McLeod Gauge Research Articles

Quick, inexpensive, and convenient protection for mercury-filled McLeod gauges

Quick, inexpensive, and convenient protection for mercury-filled McLeod gauges

Measurements of Total Cross Sections for the Scattering of Potassium Atoms by Rare Gases

Abstract Total cross sections for the scattering of potassium by He, Ne, Ar, Kr, and Xe have been measured in the thermal energy range. The velocity of potassium beam is selected by a velocity selector of a slotted disk type. To obtain accurate total cross sections, the pumping effect of a McLeod gauge for the pressure measurements, the fringe effect of the scattering chamber, corrections for angular resolution and for thermal motion of the target gases are taken into consideration. A new type of scattering chamber with a variable length is used to eliminate the error of the effective scattering path length. Velocity dependence of total cross sections has been obtained and glory undulations are observed for potassium-xenon systems. For argon, krypton, and xenon the van der Waals constants are calculated from the absolute total cross sections. The values of CK–Ar, CK–Kr, and CK–Xe are 237, 354, and 535×1060 erg·cm6, respectively, and are in good agreement with the theoretical ones of Dalgarno and Davison. For neon the velocity dependence of the total cross section can not be explained by the van der Waals potential, −C⁄rs, where s=6. For helium the correction factors for thermal motion of the scattering gas are large and the value of s for attractive potential can not be determined unambiguously.

Precise Measurements of Total Cross Sections for theScattering of Potassium Atomsby Rare Gases. (Part I)

The molecular beam apparatus used for the measurements of total collision cross sections of potassium and rare gases is shown in detail. A new type of the scattering chamber with a variable length is used to eliminate the error of the effective scattering length. To measure the pressure of the rare gases in the scattering chamber an ionization gauge is connected directly to the scattering chamber. The sensitivities of the ionization gauge for the rare gases are calibrated by a McLeod gauge. To eliminate the mercury drag effect in the calibration, the cut-off position of the McLeod gauge is cooled with dry ice. The magnitudes of the effect are in good agreement with those calculated by Takaishi's theory. Effective total cross sections for the scattering of potassium by He, Ne, Ar, Kr, and Xe have been measured in the velocity range, 400-1200 m/sec, of potassium beam. Corrections for the angular resolution are calculated by a general method derived by the present authors. The correction values are compared with those calculated with Busch's formula. To estimate true total cross sections, the effective total cross sections are corrected for the thermal motion of the scattering gases by means of the method of Berkling et al. The velocity dependences of the total cross sections in the potassium-argon, potassium-krypton, and potassium-xenon systems are shown to be explained by the Van der Waals potential, —C/γ5, where s=6. Glory undulations are observed for the potassium-xenon system. For neon the velocity dependence can not be explained by s=6. For helium the correction factors for the thermal motion of the scattering gas are large and the value of s can not be determined unambiguously. To estimate the Van der Waals constants from the total cross sections, the SLL approximation formula is used and the values of 231, 337, and 515×10-60erg·cm6 are obtained for CK-Ar, CK-Kr, and CK-Xe, respectively. These values are in good agreement with the theoretical ones of Dalgarno and Davison. For the potassium-xenon system the potential parameters of L-J(12, 6)are estimated from the glory undulations, giving 2.3×10-14 erg and 4.7Aforeandγm, respectively.

Safety enclosure for mercury filled McLeod Gauge.

The enclosure presented both protects the gauge and contains the mercury if breakage should occur.

Sublimation Pressures of Solid Ar, Kr, and Xe

Results of an experiment to measure the sublimation pressures of Ar, Kr, and Xe over wide temperature and pressure ranges are presented. Data are reported from near the respective triple points to about (2.3 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}6}$ Torr, 25.506 K) for Ar; (2.1 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ Torr, 45.130 K) for Kr; (3.8 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ Torr, 70.075 K) for Xe. Pressures were measured with a Hg manometer, a McLeod gauge, and a calibrated Bourdon gauge. The data have been corrected for thermomolecular flow and streaming. Temperatures were measured with a N.B.S.-calibrated Pt resistance thermometer. The application of the law of corresponding states is investigated. Values for static lattice energy, geometric mean of the lattice vibrational spectrum, heat of sublimation, and lattice vibrational energy are calculated using theoretical sublimation pressure curves.

