A Thermocouple Reference Bath with Long Term Stability The precision measurements with can he no greater than th e precision . .and accuracy of temperature .. . . . . thermocouples . . . .. and accuracy ot the temperature of the thermocouple reterence lunctlon.' 'lhe ice polnt, mamtalned by means of an chosen for the reference temuerature. The freauencv iee-water mixture in a Dewar flask,. is eenerrtllv .. , . with which the ior i l l the l)cwnr fla5k must be rt-plenishml vsri?.; wirlr lla sir? of the Iktiar, the number of j w ~ t i o winmwrs~din 11, anhirl,t wmperature, and other fnrlors, bur eavh limp rhe ju~wrioniare diirwbrd ro replr!~irln11w ice, tempernrim measurement and control are perturbed. During the course of measurements of relative water activities in concentrated solutions with a high temperature isopiestie ~ p p a r a t u swe , ~ noted temperature drift of several tenths of a degree per day for one t o two days after replenishing ice in the Dew&=flasks. (This drift wssohserved by means of aplatinum resistance thermometer in the apparatus.) Furthermore, i t was necesssry t o replenish the ice about twice a week, severely limiting the time during which it was possible t o ascertain constsnoy of temperature in different parts of the apparatus. After considering various alternatives, including the use of commercial thermostated reference ehamhers,"e arrived a t the following simple solution. The 4-qt Dewm flask, containing an icewater mixture in which were immersed the reference junctions of our four measuring thermocouples and eighteen-junction control thermopile, was enclosed in a polyethylene bag whose top was taped around the thermocoupleleads, and wasplaced in an ice chest (about 2 fta). Ice was added t o the chest two or three times weekly, hut i t no longer was necessary t o disturb the reference junctions in the Dewar, and drift was reduced below O.Ol"/day. I t was not found necessary t o retain water in the ice chest or t o maintain intimate contact of an ice-water mixture outside the Dewar, since the cooling of the leads and the walls by the melting ice in the chest was sufficient t o greatly reduce the rate of melting in the Dewar. The Dewar was checked, weekly a t first, then monthly, t,he~ m~nntitv of ice. n -fact. six months. the icewit,h no visihle chance . ~ in ~ x~~~ ~" ~ ~ I. , a t the conclusion of ~- a. series of ~- runs lastine ~- -~~ e~~~~ ~~, water mixture in the Dewar appeared unchanged, and did not require replenishment. Simply insulating the I>ewar would not have accomplished this since it wouldnot cool theleads that provide a heat sink t o ambient temperatur es. For applications with only a small number of reference junctions, commercial thermoelectrio devices mi@ :ht he convenient (although probably not as accurate a s the system described here). A small electric refrigerator also might be used t o thermostat the Dewar flask. However, the sim~licitvand effectiveness of using the ice uoint as 8Inoncycling seconllarv thermostat for the reference iunitions in'the &-water mixture in the Diwar seemed so abvious, after the fact, Research ENute of f flux and a reducing flame for preparing a thermocouple sensing junction applies only t o the base metal thermocouples in Table 1. Platinum thermocouples should not be exposed to reducing atmospheres. "BRAWNSTEIN, J., AND BRAUNSTEIN, H., Southeastern Regional ACS Meeting, Richmmad, Va., Nou. 6-8,1969,IPaper No. 220; MCDUFFIE,H. F., BIEN,P. B., RAY,R. W., AND BRAUNSTEIN, J., Reactor Chem. Diu. Ann. Progr. ikept., ORNL-4400. o. 92: SOLDANO. B. A,. STOUOHTON. R. W.. FOX.R. J.. AND SCATCHARD. G.. in "The Structure of Eleck , p. 224.' &olyte ~oluk&';(~ditor: HAM&, W.) .John wiley & sons, ~ e w ~ o i o r1959, * Omega Engineering Inc., Box 4047, Springdde Station, Stamford, Conn. ~
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Volume 4;
Number 9, Septembel
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