Gas-operated Pressure Balance Research Articles

Rarefied gas flow in pressure and vacuum measurements

Flows of a gas through the piston-cylinder gap of a gas-operated pressure balance and in a general vacuum system have one aspect in common, namely that the gas is rarefied due, respectively, to the small dimensions and the low pressure. The flows in both systems could be characterised as being in either slip-flow or transition regimes. Therefore, fundamental research of flow in these regimes is useful for both pressure and vacuum metrology, especially for the gas-operated pressure balance where a continuum viscous flow model is widely used for determining the effective area of the pressure balance. The consideration of gas flow using the most suitable assumption would improve the accuracy of such a calculation. Moreover, knowledge about rarefied gas flow will enable gas behaviour in vacuum and low-flow leak detection systems to be predicted. This paper provides useful information about rarefied gas flow in both slip-flow and transition regimes.

An initial investigation of the damped resonant behaviour of gas-operated pressure balances

The aim of this investigation into the damped resonant behaviour of the piston of gas-operated pressure balances is to provide insights into ways of improving the short-term performance of gas-operated pressure balances. Initial measurement results are reported for the damped resonant behaviour of two different pressure balances as a function of system volume. The pressure balances are operated using air in absolute mode at 100kPa and the system volume is varied from about 10mL to 250mL. Values for the period, the logarithmic decrement λ and the Q of the oscillation are obtained by fitting the measured damped oscillatory motion of the piston height to a classical damped oscillator model. Results show that the damping is dominated by non-adiabatic pressure variations in the system volume and not by viscous damping of the vertical motion of the piston in the cylinder. The settling time for the oscillations to damp out is primarily determined by the period of oscillation with a lesser influence from variations in λ. Implications for improving pressure balance performance are discussed.

FEA calculation of pressure distortion coefficients of gas-operated pressure balances – EURAMET project 1039

Finite element analysis methods were developed and applied to gas piston–cylinder units (PCUs) of piston- and cylinder-floating configuration (2, 5, 10 and 20)cm2 nominal effective area, operated in gauge and absolute mode at pressures (0.06–7.5)MPa to determine their zero pressure and pressure-dependent effective areas, as well as pressure distortion coefficients (λ) with associated uncertainties. Real dimensional properties of the PCUs were used. λ were found to be independent of gas (ideal, N2, He) within the viscous flow model, but strongly dependent on the gap shape, operation mode and elastic properties. Results demonstrate good agreement for λ, with its uncertainty for different PCUs varying between (0.03 and 0.21)×10−6MPa−1 corresponding to maximum relative uncertainties in pressure of (0.07–0.34)×10−6.

Performance aspects of gas-operated pressure balances as pressure standards

In this paper, the aspects of gas-operated pressure balances that affect their performance are discussed. These aspects include both the features that make gas-operated pressure balances suitable for use as pressure standards and the factors that must be managed to achieve good performance. As key features, gas-operated pressure balances are pressure generators, have excellent relative short-term stability (∼5 × 10−9) and long-term stability (∼1 × 10−6), and can be used as primary standards. Factors to manage include harmonic oscillation of the piston in the cylinder, pressure balance temperature and operating height, and geometric imperfections. Three applications are used to illustrate the performance aspects of gas-operated pressure balances: a primary standard, a standard for differential pressure from <1 Pa to >1000 Pa and a standard for absolute pressure from <10 Pa to 10 kPa.

Viscous drag forces in gas operated pressure balances

Accurate determination of all forces acting on the piston in the gas operated pressure balance is a long-standing problem in high precision pressure measurements. A major contribution, responsible for the difference between the actual and the effective areas of the piston, is due to the drag force on the piston, and at a fundamental level surprisingly little is known about this force. We present here a first detailed analysis of the hydrodynamics shear forces acting at the gas–piston interface on a molecular level, using effective intermolecular potential functions and a large-scale non-equilibrium molecular dynamics simulation to model the flow of several gases. We show using the hydrodynamic approach that the drag force on the piston for force-driven plane steady-state flow is exactly one half of the external driving force independent of the boundary conditions, justifying a widely accepted assumption in pressure balance theory, and verify this result in computer simulation for a range of gas densities. The natural fall rates using different gases as the pressure transmission medium were measured experimentally for a range of generated pressures and estimated from computer simulation using force-driven Poiseuille flow. Both results, which are in excellent agreement, show pronounced species dependence. This, however, cannot explain the gas species dependence of the effective area.

