Topics in..
. Chemical Instrumentation
Edited b y S. Z. LEWIN, New York University, N e w York 3, N. Y.
These articles, most of which are to be contributed by guest authors, are intended to serue the readers of lhis JOURNALby calling allenlion lo new developments i n the theory, design, or availabilily of chemical laboratory instrumenlalion, oi by presenling useful insights and explanations of topics lhat are of practical imporlance to lhose who use, or teach lhe use of, modern inslrumenlalion and inslrumalal techniques.
XII. Pressure Measurement.
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Part One
Seymour T. Zenchelsky, School o f Chemistry, Rutgers University, New Brunswick, N. I . V;trk,os devices are used for themeasurernent of pressure, depending chiefly an the pressure range of interest and the precision desired. Since most work in rhemistry involves pressures below s t rnospheric, this series will restrict itself accordingly. Despite this fact, the numher of pressure-measuring devices is still quite large (I). Thus it will be necessary t o further limit these articles to those dwices which are readily available commereixlly. Obviously, a very useful classification would be based upon the applicable pressure range, such as is shown in Figure 1. However, in the interests of
distortion of a solid surface and is usually followed by observing the displacement of a pointer attached to it. This is the Bourdon-gauge principle, and many designs based upon it have been described. However, most numerous are those devices w-hieh depend upon observation of the kinetie-molecular behavior of gases (I). This third method utilizes sucll properties of gases as viscosity, heat conductance, ionization, or compressibility, as a means far deducing the pressure. Of the three methods, only the last one is absolute; i.e., the pressure may he calculated from the gas properties if all the required parameten are known. I n practice, however, these are not slways available. On the other hand, while the first two are reallv differential
a very good approximation in practice.
Liquid-Column Gages
Figure 1. Ranges of applicability of commonly "red types of pressure-measuring devices lopproximotel: (11 U-tube Manometerr; 12) Mechanical Goger; (31 Thermal Gage.; 141 Mcleod Gages; (51 Ionization Goger.
orderly exposition and the avoidance of repetition, the fallowing discussion will be arranged primarily according to the principle of measurement involved. Three basic methods have been used. The first consists of observing the height of a liquid column, ususlly mercury. A very familiar example is the simple U-tube manometer, but many variations are known. Tho second method involves the
U-tube Manometer. The difference between the levels of two interconnected columns of liquid is directly proportional to the difference between the pressures exerted upon them, assuming equal eapillsry effects in both tubes. Thua, by canneet,ing one end (the reference end) of the U tube to a vacuum (zero pressure), the manometer becomes an absolute device. In fact, for practical purposes, it is only necessary that the pressure a t the reference end be very much smaller than t,he one being measured a t the other end. But this goal iis impossible to achieve if a. manometric liquid with high vapor pressure is used, particularly if verv low oregeures are t o be measured.
In this seriesaccording to common practice-the unit of zbsolute pressure will be taken as that exerted by a one-
5. T. Zenchelrky is Associate Profersor ol Chemistry at Rutger. University. He W.! educoted at New YorkUniverrity (0.5.. 1941 MS., 1947; Ph.0.. 19521. Heleaches m c doer rerearch in molylical and phyrico chemirtry.
