Spinning Rotor Gauge Research Articles

Static expansion primary vacuum standard — Part 2: Characterization of two spinning rotor gauges

The Part 1 of this paper discussed the determination of the volume ratio of the Static Expansion Primary Vacuum Standard (SES). Part 2 of the paper discusses in detail the use of the SES thus characterized for generating primary pressures. This standard has been used to calibrate Spinning Rotor Gauges (SRGs) at a number of pressures in the range 0.1 Pa to 1 Pa in steps of 0.1 Pa. The Gauge Coefficient at each of these pressures, defined as equal to the ratio of the indicated pressure to the true pressure is then plotted against the true pressure. The resulting straight line plot has a negative slope and its intercept equals the gauge coefficient. Two SRGs, NPL-0 and NPL-2 thus calibrated are used (i) as a device for measuring the pressure rise in the flowmeter of the National Physical Laboratory, India (NPLI) orifice flow system and (ii) as a secondary standard for the calibration of the user gauges.

Static expansion primary vacuum standard — Part 1: Determination of the volume ratio of the expansion stage

Measurement of volume ratio forms an essential part of the characterization of a static expansion system (SES) for the generation of pressures in the medium and high vacuum region. Because of the complexity of the vacuum chambers, it is not possible to determine the absolute volumes of the chambers involved. Hence an expansion process is resorted to, in which the gas enclosed in the small volume at a known pressure is allowed to expand, under isothermal conditions, into the large volume. The accuracy of the volume ratio thus determined depends on the stability of the vacuum gauges used for the measurement of initial and the final pressures and the ambient temperature. High accuracy resonant silicon gauges (RSGs) of two different ranges, one 130 kPa full scale and the other 1 kPa full scale for measurement of the initial and final pressures are found to be useful. This paper reports the results obtained with these RSGs for the measurement of the volume ratio of the National Physical Laboratory, India (NPLI) SES which is nominally 2820. With the use of the RSGs, and calibrated platinum resistance thermometers (PRTs) mounted inside the vacuum chambers, it has been possible to measure the volume ratio within the relative expanded uncertainty (k = 2) of 0.0014. The results of the volume ratio determination exercise are given in this paper while the Part 2 of this paper will discuss the procedure for the calibration of spinning rotor gauges on the Static expansion primary standard.

Capillary flow meter for calibrating spinning rotor gauges

This article describes a capillary flow meter whose maximum flow rate of 0.2μmol∕s (∼0.2cm3∕min at ambient conditions) covers the range that is useful for calibrating spinning rotor gauges. Knowing the input pressure, output pressure, and temperature of the capillary yields the gas flow rate with a relative standard uncertainty as small as 0.04%. The flow meter, which requires no moving parts aside from valves, comprises a ballast tank, a coil of quartz capillary with an inner diameter of 0.1mm, and a commercial instrumentation package. Measurements near 0.1μmol∕s showed agreement with an independent primary flow meter to within 0.2%, comparable to the combined relative standard uncertainty of 0.11%. Additional measurements showed that operating the capillary flow meter with an exit pressure less than 1kPa allowed the flow to stabilize within minutes. However, the small exit pressure caused an unexpected enhancement of the slip correction in the hydrodynamic model of the capillary.

Microelectromechanical sensors for measuring gas pressure

New prototypes and concepts of micro sensors for measuring gas pressure have been developed by using the fabrication technologies for Micro Electro Mechanical Systems (MEMS). The realization of such micro-structured sensors requires sofisticated fabrication processes such as thin film deposition, photolithography and etching techniques. This approach of MEMS sensors for gas pressure is demonstrated by few examples, such as micro-Pirani gauges, resonant vacuum micro gauges and micro spinning rotor gauges.

