Tokihiko Kobata

Final Report on Key Comparison CCM.P-K7 in the range 10 MPa to 100 MPa of hydraulic gauge pressure

This report describes a CCM key comparison of hydraulic pressure standards of nine National Metrology Institutes that was carried out in the period from November 2002 to June 2004 in order to determine their degrees of equivalence in the range 10 MPa to 100 MPa of the gauge pressure. The pilot laboratory was PTB. The primary pressure standards were pressure balances of different design equipped with piston-cylinder assemblies operated in freedeformation, controlled-clearance or re-entrant operation mode. The transfer standard was a pressure balance equipped with a piston-cylinder assembly and a mass set. The pressuredependent effective areas of the transfer standard at specified pressures were reported by the participants and led to the reference values calculated as medians. All participants’ results agree with the reference values and with each other within the expanded uncertainties calculated with a coverage factor 2, most of them even within their standard uncertainties. In addition, the results were analysed in terms of the zero pressure effective area and the pressure distortion coefficient. Also for them agreement within expanded uncertainties (k=2) is observed. The results of the comparison demonstrate equivalence of the laboratory standards and support their measurement capability statements.

Accurate determination of equilibrium state between two pressure balances using a pressure transducer

To determine an equilibrium state between two pressure balances accurately, the measurement method using a precise pressure transducer and two air-operated constant volume valves (CVV) is proposed in this paper. The advantages of the proposed method are as follows: (1) by the usage of two air-operated CVV, the pressure generated by the pressure balance can be connected and disconnected quickly to the transducer without volume change in the hydraulic circuit or heat transfer from the operator, (2) by managing the time intervals between measurements equally, the method proposed can compensate for the effect of the drift component in the successive values measured by the transducer used and (3) the short time stability of the pressure generated by each pressure balance used can be evaluated quantitatively at each pressure. From the measurement results, it was revealed that the equilibrium state could be determined accurately using the method proposed, and the differences between this method and the conventional fall-rate method were sufficiently small.

Method of evaluating frequency characteristics of pressure transducers using newly developed dynamic pressure generator

A dynamic pressure generator using a rotating valve has been developed to evaluate the dynamic characteristics and performance of pressure transducers. The waveform of the generated dynamic pressure is square, and the amplitude and fundamental frequency can be varied. In this paper, the generator using a novel rotating valve and the method of evaluating the frequency characteristics of pressure transducers using the generator are described. With the amplitude of the dynamic pressure of 10 kPa and varying the fundamental frequency in 1-Hz increments between 10 and 50 Hz, we measured the generated dynamic pressure using two different pressure transducers, and to obtain the amplitude spectrum of the measured waveform using frequency analysis. Then, the magnitude of the harmonic components in the spectrum are shown as a function of frequency. From the simultaneous outputs of two transducers, the frequency characteristics of one transducer was compared with that of the other.

Effects of pressurization procedures on calibration results for precise pressure transducers

The output of electromechanical pressure gauges depends on not only the currently applied pressure, but also the pressurization history. Thus, the calibration results of gauges are affected by the pressurization procedure. In this paper, among several important factors influencing the results, we report the effects of the interval between the calibration cycles and the effects of the preliminary pressurizations. In order to quantitatively evaluate these effects, we developed a fully automated system that uses a pressure balance to calibrate pressure gauges. Subsequently, gauges containing quartz Bourdon-type pressure transducers were calibrated in a stepwise manner for pressures between 10 MPa and 100 MPa. The typical standard deviation of the data over three cycles was reduced to a few parts per million (ppm). The interval between the calibration cycles, which ranges from zero to more than 12 h, exerts a strong influence on the results in the process of increasing the pressure, where at 10 MPa the maximum difference between the results was approximately 40 ppm. The preliminary pressurization immediately before the calibration cycle reduces the effects of the interval on the results in certain cases. However, in turn, the influence of the waiting time between the preliminary pressurization and the main calibration cycle becomes strong. In the present paper, we outline several possible measures for obtaining calibration results with high reproducibility.

Characterization of quartz Bourdon-type high-pressure transducers

The characteristics of commercially available pressure monitors were evaluated. The pressure range of these pressure monitors is up to 100 MPa and the resolution of the transducer is 0.1 kPa. The pressure monitor includes a quartz Bourdon-type high-precision electronic pressure transducer inside the body. The sensing element of the transducer is a precision quartz crystal resonator and the frequency of oscillation varies with pressure-induced stress. In total, six pressure monitors were prepared for this study. During 15 months, the six monitors were calibrated simultaneously using a hydraulic pressure balance 11 times in total. The effects on the readings of the monitors by setting parameters and environmental conditions were evaluated. The important characteristics for the pressure transducer such as the temperature coefficient of the span reading and long-term stability were also evaluated quantitatively. From the evaluation results, the behaviours of the transducers during the period were well characterized. The standard deviation of the relative deviations from the least-squares best-fitting function for each pressure transducer was less than 6 × 10−6 in the pressure range of 30 MPa to 100 MPa and had a maximum of 11 × 10−6 at 10 MPa. In this article, the details of the characterization are described.

