Capacitance Manometer Research Articles

The operation of a capacitance manometer at 300 °C

The performance of a capacitance manometer operated at 300 °C is described. Calibration against a deadweight primary pressure standard indicates that the stability over a period of 3 months is better than 0.25% of the measured pressure over the pressure range 15–1000 Torr (2.04–131.6 kPa).

Effect of gas composition on vacuum measurement

Accurate pressure measurement with most types of vacuum gauges is critically dependent on a knowledge of the composition of the gas being measured. This paper will discuss the gas dependence of measurement with a number of commonly used types of vacuum gauges. Particular emphasis will be placed on recent work in the author’s laboratory which has shown how the accuracy of capacitance manometers can be affected if the possibility of thermal transpiration is ignored. Methods have been derived and proven for applying corrections when capacitance manometers are used with a wide variety of gases. The dependence of other types of vacuum measuring equipment on gas composition will also be discussed and recent investigations with spinning rotar gauges and ionization gauges will be reviewed. In the paper the potential of small mass spectrometers for quantitative gas analysis will be discussed. The most recent results on the evaluation of these instruments will be given for both small quadrupoles and magnetic sector instruments. The various methods of calibrating these instruments will be discussed with particular emphasis on the requirements of process industries and the monitoring of environmental pollutants.

Cross sections for ionization of gases by 5-4000-keV protons and for electron capture by 5-150-keV protons

Using the parallel-plate-capacitor method and a capacitance manometer to determine pressures, total cross sections for the production of positive and negative charges were measured for 5-4000-keV-proton impact on He, Ne, Ar, Kr, ${\mathrm{H}}_{2}$, ${\mathrm{N}}_{2}$, CO, ${\mathrm{O}}_{2}$, C${\mathrm{H}}_{4}$, and C${\mathrm{O}}_{2}$. From these, ionization and electron-capture cross sections were obtained and fitted to semiempirical equations describing the energy dependence in terms of a few parameters. At high energies very good agreement is obtained in the comparison of the ionization cross sections to earlier proton- and electron-impact measurements and with theoretical treatments where they are available, but discrepancies exist for some targets at low energy. Above 10 keV the electron-capture cross sections are in agreement with earlier work for all the targets except CO and C${\mathrm{H}}_{4}$ for which they are (20-40)% higher.

A simple pressure gauge for tritium gas with high accuracy in the range 10−3 ≤ p ≤ 10 bar

A simple pressure gauge has been constructed to determine the pressure (p) of β− decaying radioactive gases in the range from 10−3 to 10 bar by measurement of the decay current. The mechanical and electronic parts of the gauge are described in detail. The main advantages of the new sensor head are: low building cost, UHV compatibility, bakeable to 250°C, robust construction and insensitiveness to vibrations. Calibration of the sensor head (output signal versus pressure) was done for pure tritium by means of single sided capacitance manometers. In the range 7 × 10−3 ≤ p ≤ 1.5 bar the pressure values determined with the new gauge differ by less than 0.5% from the values obtained by the capacitance manometer. The use of this gauge to determine the amount of radioactive gas in a mixture containing stable gases is discussed and an in-line configuration of this gauge is described.

Thermal transpiration correction in capacitance manometers

Capacitance manometers with sensors maintained at temperatures above ambient show a non-linearity in their pressure response. It is shown that the magnitude of this effect is consistent with the thermal transpiration effect calculated from simple kinetic theory. In addition, good agreement is found with the empirical model of Takaishi and Sensui 5. This model provides a simple method of calculating the response of a capacitance manometer, for a given gas, from a previous calibration with any gas.

Water Activity Measurements with A Capacitance Manometer

ABSTRACTThe design, use and performance of a capacitance manometer for the measurement of water activity (aw) are evaluated. The device consists of a sensitive capacitance sensor for measuring the vapor pressure of a sample. This value is then divided by a known value for the vapor pressure of pure water at a specific temperature to obtain aw. Precision and accuracy of this Instrument are determined and compared to a conventional electric hygrometer. These parameters are similar to the hygrometer; however, the principal advantages of the capacitance manometer may occur in circumstances in which direct measurements of aw are required and situations in which contamination of hygrometric sensors could occur.

