An Electronically Timed Gas Viscometer Fitzgerald 6. Bramwell' Brooklyn College of CUNY, Brooklyn, NY 11210 Fitzgerald Bramwell P.O. Box 1301. Sag Harbor. NY 11963 A cost effective relative gas viscometer described by Shoemaker et al., ( I ) , has significant advantages in terms of utility and precision over the design of other relative gas viscometers used for instructional ournoses . . (2.3). . . . In this viscometer, gas flows through a capillary under a pressure gradient resulting from mercury displacement. We have modified the Shoemaker design in two ways to produce a low cost relative aas viscometer. shown in the figure. of hieher vrecision and sHfer design. ~ i ; s t ,three tungsten eiectroies t;ave been mounted in the viscometer usina graded alass seals2. Second. a hall trap has been inserted hetween th;? mercury reservoi; and all external stopcocks. The experimental procedure requires charging the viscometer with a gas to displace mercury from its equilibrium position to below electrode two. The cas is then allowed to escape from the viscometer through a nirrow bore capillary, (--0.004 in. i.d.)3. By measuring the time reauired for the disnlaced mercurv to nass from electrode two to klectrode three, 'the flow rate i f gas through the capillary (and thereby the relative viscosity) can he determined in a reproducible and precise manner. Improved precision results from an electronic timing circuit which can he constructed easily in a few hours from a pair of 12 V ac relavs, a simvle electric clock. and a 12 V ac transformer. hec circuit diagram, also shown in the figure, uses circuitry and materials that are low cost, rugged and reliable. Electrode one, which is always in contact with the mercury reservoir, and electrode three cany the same low voltage (=12 V) ac potential. During an experiment, when rising mercury contacts electrode two, a relay is activated which starts an electric clock. When the mercurv reaches electrode three. a second relay is tripped which sto"ps the clock and provides a vrecise measurement of the elavsed time from which the rate bf gas flow through the capill&, and thereby the relative coefficient of viscositv can be comvuted. Low voltage, low current ac is used to prevent polarization of the viscometer electrodes and to minimize anv shock hazard. As a further safety precaution, the visrometer is mounted in a Lucite* box which is deep muugh to isalace the electrode contacts but shallow enoughto allow easy access to the stopcocks. T h e electronic timing circuit should be used with great c a r e when timing combustible gases because occasional sparking i s observed a t t h e electrodes. The inclusion of a hall tran between all stoncocks and the mervury reservoir 19 a ,~yniflcant irnprt~vernent In deslgn safetv. Chamma the vlworneter r hrough the rad inlet stoocock or outgassing tge system through thethreeway capillar;/vent stopcock requires the relatively rapid movement of several pounds of mercury through Pyrex@apparatus. The inclusion of a ball trap reduces the probability of inadvertent mercury spillage. The results of student measurements over two semesters on a variety of different gases are within 0.5% of literature values, (4). This viscometer is used in conjunction with an -
-
-
I Author to whom correspondence should be addressed. >Suitable grade gla& seals can be made from uranyl glass, or Corninga 7720 glass. Dropping mercury electrodes or broken thermometers are a gwd source of precision bore capillary tubing.
680
Journal of Chemical Education
--
Find Mercury Level Electrode 3
Clock
r=--l
An electronically timed relative gas viscometer with lhe essacialed circuit diagram.
experiment desnibed hy Shoemaker et al., (I).Our design has several distinct instructional advantages. First, students are exposed to practical applications of elementary electronics. The circuit design provides a good starting point for introducing more elegant digital logic circuits. Second, this design permits more efficient use of limited student laboratory time. Watching mercury rise in a viscometer has been described as "akin to watching water boil." Electronic timing allows students to utilize the "mercury rise time" for setting up other experiments or for data reduction. Third, because the electronically timed viscometer is more precise and less tedious to use than the manually timed viscometer, students are able to experiment with a greater number and variety of gases. Acknowledgment The authors wish to thank Mr. Ottmar Safferling of the Brooklyn College Glass Shop for construction of the viscometer and Mr. Ira Senzon of the Brooklyn College Electronics Shop for construction of the electronic timing circuit. Lierature Cited (11 Shoemaker, 0. P., Garland. C. W., and Steidcld, d I., "Erperimenvi in P h y a i d Chemiatry,l3rd Ed., McCrsw-Hill,he.. New York. 1974, pp. 10%113. (21 Salzbew, H. W., J. CHEM. EDUC.,12,663,119651.
(31 Ms1inanskas.A. P., Whisenhunt. Jr., S. L a n d Searcy.J. Q . J . CHBM..EDUC., 46.781, (1969). (4) Partingon, J. R., "An Advaned Trestiae on Phyaicd Chemistry: Lonpmana. London. 1949.
uol. I, pp. 856ff..
