A precision McLeod Gage for volumetric gas measurement

A Precision McLeod Gage f o r V o l u m e t r i c Gas M e a s u r e m e n t H. H. P O D G U R S K I and F. N. DAVIS Edgar C. Bah* Laboratory for Fundamental Research, United States Steel Corporation Research Center, Monroeville, Pennsylvania, U.S.A. The modified McLeod Gage described is designed to measure pressures between 10-3 and 10 -4 Torr with an accuracy o f 4-2 per cent. A large-bore side arm isolated from the gage by a cut-off replaces the usual open-end capillary. This feature provides the means for calibrating the variations in capillary depression o f mercury at every position along the length o f the remaining capillary without contributing to the volume o f the gage. A tapered plug used to seal the capillary end eliminates sticking even at 10-7 Torr and also extends the range o f stable calibration in the ground capillary bore up to the end seal, thus extending the usefulness o f the gage to a lower pressure limit. With these innovations, a gage o f 200 ml capacity with a capillary o f one-half ram bore was found adequate for the accuracy sought. Solution o f helium in the pyrex capillary accounts for errors estimated between 1 and 3 per cent. ; hence .for volumetric calibration neon is preferable to helium.

Introduction

means (turbulence) may well occur to a greater degree at the interface experiencing greater changes in area. Presumably a greater contamination level resulted at the meniscus in the sealed-end capillary above the gage bulb and the wetting of the capillary wall by mercury was enhanced. With no reference point available in the conventional gage by which to calibrate the wetting behavior in either capillary, no dependable corrections could be made. The new gage design depicted in Fig. 1 solved these problems. The basic contributions from this investigation are : (1) Establishment of the need for calibration curves describing the wetting character of mercury at every position in the gage capillary. (2) Isolation of the large dead space or volume of the reference arm replacing the open-end capillary. (3) Use of a tapered plug seal on.capillaries ; this eliminates sticking and extends the working range of the gage to lower pressures. (4) Demonstration of helium solubility as the source of error when measuring helium pressures.

An error of 5 per cent or more can be expected when pressures are measured at 10 -4 Tort with a conventional McLeod gage having two matched capillaries and a one liter c a p a c i t y ; y e t this gage is frequently employed to calibrate ion gages down to pressures approaching 10 -6 Torr. Such calibrations must be in considerable error. When the atmosphere in the system co/lsists essentially of helium, more than 1 per cent of the gas sample compressed in the gage dissolves in the pyrex capillary during the time interval taken to measure the pressure. This constitutes one of the known sources of error. The variation in capillary depression of mercury noted at different positions along the length of a gage capillary proves of greater concern. Working below 10-6 Torr with a gage equipped with matched precision bore capillaries (ground), the mercury level was observed to rise to a higher rest position in the sealed end capillary than in the matched open-end capillary. Furthermore, the difference in level varied with rest positions taken along the length of the capillary bore. If this source of error is neglected, the measured pressure values will be too low, conceivably by about 20 per cent (at 10 -4 Torr). When attempts were made to minimize contamination in the mercury, the effect became more pronounced. We believe that a difference in concentration of trace impurities at the surface of the two menisci was produced by the very nature of the gage operation. The mercury-vacuum interface is extended as it enters into the gage bulb and drops several thousandfold as it passes from the bulb into the capillary above i t ; t h i s large extension and compression of the mercury-gas interface does not occur in the meniscus entering the adjacent capillary. The concentration of soluble or insoluble impurities through adsorption or by mechanical

Gage design and assembly A survey of the literaturerevealed a gage design (adopted by Keevil, Errington, and N e w m a n 1 for use as a microgas burette) in which the capillary depression could be measured and corrected for, i.e.,a reference existed for calibrating the wetting character of the capillary tube. The open-end capillary was replaced by a large-bore tube in which the mercury level in any rest position was independent of its wetting character. With this modification Kecvil et al determined an average depression* of the mercury level in the closed-end capillary relative to the mercury meniscus position in the large-bore tube when the gage was completely

*Established with helium in gage (graphical method). 377

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evacuated. They then applied this single correction to the pressure measurement of the helium gas sample captured in the burette or gage (one liter capacity). As will be shown later, a single average correction was found to be inadequate in our work. Furthermore, the large-bore side arm also contributes an undesirable dead space. In the design that we finally adopted (Fig. 1), this difficulty was overcome by isolating the large tube with the cut-off arrangement shown between position C and N. It remained to establish a wetting calibration curve for the closed-end capillary, i.e., a capillary depression value for every position along the capillary measured at very low pressures ( < 10-6 Tort), and to demonstrate that the corrections afforded by this curve were valid at measurable pressures. Another feature added to the new gage is the end seal on the capillary. The significance of this change will be appreciated after the pressure determination procedure has been discussed.