Computer uses in vacuum equipment and processes

Complex ultrahigh vacuum processing systems for manufacturing applications at the Neutron Devices Department dictated the need for sophisticated computer-based designs. Equipment requirements involved data acquisition, instrument control and interactive operation interfacing for a single system incorporating automatic and manual vacuum valves, vacuum ionization gauges, mass spectrometers, thermocouple gauges, capacitance manometers, ion pumps, specialized heaters, and other electronics. By way of example, a design criterion is established and the major technical aspects of computer interfacing are discussed. The significance of the design is that the considerations given to the computer utilization are generally applicable to similar systems and exploits late technologies in electronic interfacing, CRT displays, speech output, and other human interface subsystems. Such systems have been successfully implemented at the Neutron Devices Department from which many general system design philosophies have emanated.

A system for vacuum gauge calibration using the comparison technique

A system for calibrating vacuum gauges in the range 10−4 to 102 Pa with an estimated uncertainty of ±8% is described. The factors affecting the design of the calibration vessel are discussed and the performance of the system enumerated. The reference gauges used are triode ionization gauges and a capacitance manometer. The reproducibilities of the reference gauges are estimated together with the other factors which contribute to the total uncertainty attached to a calibration performed on the system.

Process monitoring of tritium concentration

Tritium fueling and recycling processes for the fusion industry need measurements of tritium concentrations, at various points, for control decisions. A prototype process sampling and analysis system built to monitor tritium concentration in a thermal diffusion column (used to enrich tritium) is described. The system draws a 2‐cm3 gas sample from a selected point and analyzes tritium concentration. Concentration is determined using a beta scintillation detector to measure the tritium partial pressure in ratio to the total sample pressure measured, using a 1 Torr capacitance manometer. The percent relative (% rel) calibration accuracy of the beta scintillation detector is 1.4% rel/°C plus the electrometer zero drift. Accuracies ≤6% rel for 1 mol % of tritium reducing to ≤1.5% rel at 90 mol% tritium are routinely produced in a typical air‐conditioned laboratory. Temperature control of ±0.1 °C improves accuracies to one‐fifth the level for ambient conditions. For high purity tritium (>90 mol%), a miniature quadrupole mass spectrometer with Faraday ion detection is required to assay tritium purity by differences (100% minus nontritium components). The detection limits of 0.2 mol% for DT and 0.5 mol% for HT and HD place ultimate limits on purity measurements. The sampling and analysis system is fully automated allowing arbitrary sampling points or a timed sampling sequence. A method is also presented for direct measurement of tritium partial pressure in exhaust lines of a fusion device.

Summary Abstract: Effect of tritium on accuracy of capacitance manometers

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Master equation description of the multiphoton decomposition of ethyl acetate

In experimental observations of the multiphoton decomposition of ethyl acetate by a CO 2 laser at 1045.0 cm −1 fluences up to 4 J cm −2 were employed to dissociate 2 Pa of ethyl acetate in up to 600 Pa of N 2, He, Ne, Ar, Kr, Xe, ethylene and acetone bath gases. The fraction dissociated was measured in a closed cell, either by rate of removal of reactant, monitored by a probe laser and spectrophone, or by rate of pressure rise, monitored by a capacitance manometer. Total absorption and transient absorption changes were also measured in separate experiments. Data were analysed by finite difference solution of the energy-grained master equation, incorporating collisional effects and changes in transitional temperature. The convergence of the solutions was checked with decrease in the size of the energy grain. Microscopic reaction rates were described by an RRKM formulation with parameters from thermal experiments. Radiation absorption was described by an energy-dependent cross section with one free parameter chosen to match independent data. The collisional energy transfer function was an exponential form with the mean down transfer energy <Δ E> as a parameter. Dissociation versus pressure and fluence was fitted by one value of <ΔE> for each bath gas: N 2-820, He-550, Ne-700, Ar-930, Kr-905, Xe-920, C 2H 4-3000, CO(CH 3) 2-5000 cm −1. The technique appears to be a reliable means of obtaining energy transfer data at low temperatures. In addition to the experiments with reactant diluted in bath gas, the decomposition of undiluted reactant was also observed, at pressures in the range 1 to 800 Pa; here, reaction was terminated by collisions of irradiated molecules with molecules outside the beam; an approximate theory which appears to confirm the hypotheses is presented.

