Dr Barbara Brooks
Instrument Support Level 3
Paroscientific, Inc. Model 745
High Accuracy Pressure Standard
The Model 745 Portable Transfer Standards provide the highest accuracy pressure measurements for laboratory and metrological applications.
Digiquartz® Transducers cover 26 absolute and gauge pressure ranges up to 40,000 psi (276 MPa) with unsurpassed accuracy, reliability, and stability. The simple, intuitive front-panel user interface and versatile functionality with the rear panel RS-232 computer interface make the Model 745 the best choice for high-performance pressure calibrations.
- 0.008% Full-Scale Accuracy
- Resolution better than one part-per-million
- 20 Absolute Pressure Ranges from:
- 0-15 psia (0.10 MPa) to 0-40,000 psia (276 MPa)
- 6 Gauge Pressure Ranges from:
- 0-15 psig (0.10 MPa) to 0-200 psig (1.38 MPa)
Resolution of up to one part-per-billion of full scale and typical accuracy of 0.01% of full scale are achieved even under difficult environmental conditions.
Over 30 full-scale pressure ranges are available – from a fraction of an atmosphere to thousands of atmospheres (+/-2 psig to 40,000 psia).
The superior performance of the Digiquartz instruments 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
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 180deg phase opposition such that very little energy is transmitted to the mounting pads. The high-Q resonant frequency, like tuning 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.
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 that applies tension to the quartz crystal.
Temperature sensitive crystals are used for thermal compensation. The mechanisms are acceleration compensated with balance weights to reduce the effects of orientation sensitivity. 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 highly stable TCXO counter timer circuitry to measure transducer frequency or period outputs and to store the
linearization and thermal compensation algorithm, calibration coefficients, and command and control software that process the outputs in a variety of digital formats.
Data from instruments that are part of a laboratory do not archive their data to CEDA and so do not have to conform to any AMOF standard.
Data can be provided for the period of time that the user has accessed the facility.