Final report on supplementary comparison APMP.M.P-S2 (bilateral comparison) at a nominal pressure of 0.05 Pa

A supplementary bilateral comparison was carried out to compare two different pressure standards at a nominal pressure of 0.05 Pa. The two standards used different principles for generating the nominal pressure; one based on the static expansion method (NPL, India), and the other based on the orifice flow or dynamic expansion method (NIST, USA). The transfer standard consisted of a pair of spinning rotor gauges whose accommodation coefficients were measured at a nominal pressure of 0.05 Pa at the two laboratories. The nominal pressure value for the comparison was chosen because (i) it can be easily generated by the respective standards with high accuracy and (ii) it had not been covered in the earlier two key comparisons, namely, CCM.P-K3 and CCM.P-K4, which the two NMIs had participated in. NPLI served as the pilot lab, and provided the stainless steel rotors; each lab used its own thimbles and electronics. The rotors were hand-carried from NPLI to NIST at atmospheric pressure in small, insulation-packed glass vials to prevent the rotors from moving during transit; otherwise no special transit precautions were taken.The pressure value of 0.05 Pa generated by the NPLI and NIST showed good agreement within their combined uncertainties, proving thereby that the two standards were equivalent to each other for the pressure of 0.05 Pa.Main text.To reach the main text of this Paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the APMP, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Factors affecting the reproducibility of the accommodation coefficient of the spinning rotor gauge

The spinning rotor gauge is the most stable device available for measuring pressure between 10−4 and 10−1Pa. The calibration of these gauges is determined by measuring an effective accommodation coefficient, which is used in conjunction with an equation derived from the kinetic theory of gases to calculate the pressure seen by the rotor gauge. To assess the reproducibility of the spinning rotor gauge (and hence its accommodation coefficient), eight spinning rotor gauges were calibrated numerous times simultaneously with the primary midrange vacuum standard at the National Institute of Standards and Technology over a 15month period. Events encountered by a rotor during its normal operation were investigated for their effects on the accommodation coefficient of the rotor. The effects of baking as well as “crashing” of the rotors were also investigated. We found that the accommodation coefficient may be dependent on the orientation of the rotor’s suspension as a consequence of anisotropically distributed blemishes on the rotor’s surface. Consequently, the accommodation coefficient will not reproduce well when the rotor is resuspended because the rotational axis is not replicated. Data on the accommodation coefficient obtained from ordinary rotors as well as rotors specifically prepared with a band of scratches to exaggerate the anisotropic nature of the blemishes will be presented.

Microstructured vacuum gauges and their future perspectives

New prototypes and concepts of microstructured vacuum gauges have been developed by using the fabrication technologies for micro electro mechanical systems (MEMS). The realization of such microstructured gauges requires sophisticated fabrication processes such as thin film deposition, photolithography and etching techniques. This approach of MEMS vacuum gauges is demonstrated by a few examples. Micro-Pirani gauges are based on the principle that the heat transfer between two surfaces is proportional to the number of molecules (and hence the pressure) transferring the heat, when the mean free path in the gas is larger than the distance between the surfaces. In contrast to conventional Pirani gauges with heated thin wires, in a micro-Pirani gauge the heat transfer takes place between an extremely thin heated membrane and the surrounding. The thin membrane (200-nm thick) is realized by deposition of siliconoxide/siliconnitride, photolithography and anisotropic etching of silicon wafers. Heating is performed by a meander-shaped aluminum thin film heater in the center of the membrane. This micro-Pirani gauge shows a high sensitivity in the pressure range between 10 −4 and 100 mbar. By using a silicon “microbridge” with 10 μm small gap between heated membrane and surrounding, the pressure sensitivity of the chip is extended up to 1000 mbar. Similar concepts are presented and discussed with respect to the miniaturization of spinning rotor gauges. The new concept is based on the application of microfabricated disks (instead of spheres) and of electrostatic instead of magnetic driving forces. The extension of the sensitivity range for miniaturized spinning rotor gauges is also discussed. Finally, new perspectives for mechanical vacuum gauges are demonstrated. By application of micromechanical processes, very thin stress-compensated membranes can be fabricated which enable sensitive mechanical gauges even for pressures in the high vacuum range. First, experimental results with respect to these membranes are represented.

Experimental verification of a Monte-Carlo simulation of the gas density in a vacuum chamber

An investigation was performed to check the results of a Monte-Carlo simulation of the gas density variations inside a vacuum chamber used for calibration purposes. The simulations revealed variations of up to 0.33% between gas densities at different calibration gauge ports. In total seven spinning rotor gauges were used to detect the differences experimentally. The mean values of the differences that could be determined with an uncertainty of 0.1% of the mean gas density only are consistent with the results of the Monte-Carlo simulation and give confidence in the simulation overall.

