital quartz crystal
ducer. The stainless-steel header which contains pressure sensors featuring resolution better than
the electrical feed-through to the temperature- one part per million and accuracy better than 0.01
compensation compartment is also welded into percent. This remarkable performance is
the structure of the transducer. achieved through the use of a precision quartz
This welding, in conjunction with the ceramic crystal resonator whose frequency of oscillation
firing technique used for the electrical feed- varies with pressure-induced stress. Quartz crys-
through connections, provides secondary con- tals were chosen for the sensing elements because
tainment security of 50 MPa for absolute gauges of their remarkable repeatability, low hysteresis,
and those with a sealed reference chamber. and excellent stability. The resonant frequency
Sensors of this type are available with ranges outputs are maintained and detected with oscilla-
from 0 to lOOkPa up to 0 to 60MPa with max- tor electronics similar to those used in precision
imum non-linearity and hysteresis of 0.25 to 0.15 clocks and counters.
percent respectively and a repeatability of 0.05 Several single or dual beam load-sensitive reson-
percent of span. ators have been developed. The single-beam reso-
The maximum temperature effect is 0.15 percent nator is shown diagrammatically in Figure 9.29. It
of span per Kelvin. depends for its operation on a fixed beam oscil-
lating in its first flexural mode with an integral
9.2.4.3 High accuracy digital quartz crystal isolation system that effectively decouples it from
pressure sensors the structures to which it is attached. The entire
sensor is fabricated from a single piece of quartz
Performance and utilization factors may be used to to minimize energy loss to the mounting surfaces.
differentiate between digital and analog instrumen- The beam is driven piezoelectrically to achieve
tation requirements. Performance considerations and maintain beam oscillations. Figure 9.29
include resolution, accuracy, and susceptibility to shows the placement of electrodes on the beam
environmental errors. Utilization factors include and Figure 9.30 illustrates the response to the
ease of measurement, signal transmission, equip- imposed electric field from the oscillator electron-
ment interfaces, and physical characteristics. ics. The Double-Ended Tuning Fork (DETF)
Some advantages of digital sensors relate to the
precision with which measurements can be made
in the time domain. Frequencies can be routinely
generated and measured to a part in ten billion,
whereas analog voltages and resistances are com-
monly measured to a part per million. Thus, digital- Crystal Electrode
type transducers have a huge inherent advantage Charge
in resolution and accuracy compared to analog
sensors.
Frequency signals are less susceptible to inter-
ference. easier to transmit over long distances,
and easily interfaced to counter-timers, telemetry Figure 9.29 Resonant piezoelectric force sensor
and digital computer systems. Courtesy, Paroscientific Inc.