Fredrik Arrhén

In this paper, a development of the shock tube at RISE, the National Metrology Institute of Sweden, to extend its capability to the high-pressure regime is presented. The shock tube was developed to be operated in three different configurations: conventional, with an amplification system and with a converging cone. In the conventional and with the amplification system, the well-established shock tube analytical solution was used to calculate the reference pressure, while in the converging cone, a numerical simulation was applied. To demonstrate the capabilities and limitations of each configuration, a device under test (DUT) was characterized. The results show a good agreement in the DUT dynamic response calculated using the three configurations in the overlap regions between them. The uncertainty in measurements was estimated for each configuration. The three configurations complement each other to reach a pressure range from 0.1 MPa to 25 MPa and a frequency range from 0.5 kHz to 500 kHz.

The design, performance characteristics and validation of a next generation reference facility for carbon dioxide (CO₂) in air standards based on manometry is presented. Novel attributes of the facility include automated operation, avoidance of significant pressure corrections for measurements on extracted CO₂, fully characterized trapping efficiencies, and reduced measurement uncertainty. Improvements in system performance have been achieved using specific materials, notably Silconert^®-treated stainless-steel, providing increased mechanical stability whilst minimizing carbon dioxide adsorption on surfaces, and avoiding use of elastomer O-rings, which would lead to losses of CO₂. Full automation of the cryogenic extraction process of CO₂ from air has been achieved, avoiding any manual intervention within the temperature stabilized section of the facility, and allowed full characterization and correction for trapping efficiencies and trace water measurement. The facility has been validated across the CO₂ in air amount fraction range of (380–800) µmol mol⁻¹ using standards with values traceable to the reference value of the CCQM−K120 (2018) comparison. It was demonstrated to operate with a standard measurement uncertainty of 0.09 µmol mol⁻¹ at 400 µmol mol⁻¹. The automation allows five measurement results per day to be produced with a typical standard deviation of the mean at or below 0.02 µmol mol⁻¹. The facility will be used as a stable reference in the future BIPM.QM−K2 ongoing comparison, to compare consistency of amount fraction values in different CO₂ in air standards. The CO₂ amount fraction in two ensembles of nine BIPM standards covering the same range will also be measured with the facility to provide their SI-traceable values, and further monitored on a longer time scale. Each ensemble will constitute a CO₂ in air scale to be compared with other available scales such as WMO.CO2.X2019 through the BIPM.QM−K5 comparison.

In this paper, we introduce a robust method for dynamic characterization of pressure measuring systems used in time-varying pressure applications. The dynamic response of the pressure measuring systems in terms of sensitivity and phase as a function of frequency at various amplitudes of the measurand can be provided. The shock tube which is the candidate primary standard for dynamic pressure calibration at the National Laboratory for pressure, Sweden, was used to realize the dynamic pressure. The shock tube setup used in this study can realize reference pressure with amplitudes up to 1.7 MPa in the frequency range from below a kilohertz up to a megahertz. The amplitude of the realized step pressure was calculated using the Rankine–Hugoniot step relations. In addition, the accurate time of arrival of the generated shock at the device under test (DUT) was measured using an optical probe based on shadowgraphy. The optical detector has a response time in nanosecond time scale which is several orders of magnitude faster than the response time of any pressure measuring system. Hereby, the latency between physical stimuli and response of the DUT can be measured. By the knowledge of the amplitude and the accurate time of arrival of the reference step pressure, the transfer function of the DUT can be calculated and presented in Bode diagrams of sensitivity and phase response versus frequency. The uncertainty in sensitivity and phase measurements was estimated. The information provided by this work is useful for developing reliable models of dynamic pressure measuring system and provide accurate information about their dynamic response. That in turn will contribute to establish a traceability chain for dynamic pressure calibration.

Accurate dynamic pressure measurements are increasingly important. While traceability is lacking, several National Metrology Institutes (NMIs) and calibration laboratories are currently establishing calibration capacities. Shock tubes generating pressure steps with rise times below 1 μs are highly suitable as standards for dynamic pressures in gas. In this work, we present the results from applying a fast-opening valve (FOV) to a shock tube designed for dynamic pressure measurements. We compare the performance of the shock tube when operated with conventional single and double diaphragms and when operated using an FOV. Different aspects are addressed: shock-wave formation, repeatability in amplitude of the realized pressure steps, the assessment of the required driver pressure for realizing nominal pressure steps, and economy. The results show that using the FOV has many advantages compared to the diaphragm: better repeatability, eight times faster to operate, and enables automation of the test sequences.