Offset scatter reduction of spinning rotor gauges by vibration isolation

The spinning rotor gauge is an accurate vacuum gauge of good long-term stability. Many efforts have been aimed at the reduction of the uncertainties of this device. Here we examine the possibility of reducing the uncertainty in the low-pressure range caused by the scatter of the pressure-independent residual drag (offset). Random vibrations exposed to the system produce random eddy currents and lead to an additional scatter of the offset value. We tested different configurations, where the suspension head only was isolated from vibration or where both suspension head and stainless steel thimble were isolated. Data sets containing time, frequency of the rotor, relative deceleration rate and temperature were taken at residual pressure conditions. It was found that vibration isolation of the suspension head alone does not cause a significant reduction of the offset scatter, while vibration isolation of suspension head and thimble reduces the scatter by a factor of down to 1 30 . Surprisingly, it was found that the size of the reduction depends on the controller suspension head unit used for the measurement.

Generation and pressure measurement of extremely high vacuum (XHV) using a TiN coated chamber

A TiN coated chamber system has been developed to reduce outgassing from the walls of the vacuum chamber. The outgassing rate of the XHV chamber measured by the throughput method was 1 × 10−13 Pa m s−1. The outgassing rate measured by the pressure rise method using an extractor gauge (EXG) and a spinning rotor gauge was 3 × 10−10 Pa m s−1 and 2 × 10−13 Pa m s−1, respectively. The outgassing rate measured by the pressure rise method using the EXG is not correct because of outgassing from the gauge itself. The ultimate pressure measured by the EXG and an AxTran gauge was 3 × 10−10 Pa and 5 × 10−11 Pa, respectively. Pressure measurement using the EXG was limited to about 3 × 10−10 Pa because of factors such as x-ray effects, effect of electron stimulated desorbed ions and outgassing from the gauge itself. The results of outgassing and ultimate pressure measurement indicate that the coating of TiN film on surfaces of the vacuum chamber is effective in reducing the outgassing rate from the walls of the chamber.

Realization of the medium and high vacuum primary standard in CENAM, Mexico

A medium and high vacuum primary standard, based on the static expansion method, has been set up at Centro Nacional de Metrología (CENAM), Mexico. This system has four volumes and covers a measuring range of 1 × 10−5 Pa to 1 × 103 Pa of absolute pressure. As part of its realization, a characterization was performed, which included volume calibrations, several tests and a bilateral key comparison. To determine the expansion ratios, two methods were applied: the gravimetric method and the method with a linearized spinning rotor gauge. The outgassing ratios for the whole system were also determined. A comparison was performed with Physikalisch-Technische Bundesanstalt (comparison SIM-Euromet.M.P-BK3). By means of this comparison, a link has been achieved with the Euromet comparison (Euromet.M.P-K1.b). As a result, it is concluded that the value obtained at CENAM is equivalent to the Euromet reference value, and therefore the design, construction and operation of CENAM's SEE-1 vacuum primary standard were successful.

Axial-symmetric transmission gauge: Extension of its pressure measuring range and reduction of the electron stimulated desorption ion effect in ultrahigh vacuum

The basic characteristics of the axial-symmetric transmission gauge (AT gauge), that were modified for measurement over a wide pressure range, are reported. A Faraday cup ion collector is used instead of a secondary electron multiplier to extend the measurable pressure range to 10−3Pa. The sensitivity of the gauge was determined by a conductance modulation method, and by direct comparison against an extractor gauge and a spinning rotor gauge as well. When operating in O2, there was systematic disagreement between pressure readings with the AT gauge and the extractor gauge. This is due to the difference in the elimination efficiency of the electron stimulated desorption ion.