Gage calibration and operation In normal operation, arm A in the modified gage (Fig. 1) is free of gas (P <10-6 Torr) and the mercury level is raised from position N. From a single value of AR~(cor)* a pressure can be calculated according to conventional methods. As previously stated, the correction, eli, for a particular Ri (any position in the capillary read on Scale B) has been found to vary along the length of the capillary ; hence, the calibration curves of the sort illustrated in Figs. 2A and B were obtained. To insure their usefulness they are frequently checked for time stability. More will be said about this point later. When corrected ARi values are used, the error in pressure determination is reduced to reading errors. If these errors can be assumed to be random in nature, several compression readings in a single pressure determination will increase the accuracy. To take full advantage of multiple compression readings, the scale position (Ri) is graphed vs. the reciprocal of

*dR~(cor) = AR~ --d~. AR~ is the differencebetween the meniscus levels in arm A and arm B ; di is the capillary depression correction.

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ARi(cor) as in Fig. 3. The corrections, di, now a function of Ri, are read from a calibration curve. The slope, C, of the graph need only be multiplied by the gage factor, F, to yield the absolute pressure*. Although the procedure is time consuming when first employed, many time saving steps may be taken once certain ]gage characteristics have been evaluated. Much of this information is obtained from the same graphs. The most regular and most time-stable calibration curves were obtained in precision-bore capillaries with internal surfaces ground according to the method suggested by Rosenberg 2. Annealing a capillary above 525°C destroys the benefit afforded by previous preparation. So, sealing the capillary end by fusion after grinding destroys the calibration stability some distance from the point of the seal. The effect obviously is quite undesirable as it does not allow reliable high compression values of ARi(cor) to be obtained in the low pressure range of the gage. To effect this end seal, as illustrated in Fig. 1, a drawn tapered pyrex rod is inserted into the end of the capillary and fused only at its end. Thus sealed, an unannealed region for calibration is retained up to the position Re. The value of this procedure is reflected by the stability and uniformity of the calibration curves in the vicinity of Re. See Figs. 2A and B. N o time stability was attainable in the calibration curve near Re, regardless of tube diameter, for the conventional end seal. The volume of the annular space about the plug is not

neglected, however, as the extrapolation of the straight line through the ordinate Re yields Re(app) rather than Ri'~. Re(app) must be established with certainty inasmuch as it contributes to the gas volume in the capillary above the mercury meniscus. Once established, fewer compression readings are needed to calculate a pressure accurately. This is particularly true when the gage is employed at its upper pressure limit and when a single compression point is taken. Graphing compression data, as in Fig. 3, although time consuming, does reveal several interesting points which otherwise might remain unnoticed. Straight line curves intersecting the ordinate at the same Re(app) for all gases establish unambiguously the applicability of the calibration curve for correcting ARi values. Deviations from linearity indicated by the dotted curve in Fig. 3 suggest immediately the need for re-establishing the calibration curve. One exception to this test arises if the mercury is moved up into the capillary too rapidly during compression of a gas sample. Under this condition of gage operation an electric charge:; builds up in the gage and nonlinearity results. The effect can be eliminated by allowing the mercury to rise slowly to the positions Ri. Experience will establish a reasonable rate to be used. Since the rate of rise is necessarily slow the practice of admitting pure nitrogen slowly into the mercury reservoir via a fine needle valve was adopted. To check the validity of corrections afforded by calibration curves, volumetric calibrations were conducted with neon

* F r o m the ideal gas law [Re(app) --Ri! × ARi(cor) = C where [Re(app) --Rt] is p r o p o r t i o n a l to the volume of gas compressed in the capillary at pressure ARi(cor). System pressure = C × F, where C = slope and F (gage factor) = ~r × (radius of capillary)Z/(gage volume). Re(app) is the a p p a r e n t position o f the end seal in the capillary on Scale B (established by the point of intercept on the ordinate of the curve in graphs o f the type illustrated in Fig. 3). ~Re and Re(app) will nearly coincide if the capillary tube is very uniform and the a n n u l a r space a b o u t the plug is kept small. ~A mercury t h e r m o m e t e r suspended near the gage will be attracted to the gage a r m s and held there when the mercury level is m o v e d rapidly up and down in the gage. An electrical discharge can be observed at certain gas pressures in the gage capillary with similar m o v e m e n t of the mercury level.