Calibration apparatus for precise barometers and vacuum gauges

In order to calibrate highly accurate absolute pressure gauges, such as barometers and vacuum gauges, laser, or ultrasonic mercury manometers have been used. However, the complexity and cost of manometers have greatly reduced the use of this method in most calibration laboratories. As a substitute, a gas-operated pressure balance is used to calibrate precise gauges. In such cases, many commercially available pressure balances are unsuitable because the necessary exposure of the piston, cylinder, and masses to the atmosphere causes contamination problems and allows dust particles into the gap between the piston and cylinder. In this article, a weight-loading device is described that allows masses to be changed in situ without breaking the vacuum. This device makes it possible to add or remove weights easily during the calibration, thereby greatly reducing the time between observations. Using this device, we efficiently calibrated a precise quartz resonance barometer (Paroscientific, model 760-16B) over a p...

New weight-loading device for calibration of precise barometers

In order to calibrate highly accurate barometers, laser/ultrasonic mercury manometers have been used. However, the complexity and cost of mercury manometers have gradually taken them out of use in most calibration laboratories. As a substitute, a gas-operated pressure balance is used to calibrate precise barometers. In such a case, many commercially available pressure balances are not desirable because the consequent exposure of the piston, cylinder and masses to the atmosphere causes the problem of contamination and the ingress of dust particles to the gap between the piston and cylinder. In this paper, a novel weight-loading device for changing the masses in situ without breaking the vacuum is described. This device makes it possible to add or remove weights easily in the absolute mode, thereby greatly reducing the time between observations. Using this device we could easily calibrate a precise quartz resonance barometer (Paroscientific, Model 760-16B) from 940 hPa to 1050 hPa.

Video-camera-based sensor to measure gas/oil interface heights for pressures up to 17 MPa

This paper describes a gas/oil interface height sensor for determining the fluid head corrections when comparing a gas-operated pressure balance with an oil-operated pressure balance. The sensor can also be used to establish the balance point of the comparison from the rate of change of the interface height. In the sensor, a video camera views the interface within a transparent pressure cell. The cell is made from a polished polycarbonate cylinder, outer diameter 50 mm, inner diameter 10 mm, designed for a working pressure of 17 MPa. A safety shield surrounds the cell. The video camera magnifies the image of the interface and a reference scale in the cell, allowing them to be viewed remotely. The sensor can measure interface heights with a standard uncertainty of 0.05 mm. Results of a comparison of pressure balances using the sensor show that over the range 1.6 MPa to 11 MPa, the balance points can be determined with a standard uncertainty of less than 4 Pa.

Progress Report on an International Intercomparison in the Pressure Range 10 kPa to 140 kPa

Results are summarized from thirteen laboratories which took part in the first four phases of an international intercomparison of pressure measurement standards in the range 10 kPa to 140 kPa, carried out under the auspices of the BIPM. The transfer standard was a gas-operated pressure balance (piston gauge). All participants took measurements in the absolute mode; several also took measurements in the gauge mode. The transfer standard showed good repeatability and long-term stability, and revealed some unexpected and significant differences between some of the participants.

The Accurate Generation of Small Gauge Pressures Using Twin Pressure Balances

The accurate generation of gauge pressures in the range from below 0.5 Pa to 2000 Pa using twin gas-operated pressure balances is described. Each gauge pressure is generated by changing the mass on one of the pressure balances while they are both operating. The performance of the twin-pressure-balance instrument was assessed by calibrating differential pressure transducers. The short-term stability of the average difference in pressure between the two pressure balances was about 0.5 mPa (standard deviation), or 5 parts in 109 of the line pressure, and the overall uncertainty (99% confidence limits) in the generated gauge pressure pe was ± (0.9 mPa + 2 × 10-5 pe).

The effective area of a pressure balance at low pressures

A theory is given for calculating the effective area of a gas-operated pressure balance from the piston and cylinder dimensions when the gas in the piston-cylinder gap in the transition and molecular flow ranges. Calculations using the theory show that the effective area decreases with increasing ratio of molecular mean free path length to gap width as the nature of the gas flow changes from viscous to molecular. This calculated area tends to a limiting value (for wholly molecular flow) which depends strongly on the ratio of gap width to piston radius.

An Intercomparison Between a Primary Standard Mercury Barometer and a Gas-operated Pressure Balance Standard

The purpose of this paper is to discuss the design and performance of a primary standard barometer and a primary standard pressure balance, and to describe in detail a direct intercomparison between the two instruments within their optimum operating range. The uncertainties associated with each pressure standard are discussed together with an estimate of the uncertainties associated with the intercomparison experiment itself. At a pressure of 100 kPa the two methods agree to within 1.2 ± 1.4 Pa at the 99.7% confidence level.