Development of small differential pressure standard using double pressure balances

A calibration system for small differential pressure standards has been developed. The standard is realized by a pair of pressure balances, which generate a differential pressure of 1?Pa to 10?kPa at a line pressure of 100?kPa ? 10?kPa. The performance of the calibration system has been evaluated by calibrating several pressure transducers and positive long-term stability has been observed for each transducer for over a year. The standard deviation measured during the period was about 8?mPa in the pressure range 1?Pa to 1?kPa. These results indicate that the system has sufficient performance to calibrate high accuracy pressure transducers. The details of the system and the performance evaluation are described.

Precise determination of the pressure distortion coefficient of new controlled-clearance piston-cylinders based on the Heydemann-Welch model.

A new controlled-clearance (CC) pressure balance has been developed with the aim of improving hydraulic pressure standards up to pressures of 1 GPa. The pressure balance is equipped with a weight-loading unit that can load/unload selected weights automatically and a CC piston-cylinder that is designed for the jacket pressure to be applied independently. In this paper, the effective area A(e) of two kinds of the CC piston-cylinders for pressures of 200 and 500 MPa was examined based on the Heydemann-Welch model. The two parameters in the model, the jacket pressure coefficient and the zero clearance jacket pressure, were precisely determined by the characterization experiments, after which the pressure dependence of A(e) and its uncertainty were estimated. The uncertainty due to the pressure dependence of A(e) for the 500 MPa CC piston-cylinder was less than 7.5x10(-8) MPa(-1). To confirm the consistency of the estimations, the results of the two CC piston-cylinders were compared through a free-deformation (FD) piston-cylinder; the two estimations were in agreement with each other. Moreover, it was shown that the extent of nonlinearity in the pressure dependence of A(e) of the FD piston-cylinder can be evaluated by calibration against the CC piston-cylinder.

Measurement of the volume of weights using an acoustic volumeter and the reliability of such measurement

For the precise measurement of the mass of a weight, buoyancy correction of air for the volume of a weight is necessary. Although hydrostatic weighing is the most accurate method currently used in determining volume, it is a relatively complicated process requiring the weight to be wetted in a reference fluid. Since more weights with high accuracy of mass are being used, a more sophisticated method of determining volume that satisfies uncertainty requirements has been demanded to improve the efficiency of calibration. In this paper, an acoustic volumeter is proposed for the measurement of the volume of weights. An acoustic volumeter can measure volume in atmosphere in a simple manner. Consequently, it is not necessary to consider the contamination and mass change due to the liquid used in the hydrostatic weighing method. In practical applications, a procedure for measuring the volume of weights ranging from 100 g to 10 kg using an acoustic volumeter is proposed. The volumes obtained using an acoustic volumeter were compared with those obtained by the hydrostatic weighing method, and the reliability of the measurement was evaluated with a relative uncertainty below the order of 1 × 10−3. From the measurement results, it was shown that the use of an acoustic volumeter is effective for the measurement of the volume of weights.

Reproducibility of calibration results by 0-A-0 pressurization procedures for hydraulic pressure transducers

This paper focuses on 0-A-0 pressurization for the calibration of electromechanical pressure gauges in order to obtain reproducible results by various procedures. In 0-A-0 pressurization, pressure is released to atmospheric pressure after finishing each measurement at each calibration pressure, and the output reading at atmospheric pressure is used for offset correction. Pressure gauges are calibrated under various pressure sequence and time interval conditions in the pressure range of 10 MPa to 100 MPa using a fully automated calibration system that uses a pressure balance as the standard device. Results for a quartz Bourdon-type pressure transducer are mainly reported. The hysteresis obtained by 0-A-0 pressurization is found to be nearly one-tenth of that obtained by stepwise pressurization. The effect of the previously applied pressure is less than 10 parts per million even for a random pressure sequence. Key points for calibration procedures are proposed from the calibration results for different time intervals. When the appropriate procedure is used, 0-A-0 pressurization largely reduces the effect of pressurization history, thus yielding highly reproducible calibration results.

Improved methods for comparing gas and hydraulic pressure balances

Two methods for comparing the pressures of different media generated by gas and hydraulic pressure balances are examined in order to improve the consistency between gas and hydraulic pressure standards. One method uses a gas?liquid separator and the other uses a liquid-lubricated pressure balance. The pressure range examined is 0.5?MPa to 7?MPa. For both methods, a precise pressure transducer is used as a comparator to determine the equilibrium state between the two pressure balances. In this paper, the measurement results and the uncertainties obtained for the respective methods are presented. It is shown that the results obtained by using both methods agree within the combined standard uncertainty over the entire pressure range. Thus, both methods examined in this study are found to be useful for comparing gas and hydraulic pressure balances accurately.