Topics in..
. Chemical instrumentation
Edited by 5. Z. LEWIN, N e w York Universify, New York 3, N. Y .
These articles, most of which are to be contributed by guest authors, are intended to serve the readers of this JOURNALby calling attention to new developments i n the theory, design, or availability of chemical labwatwy instrumenlation, oi by presenting useful insights and explanations of topics that are of practical importance to those who use, or teach the use of, modern instrumentation and instrumental techniques.
XII.
Pressure Measurement. Part Two Seymour T. Zenchelsky, School of Chemistry, Rutgers University New
Bourdon tubes areless sensitive than the other mechanical pressure-sensing devices, and are normally used for measurements above one-atmosphere (approximately 760 mm Hg). Direct-reading gages are available from many laboratorysupply companies, with Bourdon tubes made of various metals and allovs (Fie. " 22). More sensitive Bourdan-gages, made of glass or quartz, are available from fabricators of custom laboratw-glassware. The Wordengage (Worden Laboratories, Houston 27, Texas) consists of a quartz helix which possesses a sensitivity of 360-degrees of per atmosphere
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The preceding installment dealt with liquid-column gages; this one treats meehsnical and thermal gages.
Mechanical Gages
and the gas whose pressure is being measured. I n the case of mercury, the solid products of such reactions frequently deposit on the inner walls of the tubesand reduce the accuracy of measurement. This in t,he reason for the ereat concern with c.t~r-nf-~lwning 5 fc.iture I" the h i q of 3ud1,lev,led, ,,.lV8l g>,,w, 0" 11,v o t l w I~:~ncl, diwiwitc t1.c: ~c,:mo~t~t~triv lisluid, exposing to the gas a solid surface which can be metal, glass, or quartz. Such gages depend upon the distortion of the solid surface when unequal pressures are exerted an its two sides, thus these gages may be used as either absolute or differential manometers. Since a t equilibrium the farces acting on both aides of the surface must be equal, i t follows that the sensitivity of the gage will increase w ~ t hincreased surface area. Four types have been used: (1) the Bourdon tube, ('2) the bellows, (3) the capsule, and (4) the diaphragm. These are illustrated in Figures 15, 16, 17, 18, and 19. For sufficiently large pressure-differentials, the distortion may be measured hy means of the displacement of a pointer which is linked mechanicnlly to the surface, otherwise displacement transducers are needed. B o z ~ r r hTuhe. The basic Bourdon tube consists of a. leneth of tuhins. in -. elliutical . cross-section, which is bent into an arc and sealed a t one end. If the open end is connected to a source of high pressure, the cross-section of the tube will tend to hecome more nearly circular, resulting in an increase of the radius-of-curvature of the arc. Thus, if the open end of the tuhe is constrained, the closed end will be displaced. Greater sensitivity is obtained by increasing the lengt,h (+
Figure 15.
Wdloce and Tiernon Bourdon-tube gage mechmirm.
tube in order to increase its area, but space is conserved by winding the tuhe into a spiral or helix (Fig. 20 and 21). The effect of barometric pressure (on the outside of the Bourdon tube) is either compensated by the use of a second Bourdon tube, which is open to the atmosphere, or eliminated by sealing the instrument case a t fixed pressure.
(Fig. 23). This motion may be followed either by observing an attached pointer through microscope or by observing the displacement of a light-beam which is reflected from an attached mirror. The latter method provides greater sensit,ivity through the available optical leverage. ,
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Chemical instrumentation
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.F,.- heatedwrfaw is required. Ambient-tempernture rompensst,ion can be r h t e i n d I,? using :L serond sensing-resistor, sealed in a good vacuum, in a n adjacent nrnl u l the bridge. The readout consists either nf tlrc hridgc-unbalance voltage or of the current necessary to rest,ore the bridge to halimt,~. The latter may be acconlplisli~d wutc,matirally hp means of a ieedhnck network or servo system. These gages genrr:tIIy rover the range, 1-1000 microns.
Figure 36.
Co8lrolidoted Vacuum Pironi gage.
Consolidated Vacuum Corporation 8111,plies several models, including the unbalanced-bridge type as well as the selfbalancing-bridge type. They are available ior single or multiple-station operation as well as for recording and control (Fir. 36), wit,), sensing tubes made of eitlwr metal or glass. Other Pirani gages R I P available from several sources: N R C Equipment Corporation; Hastings-Rs>-dist, h e . ; Jelrus (available from WilkensAnderson Co., Chicago 56, Illinois). The find section of this article will dexl with ionization gages. A hibliogrspl~y will be included with that section. I A ~ ~ w ~ n n s oE. ? . ,A,, J . Sn. Z,islr.., 7, 217 (l!I311).
Volume 40, Number 7 0, October 7 963
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