380

H. H. PODGURSKIAND F. N. DAVIS

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effects outgassing. Prior to raising the furnace temperature to 400°C, the mercury contained in the gage reservoir is heated while both the gage and reservoir are evacuated, care being taken to condense and return the volatilized mercury to the reservoir. Finally, the mercury level is always adjusted in the gage by pressurizing the reservoir with clean dry nitrogen to avoid unnecessary contamination of the system under normal operation. The use of a 1 per cent H F solution as a cleansing agent proved disastrous even after extensive rinsing with distilled water. The pyrex glass cleaned in this manner " r e a c t e d " with mercury vapor during the bake-out operation as was evidenced by the enhanced and irregular wetting of the capillary wall by the mercury during calibration. The mercury actually was observed to rise higher in a ½mm capillary bore than in an 18 mm bore side arm and in a very erratic manner.

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and xenon gas at system pressures between 10-4 and 10-2 Torr corersponding to gas pressures in the gage between 5 and 250 Torr during measurement. The calibration of a glass burette gave values within 0.1 per cent of the volume subsequently determined with mercury. The graphs of the compression data (as in Fig. 3) for the neon and xenon were linear and extrapolated to the same Re in all instances. On the basis of gas adsorption theory these results leave no doubt that the calibration curves for mercury wetting of the capillary wall obtained at pressures below 10-6 Tort were valid with gas in the gage.

Gageconditioning Several cleaning procedures were investigated to prepare the gage for final calibration : two bear mentioning. The one adopted entailed the use of concentrated nitric acid followed by a thorough rinsing with distilled water. The gage, which (except for the mercury reservoir) is housed in an oven, is baked at 400°C. While baking, with the mercury level set at position C, the gage is alternately evacuated and dosed with oxygen (Po2--~5 Torr). This operation burns out nonvolatile carbonaceous materials from the system and

solution

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Preliminary calculations* on the possibility of obtaining a significant loss of helium by solution or diffusion into or through pyrex glass when measuring pressures in a McLeod Gage did not reveal cause for-concern. Experimental observations did not confirm this conclusion. Very pure helium gas samples (measured in the closed-end capillary), when expanded into the closed evacuated manifold of our adsorption apparatus for dead-space determinations, gave values about 1--~ per cent lower than when neon and krypton were employed. Agreement existed between the values obtained with the latter two gases. Apparently, helium was disappearing from our system because of solution in the pyrex capillary during the interval between the two compression operations when measuring the two consecutive pressures. The minimum error expected in a single pressure measurement must be greater than 1 - i per cent, but could not be determined directly. The helium solution rate subsequent to the first pressure reading was measured by noting the loss of pressure with time. The results of the measurements are graphed in Fig. 4. The dashed curve was calculated by using the solubility value and diffusion constant for helium given by Alpert and Buritz3. The calculation was made using an equation4 for the conditions of nonsteady state diffusion through a cylindrical surface into an infinite medium. The observed amount of helium dissolved in the pyrex capillary over the first ten minutes, the shortest interval taken for a single pressure determination, is an order of magnitude lower than necessary to account for the estimated 1.5-3 per cent error in dead space determinations. Undoubtedly the initial rate of helium solution in pyrex depends upon the surface roughness and surface strains in the capillary under consideration and is not revealed in long-time experiments where the diffusion rate is established by permeation through a glass membrane. The inner surfaces of our capillaries were ground and not annealed. Recently, McAfees reported that tensile stresses increase the permeability of pyrex glass membranes to helium. The disappearance of helium from the gage could not, under

*Calculations based on solubility and diffusion constant determined by Alpert and Buritz3.

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FIG. 4. any circumstances, be attributed to the presence of gas impurities. The precision claimed for this modified gage applies only when pressures are measured in a closed system saturated with mercury vapor. A symmetrical* refrigerated U trap separating a mercury-saturated atmosphere f r o m one free of mercury can produce a pressure gradient across the t r a p ; the higher pressure develops in the mercury-free region. The magnitude o f this effect is dependent u p o n the gas in the system. W e have had to assume on occasion pressure differentials o f 6 ~ 2 per cent with xenon and 2 -4- 1 per cent with argon in order to account for the distribution of these gases between those parts o f a system separated by

U traps. A value of 1 per cent is calculated employing equation 6, page 178, Vacuum Technique, by S. Dushman6 for argon in a system with a pipe diameter 0.15 cm between the gage and U trap. References

z N. G. Keevil, R. F. Errington, and L. T. Newnam, Rev. Sci. Instrum., 12, 609-611 (1941). 2 p. Rosenberg ; Rev. Sci. Instrum., 9, 258-259 (1938). 3 D. All,err and R. S. Buritz ; J. Appl. Phys., 25, 202-209 (1954). 4H. S. Carslaw and J. C. Jaeger ; Conduction of Heat in Solids, p. 282, Oxford Press (1947). 5 K. B. McAfee ; J'. Chem. Phys., 28, 218 (1958). 6 S. Dushman ; Vacuum Technique, John Wiley & Sons, Inc., New York.

*Symmetrical U traps used to avoid the development of plessure gradients attributable to the thermal transpiration effect,