Hydrogen pumping by austenitic stainless steel

In the present study, hydrogen sorption and desorption kinetics close to equilibrium were investigated in a pinched-off AISI 316 steel cell by a sensitive pressure-rise method. The pressure was monitored with a spinning rotor gauge (SRG) just before the pinch-off and after it for 6months at two stabilized temperatures: 25°C and 55°C. The preprocessing of the cell (of uniform wall thickness 0.15mm volume 125cm3, and inner surface 460cm2) consisted of baking at 200°C for 109h with several evacuation cycles to ultrahigh vacuum (UHV). The quantity of released hydrogen during the bake-out procedure equaled the average concentration change ΔC=2.8×1017at.Hcm−3. After the pinch-off intentionally done in the high vacuum range where hydrogen represented the residual atmosphere, surprisingly the hydrogen pressure slowly declined from the initial p(328K)=3.7×10−4mbar, with an initial rate dp∕dt=−5.5×10−11mbar∕s and later attained a stable value, which could be termed the equilibrium. In similar reported experiments, where valving-off began in the UHV, the dp∕dt was always positive and constant over several orders of magnitude in pressure. During 6months of measurements, a sudden temperature jump from 25°Cto55°C or back was applied a few times to investigate the stability of the equilibrium or the impact on the pressure course. The most plausible explanation of the results is given along with discussion whether hydrogen permeated through the cell wall or if it was absorbed in the cell wall.

Results of the regional key comparison Euromet.M.P-K1.b in the pressure range from 3 × 10-4 Pa to 0.9 Pa

Within the Euromet region a regional key comparison (Euromet.M.P-K1.b) was carried out in order to compare national vacuum standards in the pressure range from 3 × 10-4 Pa to 0.9 Pa. The participants were the BNM-LNE (France), CEM (Spain), IMGC-CNR (Italy), IMT (Slovenia), NPL (United Kingdom), UME (Turkey), and the PTB (Germany) as pilot laboratory. The measurements were carried out from April 2000 to February 2002.Two spinning rotor gauges served as transfer standards and showed a good transport stability. The effective accommodation coefficients of the rotors had to be determined at eight target points at and between 3 × 10-4 Pa and 0.9 Pa. The uncertainty of the generated pressure in the calibration standard was reported as part of the calibration report by each laboratory. All additional uncertainties that were related to the transfer standard were evaluated by the pilot laboratory in order to have a uniform uncertainty analysis for all participants and in order to emphasize the importance of the reported uncertainty of the generated pressure.From the available data a Euromet reference value was calculated at each target pressure. The results from most of the laboratories showed a good agreement with the reference value within the combined uncertainties. A few values of three of the laboratories were significantly off the reference value.At the highest target pressure of 0.9 Pa a linkage to the lowest target pressure at 1 Pa of the key comparison CCM.P-K4 was possible by means of the results of three laboratories that took part in both comparisons.Main text.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCM, according to the provisions of the Mutual Recognition Arrangement (MRA).

Results of the key comparison SIM-Euromet.M.P-BK3 (bilateral comparison) in the pressure range from 3 × 10-4 Pa to 0.9 Pa

A medium and high vacuum primary standard has newly been set up at the CENAM, Mexico. This vacuum standard is based on the static expansion method and covers a measuring range from 1 × 10-5 Pa to 1 × 103 Pa of absolute pressure.To validate its function and uncertainty budget, in 2002 a comparison was performed with a primary standard of the vacuum metrology section at PTB which had taken part in the key comparison CCM.P-K4. In this manner, the degree of equivalence of the pressure measurements performed with this standard with those of other national primary standards of the CCM.P-K4 could be found. In addition, a link could be achieved with the national standards of the Euromet comparison Euromet.M.P-K1.b, since the latter comparison was carried out with the same protocol and the same tranfer standards, having also PTB as common participant.Two spinning rotor gauges served as transfer standards and showed an excellent transport stability. The effective accommodation coefficients of the rotors had to be determined at eight target points, at and between 3 × 10-4 Pa and 0.9 Pa.The pressures generated by the PTB and the CENAM primary standards showed good agreement within their combined uncertainties. So, there is also equivalence of the CENAM medium and high vacuum primary standard with the other national primary standards of the CCM.P-K4. The CENAM primary standard is also equivalent with the reference value of the Euromet.M.P-K1.b.Main text.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCM, according to the provisions of the Mutual Recognition Arrangement (MRA).

