AN ISOTHERMAL BALANCE CHAMBER WILLIAM S. CASTOR, JR.' and FRED BASOLO Northwestern University, Evanston, Illinois
M A I x T m m c E of an analytical halance a t constant,
temperature for extended periods of time is a prohlem rather frequently enconntered. An isothermal bala.ncr can be obtained by monnting the instrument in one of t,he commercially availahle incubators; however, this is not always practicable for &her structural or economic reasons. The former iliflirnlties arise from the ronsitlrration that extensions mnst he fitted to t,he balance controls, to permit weighing operations without dist,nrhing t,he thermal condition. Precise temperat,nrr rontml of the balance proper is frequently avoi(lrd hy t,heuse of u torsion balance external to the ehamher containing t,he weighed sample; however, this technique is not always desimhlr. The photographs show an in6.xpensive, readily eonstructrd chamber suitahle for maintaining a balance a t a temperatwe within 5' to 1O0('. of room temperatanre,with a tolerance of +O.IoV. The ohanher is essentially a douhle-walld box constructed of scrap lumber or crating. The larger crarks I 1'1~wnt addrrss: (!lwnic.:ll IXvi~ion,
.\rnrriran ('ymtxmid Ifivw, firginin.
Cornpnyv. Cxlco
open Chamber Showin.
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Details of Construction
were filled with a plastic roof coating, and the \ ~ o \\.as d t,hen sealed with a liqnid asphalt roof coating applied to t,he su~faresfacing the intra\\-all space. Blocks urew i n s e r t ~ hctween l the walls a t point,s where holes \\-ereto he ilrillrtl, anti the intra~l-allspace filled with a cornmercial "ponring" inm~lation. Gray porch paint \DIS nse~lon the external surfares of t,he chamber illustrated. The interior as tarred; hut this did not prove very satisfilcto~y,so hlack paint was substituted in a second. larger hox of the same type. Several thicknesses of woolen rloth mew taclccd to the front edge to serve as padding hetween hox and dnor. Windows mere provided in the door for observation of the beam, vernier, and pointer scale of the Ainsworth LCU balance nsed. This type of balance was chosen from those at hand because all chain readinas are made
-" bulbs in parallel mounted 'ymmetrirally near the interior front and i~ottomedges of the hox
Closed Chamher S h o w m g Wlndows and Controls R e q u ~ r e d+or Belence opnration
380
JULY. 1951
381
to provide the light necessay for weighing ohservat,ions. hrass tube welded toacollar which fitted over the original Thry were shut off when not needed; leaving them on knob and was fixed in place with set screws. A rod ran for prolonged periods caused an increase in temperature. through it and was srrewed to the threaded shaft from Temperature control mas obtained by cycling water the pan-arrest control. The original wheel or knoh was from a constant-temperature habh through about 200 then affixed to each rod. To permit opening of the balance case without its feet of l/r-inch soft copper tuhing nailed to the interior walls of the insulated chamher. That on t,he inside of removal from the isothermal chamber, the glass door t h door ~ was joined to the rest by rubber tuhing. Ex- was replaced by one of celluloid. This could he rolled t,ernal tuhing was wrapped with asbestos cord. To hack over the top of the case. The balance was leveled avoid t,he formation of vapor lorks during the initial wit,h the aid of a small mirror. For our purposes it filling, one end of the tuhing lvas at,tached to an aspira- was desirable to desiccate t,he balance case. In order t,or and the other to the tap. After partial evacuation, to do so efficient,ly,the door \\.as sealed to the case with t,he t,ap was opened. Thereafter, no circulation diffi- a. plastic calking roni vhich as easily applied and recwlties were experienced with good laboratory centrifn- moved. gal pumps. A larger chamber of similar construction was fitted I'ktensions for the chain-weighing device and ridrr with removable shelves. I t senred very satisfactorily arm were brass rods of appropriate size att,ached vith as a desiccator box for the storage of samples heing wt-srrew collars. The heam-arrest extension was a dried to constant composition in various hygrostats