millimeber column of mercury under standard condit,ions and will be designated hy the symbol, mm Hg. In operational terms, this unit eorrespmds to a difference in eolunm-heights of one millimeter of mercury. If another manometric liquid is used (assuming that its vapor pressure is negligible), the diference in ohserved column-heights, in millimeters, can be converted to mm Hg by multiplying by the factor, Densityti,id/Density~~. The vapor pressure of mercury is 10-5 mm Hg a t 20°C; that f o r water is 17.5 mm Hg at, 20PC. Commercially available, absolute Utube manometers have one end sealed off and are filled with mercury under "vacuum" (less than 0.01 mm Hg). These as well as the open-end (difierential) type are available from almost all laboratory-supply houses and manufacturers of chemical glassware. One disadvantage of elosed-end manometers is the fact that cleaning and refilling with mercury are quite difficult. For this reason, several designs are available which permit simple opening and positive resealing of the reference end. I n the RGI Manometric Gage (Roger Gilmont Industries, Inc., New York, N. Y.), mercury is drawn up into the reference end which is then closed off by (Continued on page A612)
Volume 40, Number 9, September 1963
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Chemical Instrumentation means of a lunge^ aa shown in Figure 2. With the reference end open, the same device serves as a differential manometer. Tt, i s eraduated in millimeters aver a iemgth ;f 200 mm. The Zimmerli Gage (Scientific Glass Apparatus Co., Inc., Bloomlield, N. J.), shown in Figure 3, consists of a pair of U-tubes with adjacent ends connected.
URY SEAL
EM A
Fla. 2.
RGI Positive Closed-End Manometric
Gose
Figure 3. SGA Zimmerli Gage (with "vocuum" between the U-tubes for absolute-pressure meururementl.
(Continued on page A f i l 4 )
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Chemical Instrumentation I t is filled, under vacuum, in the horizontal position so that a continuum of mermry is present in all four legs (Fig. 4). When placed in the vertical position, t h e aontinuum is automatioally broken between the U-tubes, leaving an evacuated space as shown in Figure 3. Upon opening the gage to the atmosphere, the continuum is reformed (Fig. 4). For ahsolute meas-
Figure 4. SGA Zimmerli Goge (with mercury mntinuurn-in the horizontal position during Rllingl.
urements, both ends are connected t o the aystem and t h e right-hand U-tube is read. The function of the left-hand U-tube is merely that of sealing off the reference end of the right-hand U-tuhe. For differential measurements, only one U-tube is filled with mercury in the conventional manner. Several models are avai1nl)le; the Precision Model has a range of lOl)mm, readshle to 0.1 mm.
Figure 5. Monostot Absolute and Differential Mamrnekr.
(Conlinued o n page AGlfi)
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Chemical Instrumentation A very similar design is available as the Cenco 94122 Manometer (Central Scientific Co., Chicago, Ill.), but this device cannot be used for differential measurements. Still another design, utilizing a variation of the same principle, is found in the Manastat Manometer (Manostat. Corp., New York, N. Y.) which is shown in Figure 5. I t is available in two ranges, 0-200 mm and 0 3 0 0 mm, both readable to 0.1 mm. An increase in sensitivity would permit more precise differential readings as well as the measurement of lower absolute pressures (provided that the "vacuum" a t the reference end were very good). Toward this goal, numerous devices have heen used, including cathetometers, optioal levers, and displacement transducers of various kinds (I). Same of these are available--or may be obtained on special elassorder--from makers of ~reeisian " -~~ ware, such as Eck and Krebs (Long Island City, N. Y . ) The RGI Micrometric Manometer, which is shown in Figure 6 , ~
~
Figure 6.
.~
RGI Micrometric Monometer.
employs a micrometer drive to bring a stainless-steel needle into accurate contact with the top of a mercwy column. Changes in the mercury level can then be estimated to the nearest 5 X lo-' inches (1.3 X 10-2 mm). The total range i~ approximately 50 mm. Of eaune, the sensitivits- of all of the preceding devices can be increased by using a. manometric liquid of lower density, subject to the limitation on vapor pressure previously mentioned. Cartesian Manometer. Dubrovin ( d ) has described a manometer based on the Cartesian-diver principle. Such gages consist of a mercury-containing outer tube in which there floats a thin-walled glass tube. The float is open at the
(Confinued on page A618)
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Chemical instrumentation bottom and is rentered in the outer tuhe by means of loose-fitting guides. If bath the Hoat and the outer tube are evacuated, the t,op of the Hoat will he near the zero mark of Figure 7. If the outer tuhe is
Figure 7.