Dissociation pressures and enthalpies of reaction in MgCl 2 · nH 2O and CaCl 2 · nNH 3

Chemical heat pumps based on reactions involving salt hydrates, ammoniates, or methanolates are currently being developed. The efficient operation of a chemical heat pump depends strongly upon such quantities as reaction energies and pressure-temperature relations. Therefore, dissociation pressures as a function of temperature and derived reaction energies are presented for MgCl 2 · nH 2O (where n = 6.1, 6, or 4) and CaCl 2 · nNH 3 (where n = 8, 4, 2, or 1). Pressure measurements were made using a capacitance manometer over a temperature range of about room temperature to 470 K (the upper temperature varied depending on the composition being studied). By generating Φ o functions from available heat capacities complete second-law and third-law analyses have been done for MgCl 2 · 6H 2O and MgCl 2 · 4H 2O. Results for 1 6 MgCl 2 · 6H 2O yielded enthalpies of dissociation at 298.15 K of (58.3 ± 0.2) and (58.6 ± 0.1) kJ mol −1 for second-law and third-law treatments, respectively. For MgCl 2 · 4H 2O the values determined were (66.3 ± 1.2) and (67.0 ± 0.4) kJ mol −1. Values for the enthalpies of dissociation of ( 1 n ) CaCl 2 · n NH 3 were also determined; they are (44.4 ± 0.3), (42.0 ± 0.3), (67.8 ± 0.8), and (78.2 ± 0.8) kJ mol −1 for CaCl 2 · 8NH 3, CaCl 2 · 4NH 3, CaCl 2 · 2NH 3, and CaCl 2 · NH 3 respectively.

Ultimate pressure of mechanical pumps and the effectiveness of foreline traps

The ultimate pressure attainable with two‐stage, rotary vane‐type mechanical pumps is often obscured by the presence of the lubricating oil. Its reported value depends on the method of measurement. Experimental results are presented with trapped and untrapped pumps using McLeod, ionization and diaphragm gauges (capacitance manometers). Partial pressure measurements performed with both zeolite and liquid nitrogen trapped pumps continuously monitored over a two month period indicate the possibility of maintaining ultimate pressure near 1×10−3 Pa for a few months. This depends on the frequency of atmospheric exposure and the duration of operation at higher pressures. However, the usefulness of mechanical pumps at such reduced pressure is limited by the relatively low pumping speed for a given size and cost. The primary benefit is in reducing the condensable vapors by approximately one hundred times.

Total cross section measurement for e-CO2scattering down to 0.07 eV

The authors report transmission measurements made with an electron time-of-flight spectrometer. The gas pressure in the target cell was determined by means of a capacitance manometer. From 2 eV down to 0.07 eV the results agree extremely well with the predictions of Morrison et al. (1977) obtained from a coupled-channel theory.

Automatic measurement of soil-water pressure using a capacitance manometer. Technical note : J Hydrol, V46, N1–2, March 1980, P189–196

Automatic measurement of soil-water pressure using a capacitance manometer. Technical note : J Hydrol, V46, N1–2, March 1980, P189–196

Surface oxidation kinetics of sputtering targets

Films of compounds can be deposited by sputtering a metal or alloy target in an atmosphere containing a suitable reactive gas. Both Al and 90 : 10 In : Sn targets have been sputtered in argon/oxygen mixtures to obtain Al2O3 and indium tin oxide films. The experiments were carried out in a planar magnetron sputtering system with both dc and rf excitation. To investigate the kinetics of the reactive sputtering process, the time dependence of the total gas pressure was measured after a change in oxygen flow rate or sputtering power; a capacitance manometer gave accurate and reproducible results. There were simultaneous changes in the rf matching conditions when rf excitation was used. These changes can be attributed to the formation of an oxide on the target surface. The time dependence of the oxygen pressure measured for the Al and In : Sn targets have been used to compare various models of the reactive sputtering process. Fitting of the experimental values to these models yields values of the equilibrium oxide thickness on the target and these have been compared with measured values. For rf sputtering of an Al target at 500 W with flow rates of 3 mlmin and 2.2 mlmin for argon and oxygen respectively, both the calculated and the measured value of the oxide thickness is 100 nm.