A dynamic pressure reduction system used for the calibration of a mass spectrometer for argon

The probable errors involved in conventional calibration methods (ie the use of a pre-calibrated ionization gauge) are outlined and are shown to be extremely large. A dynamic pressure reduction system is described which allows absolute calibration in the pressure range 1 Nm−2 (~10−2 torr) to 10−6 Nm−2 (~70−5 torr). The reference standard of this system is a McLeod gauge having a sensitivity of 68 Nm−4 (5 The results of an argon calibration for a 60° deflection mass spectrometer are reported and the linearity of the instrument between 10−6 Nm−2 (~10−8- torr) and 10−3 Nm−2 (~10−5 torr) is demonstrated within the limits of experimental error.

Molecule–Atom Total Scattering Cross Sections: LiF on Ar, Kr, and N2

Total quantum-mechanical scattering cross sections of LiF on Ar, Kr, and N2 were measured as a function of collisional velocity in the thermal energy range, to about 1%–2% accuracy, through the use of circular beam geometry and a McLeod gauge to measure target gas density. Good absolute agreement with the Schiff–Landau–Lifshitz elastic scattering theory and the calculated 1/r6 long-range potential was obtained at only the lowest experimental velocities. At higher velocities the cross section is larger than predicted by this theory. No glory undulations were detected. Forces arising from the higher-order multipoles are insufficient to explain the results, but molecular protrusion correlates with the ordering of the deviations observed.

Fast Accurate McLeod Gauge System

The use of an orifice and a secondary mercury source near it as means for reducing the trap pumping error of the McLeod gauge is discussed. With the proper utilization of an orifice, trap pumping is not a serious hindrance to the use of the McLeod gauge as a primary pressure standard. Accuracy better than 1% with a time constant less than 30 sec can be achieved using a conventional McLeod gauge. Experiments that directly demonstrated a reduction of the trap pumping error gave good agreement with the theoretically expected reduction.

Accurate Pressure Measurements in the Region 0.1–1000 mTorr; The Intercalibration of Two McLeod Gauges and a Capacitance Manometer

The capacitance manometer appears to be a convenient and reliable pressure measuring device over the range 0.1–1000 mTorr. We find that deviations from linearity in this region do not exceed a few tenths of a percent. On the other hand, mercury streaming in a trapped McLeod gauge can introduce errors in the measured pressures which are dependent upon the pressure in a complicated manner.

Simple, contamination-free operation of a McLeod gauge

This note describes a method of operating McLeod gauges which avoids the use of a rotary oil pump and the associated risk of contamination. The method relies on the use of liquid nitrogen to produce a change of state of a suitable inert gas.

An Accurate Measurement of the Mercury Vapour Drag Effect in the Pressure Region of Transition Flow

The pressure differences caused by the mercury vapour stream from a source to a trap have accurately been measured with a diaphragm gauge. The measurements have been made for nitrogen, krypton, and xenon at a temperature of 28.0 + 0. 1°C in the pressure range from 10-3 to 10-1 Torr. All the data obtained have been found to fall well on a curve predicted with a proper value of parameter peculiar to a given system. It has experimentally been assured that the sufficient prediction is possible to make on the relative errors in low pressure measurements with McLeod gauge if proper values on the molecular parameters are available.

Molecular Velocity Determination through Gas Effusion: Intermediate Laboratory Experiment

An experiment is described to determine the average velocity of gas molecules in a container while effusion takes place through a small hole into vacuum. When the mean free path of the molecules becomes large compared to the hole diameter, the measurement of pressure as a function of time permits the average velocity to be extracted. The equipment and procedure are simple, and an agreement to within 10% of the kinetic theory of gas value has been obtained using an inexpensive McLeod gauge. The experiment has been performed using air, helium, and argon; measurements on air and helium were performed both at room and dry ice temperatures. A formula has been derived to take into account the effect of outgassing of the container walls at low pressures. Average velocities may be extracted either by a hand calculation between pairs of data points, or by using a short computer program for least-squares analysis. A derivation is given in an Appendix for the effect on the effusion of the hole being drilled in a wall of thickness small, but not negligible, compared to the hole diameter. During the initial pump-down period, while the container pressure is still comparable to atmospheric, the pressure-versus-time data can be analyzed in terms of hydrodynamic theory and used to deduce a virtual hole area for the streaming molecules. Measurements of pressure-increase with time in a sealed-off system gave poor agreement with the rate of outgassing needed to fit the effusion data.