Manortot torlesion Monometer.
then opened t,o the atmosphere, tile mercury will rise into the float, tllereb>lowering its buoyancy and causing it t o sink to the bottom of the outer tube. G e m a n n and Gagos (3) have given a therrretieal treatment of the principle of uperabion. They showed t,hat the linear displacement of the float depends upon its diameter and wall thickness. Thus, for very thin-walled Hoats, it is possibl~ to obtain a displacement ai 100 mm for a pressure change of only 10 mm Hg. Such gages are available from Roger Gilmont Industries and from Manostat Curp. The former features easy Zen,setting while the latter is usahle for both dikrential and absolute pressure measurement. Both of these gages present the pressure range, 0-10 mm Hg, on a. scale of 100 mm which is graduated in onemillimeter divisions. A very sensitive version of this device is the Betr Micrnmanometer (Znslrumenlfabriek Van Essm N.V., Delft, available from Epic, Inr:., New York, N. Y.). I t utilizes water as t,he manometric liquid, an extremely thinwalled float, and optical projection irr further magnification oi 20 diameters. Two models are available, one covering the range 0-250 mm H 2 0 (approx. I!) mm Hg) and the other covering t,he range ( C o n t i n l d on pagP Afi30)
Chemical Instrumentation 0-500 mm H,O (appmx. 38 mm Hg). The farmer can be estimated t o the nearest 0.001 mm Hg and the latter t o the nearest 0.002 mm Hg. Since the vapor pressure of water a t 20DC is about 17 mm Hg, this instrument is useful only as a differential device. McLeod Gage. Although the height of a liquid column is observed, the McLeod gage aitctuslly belongs t o the third .rabegory of pressure~lueasuringdevirest.hose depending on the kinetie-malerular behavior of gases. It is thus a true absolute-device. As shown in Figure 8,
Figvre 8. Gage.
Manom? High Precision Mdeod
the gage consists of a mercury-reservoir flask connected t o the vacuum system by means of a large-diameter tube (left,). This tube contains a capillary bypass (center tube) and is also connected t o s bulb (right) which terminates in a closed-
(Conlintred on page A622)
Chemical instrumentation end capillary. The bypass has the same bore s s the closed-end capillary, and its sole function is that of cancelling the effects of capillarity on the measurement. I n operation, a known volume of gas at t,he system pressure is compressed to a much smaller known volume and the resulting change in pressure is determined by observing the difference in mercurglevels between the bypass and closed-end capillaries. The system pressure mn then he calculated by means of Royle's law. Usually, however, the closed-end capillary is graduated directly in absolute pressure. Because the McLeod gage depends upon compression, the pressure of eondensahle vapors, e.g., water, ammonia, o r mercury, cannot be measured. Since the initial volume is determined chiefly hy the size of t,he bulh, and the final volume by the size of the closed-en11 capillary, the range of measurement can be altered by altering the sizes of either or both of those parts. McLeod gages are available in various ranges from s u p pliers of chemical glassware, who will also custom-fabricate special designs. Typical ranges are: 5 X 10.' t o 5 X 10-2 mm Hg, lo-' t o 10-' mm Hg, and 10-2 t o 20 mm Hg. The scale is nunlinear because the measured pressure is proportional t o the square of the difference in mercury levels; but by means of a slight variation in the measuring pmcedure, a portion of the range can he presented an a linear scale. Thus on the Maoostat High Precision McLeod Gage (Fig. 8) which has a square scale covering the range 5 X lo-' t o 2 X 10-' mm Hg, there is also a linear scale covering the range 5 X lo-' t o 2 X 10-I mm Hg. In the conventional McLeod gage, mercury is kept from rising into the largediameter tube and capillaries by cnnneeting the reservoir-flask t,o an auxiliary vacuum. Then the mercur." is permitted t o rise by bleeding air into the flask, thus compressing the gas in the closed-end capillary t o its final known volume.
Figure 9.
Manottot Tilting M d e o d Gage.
(Continued on page A626)
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