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Paroscientific
manufactures and sells a complete line of inherently-digital,
high precision pressure instrumentation. Resolution of better
than 0.0001% and typical accuracy of 0.01% are achieved even
under difficult environmental conditions. Other desirable
characteristics include low power consumption, high reliability,
and excellent long-term stability. Over 30 full scale pressure
ranges are available - from a fraction of an atmosphere to
thousands of atmospheres (3 psid to 40,000 psia).
The remarkable performance is
achieved through the use of a precision quartz crystal resonator
whose frequency of oscillation varies with pressure induced
stress. Quartz crystals were chosen for the sensing elements
because of their remarkable repeatability, low hysteresis, and
excellent stability. The resonant frequency outputs are
maintained and detected with oscillator electronics similar to
those used in precision clocks and counters.
Several flexurally-vibrating,
single or dual beam, load-sensitive resonators have been
developed. The Double-Ended Tuning Fork consists of two
identical beams driven piezoelectrically in 180° phase
opposition such that very little energy is transmitted to the
mounting pads. The high Q resonant frequency, like that of a
violin string, is a function of the applied load; increasing
with tension and decreasing with compressive forces. The digital
temperature sensor consists of piezoelectrically-driven,
torsionally oscillating tines whose resonant frequency is a
function of temperature. Its output is used to thermally
compensate the calculated pressure and achieve high accuracy
over a wide range of temperatures. |
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Pressure transducer
mechanisms employ bellows or Bourdon tubes as the
pressure-to-load generators. Pressure acts on the effective area
of the bellows to generate a force and torque about the pivot
and compressively stress the resonator. The change in frequency
of the quartz crystal oscillator is a measure of the applied
pressure. Similarly, pressure applied to the Bourdon tube
generates an uncoiling force which applies tension to the quartz
crystal to increase its resonant frequency. Temperature
sensitive crystals are used for thermal compensation. The
mechanisms are acceleration compensated with balance weights to
reduce the effects of shock and vibration. The transducers are
hermetically sealed and evacuated to eliminate air damping and
maximize the Q of the resonators. The internal vacuum also
serves as an excellent reference for the absolute pressure
transducer configurations.
Microprocessor-based intelligent
electronics are available with counter-timer circuitry to
measure transducer frequency or period outputs, storage of the
linearization and thermal compensation algorithm, calibration
coefficients, and command and control software to process the
outputs in a variety of digital formats. |