On elimination of the mercury pumping error effect in Mcleod gauges

A simple device consisting of a magnetic ball valve and condenser can be added to McLeod gauge manometers to obstruct the flow of mercury vapour from reservoir to cold trap thereby eliminating the pressure gradient between gauge and trap sustained by mercury streaming in conventional systems. The theory of the pumping error is considered, and a generalized low pressure diffusion model, of which the models of Takaishi and Gaede are limiting cases, is presented. Comparison of the modified gauge with a conventional one shows that the pumping error falls off more rapidly with increasing pressures than any of the models predict, but it approaches the high pressure limiting values obtained by taking the gradient to be constant in the region of Poiseuille flow.

1249. Error due to the meniscus volume in measuring low pressures with a McLeod gauge: K Nakayama, J Vac Soc Japan, 11 (5), 1968, 156–164 ( in Japanese)

1249. Error due to the meniscus volume in measuring low pressures with a McLeod gauge: K Nakayama, J Vac Soc Japan, 11 (5), 1968, 156–164 ( in Japanese)

An Accurate Measurement of the Mercury Vapour Drag Effect in the Pressure Region of Transition Flow

The pressure differences caused by the mercury vapour stream from a source to a trap have accurately been measured with a diaphragm gauge. The measurements have been made for nitrogen, krypton, and xenon at a temperature of 28.0±0.1°C in the pressure range from 10-3 to 10-1 Torr. All the data obtained have been found to fall well on a curve predicted with a proper value of parameter peculiar to a given system. It has experimentally been assured that the sufficient prediction is possible to make on the relative errors in low pressure measurements with McLeod gauges if proper values on the molecular parameters are available. Diffusion constants deduced from the experimental data are in good agreement with the theoretical expected values.

830. An improved method of sealing the compression capillary of a McLeod gauge: G R Reid, J sci instrum, (J phys E, ser 2, 1 (1), 1968, 60–61.

830. An improved method of sealing the compression capillary of a McLeod gauge: G R Reid, J sci instrum, (J phys E, ser 2, 1 (1), 1968, 60–61.

Dissociation Pressures of Various Metallic Oxides

Abstract Gases evolved during a vacuum heat treatment of various oxides were analyzed by means of an omegatron mass spectrometer. Carbon dioxide, carbon monoxide and water were detected as main gases evolved at lower temperatures. Oxygen due to thermal dissociation of ZnO, CdO, NiO, CuO, Cu2O, and WO3 was found at high temperatures. In the case of MoO3, however, evolution of oxygen was not observed. The equilibrium pressures of the evolved gases over these oxides have been measured with a McLeod gauge and a Knudsen gauge. The oxygen dissociation pressures of ZnO, CdO, NiO, CuO, and Cu2O were measured and compared with the calculated values.

A greaseless valve for use in mercury reservoirs

A greaseless valve suitable for controlling mercury from a reservoir to a high vacuum system is described. The valve has positive absolute shut-off and is capable of fine control. The construction is simple and adaptable for use with manometers, McLeod gauges, Toepler pumps and cut-offs.

The McLeod gauge vapour stream effect: R S Dadson, K W T Elliott and Daphne M Woodman, Division of Quantum Metrology, National Physical Laboratory, Teddington, Middlesex, England

The McLeod gauge vapour stream effect: R S Dadson, K W T Elliott and Daphne M Woodman, Division of Quantum Metrology, National Physical Laboratory, Teddington, Middlesex, England

Capillary depression in McLeod gauges

A source of the irreproducibility in measuring low pressure with a McLeod gauge is described. Particular attention has been paid to the capillary phenomena in narrow tubes. The form of a meridional section of a mercury meniscus in cylindrical tube has been calculated with a high-speed digital computer from the Laplace's differential equation which has an entirely theoretical basis. Experimental data for capillary tubes differing in material, bore, and condition of inner surface have been compared with the computed values. It is observed that the capillary depression extensively varies, depending presumably on the surface condition of glass. Cancellation of the capillary depression, therefore, can be hardly expected even in the conventional McLeod gauges having twin capillaries. Tapping is found to be effective in reducing the irregular variation of the capillary depression.