Pressure broadening of single vibrational-rotational transitions of acetylene at ν = 5

To understand the mechanism of pressure broadening one must have accurate values of the pressure broadening coefficients as a function of vibrational quantum number. Unfortunately, such data for polyatomic molecules are scarce. The coefficient for self-broadening of methane has been found to be the same for the R(0) and R(1) lines of the 2{nu}{sub 3} and 3{nu}{sub 3} bands and for an unidentified line of the 5{nu}{sub 1} + {nu}{sub 3} band at 6190 {angstrom} (1). Measurements have also been performed on the {nu}{sub 2} (2) and {nu}{sub 1} + {nu}{sub 3} (3) bands of acetylene. In the work described here, high resolution spectra of single vibrational-rotational lines of the 5{nu}{sub 3} band of acetylene at 15,600 cm{sup -1} have been taken to determine the coefficients for self-broadening at a much higher level of vibrational excitation. A single frequency cw dye laser (Spectra-Physics 580A) with Rhodamine B as the lasing medium is used as a narrow bandwidth light source. The laser is continuously scannable over a 10 GHz region with 30 MHz linewidth. The unfocused beam is chopped and directed through a small, nonresonant optoacoustic cell. The pressure of the acetylene (> 99.99% purity) in the cell is measured withmore » a capacitance manometer. The optoacoustic signal is detected by a miniature electret microphone placed within the cell and is processed by a lock-in amplifier. The high resolution scans are calibrated ({+-} 5%) by monitoring the dye laser output with a spectrum analyzer equipped with 2 GHz FSR mirrors. All spectra were taken at room temperature (293 {+-} 2 K). At the relatively low pressures used in these experiments, the experimental linewidths are not more than three times the Doppler width (1.1 GHz FWHM). The method of Gronwall was used to calculate values of the Voigt lineshape. With these data, the pressure broadened widths, {Delta}{nu}{sub p}, were extracted from the experimental lineshapes and are plotted as a function of pressure for the R(3), R(9), and R(15) lines in Figure 1. The lines drawn through the data points were determined by the linear least squares method. The self-broadening coefficients, {gamma}, and their standard errors are summarized in Table 1 along with the results of other workers.« less

Simple pressure gauge for uranium hexafluoride

A sensitive detector and pressure gauge for uranium hexafluoride in high-vacuum systems is described. Negative surface ionization of UF(6) occurs on ribbon filaments operated at temperatures too low for electron emission to be significant. The ion current measured on a cylindrical collector surrounding the filament assembly varies regularly with UF(6) pressure below 10(-3) Torr. Different filament materials are considered, including rhenium, thoriated tungsten, and platinum. Rhenium is found to be the most satisfactory material for operation of diode emitters as a pressure gauge. Gauge constants (in A Torr(-1)) are derived from comparing negative surface ionization currents with the response of a capacitance manometer and are shown to be independent of temperature within a reasonable operating range. The effects of exposing the rhenium filament to various gases is considered, and it is shown that brief exposure to acetylene substantially improves the operating characteristics of the gauge.

An interferometric oil micromanometer

Describes an absolute U-tube micromanometer using silicon oil for measuring pressures from 0.3 to 6 Pa with an uncertainty of less than +or-.05%. A novel polarising interferometer using a helium-neon laser and fringe-counting electronics is used on each limb to measure the movement of the oil surfaces to the nearest quarter-fringe. Oil vibrations are damped over the measurement area by reducing the oil depth to 0.5 mm. The various sources of uncertainty are discussed in detail, and the results of a comparison between the micromanometer and a capacitance manometer, previously calibrated by the series expansion technique, given.

Pressure measurement system for beam experiments in the range 0.5–10−2 Pa

A system for measurements of gas pressure in the range 0.5–10−2 Pa is described. A known pressure is produced by static gas expansion and compared with the pressure in a scattering cell using a capacitance manometer as the null detector. The pressure in the scattering cell is controlled by a separate dynamic expansion system. Sources of error are discussed and the absolute error is estimated to be 0.4% at 0.5 Pa, 0.5% at 0.1 Pa, and 2.3% at 10−2 Pa.