Local pressure in the proximity of a field emitter

A basic characterization of field emitters is usually realized in a standard UHV chamber equipped with appropriate pumps. Their high pumping speed and the remote location of the vacuum gauge prevents the monitoring of small pressure changes during the operation of a field emitter. In most potential applications, the field emitter is supposed to run in a small sealed device, where the local pressure is frequently unknown and its influence on the device’s lifetime is often underestimated. A small glass envelope equipped with a miniature ion-getter (IG) pump, spinning rotor gauge (SRG), and a cathodoluminescent screen, was prepared to study the influence of the field emitter on the residual atmosphere. This was monitored via the discharge current of the miniature IG pump, previously calibrated in situ by the SRG. Single-crystalline nanoparticles deposited onto molybdenum needles were used as field emitters. The initial pressure in a well outgassed envelope was in the UHV range, but its increase for almost three orders of magnitude was detected when the field-emission current from a few emission sites reached the one microampere range at 1 kV. A good correlation between the power of the generated e-beam and the discharge current of the miniature IG pump was observed.

The effect of ambient temperature on the sensitivity of hot-cathode ionization gauges

We measured the sensitivities of five hot-cathode ionization gauges for ambient laboratory temperatures between 23 and 31 °C. All of the ionization gauges exhibited very similar behavior, and the sensitivity could be adequately modeled with a linear function of temperature. The slopes of the fits were smaller than one would expect due to changes in the calibration chamber number density. The thermal transpiration effect, due to local heating of the gauge structures, can account for this, and a characteristic gauge tube temperature can be inferred from the temperature dependence of the sensitivity. A recent comparison of the high vacuum standards of several National Metrology Institutes (NMIs) was performed over the range of 10 −6–10 −3 Pa using hot-cathode and spinning rotor gauges as transfer standards. Among the participants, laboratory temperatures varied by as much as 5 °C. It is necessary to know how laboratory temperature affects the sensitivity of the hot-cathode transfer standards (spinning rotor gauges explicitly account for the gas temperature) so that individual laboratory results can be corrected to a common temperature.

Comparisons to establish a force-balanced piston gauge and a spinning rotor gauge as the new measurement standards of MIKES

Centre for Metrology and Accreditation (MIKES) has worked towards expanding its calibration capabilities in the absolute pressure range downwards from 0.2 Pa and lowering measurement uncertainties in the range <10 kPa using a force-balanced piston gauge (FPG) and a spinning rotor gauge (SRG). MIKES was the first national pressure laboratory to purchase a novel type of piston manometer, FPG, developed by DH Instruments Inc., USA. The effective area of the FPG was at first determined at MIKES by a comparison with a conventional pressure balance. The result was confirmed in comparisons in the same pressure range with two other laboratories, Swedish National Testing and Research Institute (SP), Sweden and Institute for Metrology and Technology—Van Swinden Laboratorium (NMi-VSL), Netherlands. Next a direct comparison with the mercury column manometer of Physikalisch-Technische Bundesanstalt (PTB), Germany in the absolute pressure range from 1 to 15 kPa was carried out. Use of the FPG for calibration of capacitance diaphragm gauges (CDGs) is also presented. MIKES now uses two SRGs as reference standards, manufactured by MKS Instruments, Inc., USA. A comparison of SRGs in the range from 0.1 to 5 Pa between MIKES, SP and Vaisala Oyj accredited laboratory was performed in 2001–2002.

Measurement of vacuum; 1950–2003

The major developments in the measurement of vacuum since the Bayard–Alpert gauge was invented in 1950 are reviewed; the American Vacuum Society was started three years later in 1953. These developments include (a) the improvement in understanding the processes causing limitations of the lowest measurable pressures (both total and partial), (b) the development of gauges, residual gas analyzers, and optical methods of pressure measurement to reduce these limitations, (c) the introduction of room-temperature electron sources (Spindt cathodes) to replace thermionic cathodes, and (d) the development of the spinning rotor gauge as a secondary standard.

Photodesorption of gases in vacuum glazing

When samples of vacuum glazing are exposed to sunlight, the pressure within these devices is observed to increase. The major gas species released have been identified using mass spectroscopic techniques, and are found to be carbon monoxide and carbon dioxide rather than water vapor as in thermally degraded vacuum glazing. Different measuring techniques, including the use of a spinning rotor gauge, were developed to study the time dependence of pressure in samples of vacuum glazing during sunlight exposure. The mechanism associated with optical stability of vacuum glazing has been studied. High temperature baking during the evacuation stage of the manufacturing process for vacuum glazing greatly improves the stability of these devices under optical illumination.