John T. Viola and William E. McDermott United States Air Force Academy USAF Academy. Colorado 80840
II
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A Recording Mercury Manometer
On occasion i t is dksirable to record a pressure that is varvine a t a rate too ereat to observe by means of a static " column of mercury, e.g., a mercury manometer. This paper describes the design and construction of a system which responds directly to rapid pressure changes and allows a recordine of the response on a strip-chart recorder. The device is simirar in concept to a recording manometer designed by Svec and Gibbs.' The technique employs a thin wire2 of relatively uniform resistance throughout its length centered axially in a U-tube containing mercury (Fig. 1). A constant source of dc voltage provides a current through the wire, which acts as one leg of a Wheatstone bridge (Fig. 2). The bridge is balanced when the mercury level is quiescent (VAZ= 0, Fig. 2). Pressure changes produce changesin the mercury level which contacts a new length of wire, 1. This results in a resistance change which causes the bridge to become unbalanced, producing an observed voltage drop ( V k ) that is proportional to the length of wire exposed to the mercury, which is in turn proportional to the pressure change. This is shown by the following analysis
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In Figure 2 the currents in the two branches of the bridge
I,, and I h are given by eqns. (1) and (2) where E = constant dc voltage and 124 is the total resistance of the slide wire. The voltage drop (VM) seen by the detector is given by eqn. (3). VAZ= Ih(562
+ A ) + IaRI
(3)
where A is the resistance of the portion of the slide wire to the contact, A. By substituting eqns. (1) and (2) into eqn. (3), we obtain eqn. (4).
If R2 >> Rr and the other resistances are constant, then eqn. (4) reduces to eqn. (5). VAZ= C - C'R,
where C, C' are constants.
(5)
Figure 2. Wheatstone bridge circuit containing wire-mercury resistance eie ment.
The observed voltaee dron is therefore linearly . ~roportional - to R,, the resistance of the Are, which is linearly proportional to the ex~osed leneth of wire. which in turn varies lmearlv with The volGge may bk observed with a multimeter and tbroueh analoe output to a recorder. ~ h e s ~ s t e mcalibrated & easily by inducing a displacement of the mercuw in the U-tube, measuring the difference in height of the two mercury coiumns, and noting the correspondingvoltage on the recorder. The voltage output of the power supply is adjusted to produce full scale recorder deflection a t the maximum expected pressure Thr system can be cahhra;ed dynamically hy expanding air into an evacuated chamber that 1s connected tu the manometer, then pumping out the chamber at a controlled rate as the pressure is observed and recorded. In a specific application the device was used to determine the rate of absorption of chlorine onto a surface coated with aqueous sodium hydroxide. The recording manometer was used as a closed-end manometer by pumpingon one leg of the mercurv. " The other lee containine the wire was exwsed to the absorption chamber which also was pumped. The absorption chamber numo . . valve was closed and a valve to the chlorine reservoir of known pressure (100-150 torr) was opened. The mercury levels immediately responded to the rapid increase in pressure then decayed as the chlorine was absorbed, thus reducing the pressure. The pressure decayed linearly with time according to the equation:
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(P-Pd=Po-at
(6)
In a typical experiment Po was 50-80 torr (the chlorine reservoir volume was approximately the same as the volume of the absorption chamber), and the surface area covered by NaOH was about 50 cm2. The quantity Pfrepresents the final pressure of the system after all chlorine was absorbed. I t was about 10 torr and corresponded to the equilibrium vapor pressure of the NaOH solution a t room temperature. Under these conditions a was about 1.0 sec-'. The linear decrease in the pressure with time suggests a diffusion-limited process. Svec, H. T., and Gibbs, D. S.,Reu. Sci. Instr., 24,202 (1953). 28 gauge bare nickel chromium wire cable cord, manufactured by Consolidated Companies, Chicago, Ill. 1
Figure 1. Recording manometer. 670 / Journal of Chemical Education
