GAS T H E R M O M E T E R F O R AN I N S T R U C T I O N A L L A B O R A T O R Y

N. A . L i s e n k o v UDC 536.511

The constant-volume gas the rmomete r is apparently still included among the projec ts in an ins t ruc- tional physics labora tory . Several models have been proposed in which dead space has been eliminated [1, 3], m e r c u r y has been replaced by other liquids [2, 3], and the water thermosta t has been replaced by an air thermosta t [4, 5]. The air thermosta t has several advantages, although it is difficult to achieve prec i se tempera ture control with it at 35-40~ In the model proposed below an attempt has been made to completely eliminate the need for maintaining a constant temperature .

The p r e s s u r e in volume 5 (Fig. 1) is equated by means of null gauge 4 to the eounte rpressure in a pneumatic sys tem consisting of a water gauge 9, a buffer volume 10, and a pump. The rigid glass vesse l must have a geomet ry ensuring a uniform convective flow throughout the volume. A 1 l i ter conical f lask is used for this purpose in our apparatus. The working liquids in the null gauge are polyethylsiloxane liquids Nos.2 and 3, polymethylsiloxane liquid No. 10 [6], and VKZh-94 [3].

The rmomete r 6 with a scale of up to 40-50 ~ (preferably with 0.1 ~ divisions) is placed within the flask. In this postion it is in di rect contact with the medium whose temperature is measured, and there is no need to make a cor rec t ion for the projecting column. The thermometer is conveniently read by means of a mea - sur ing microscope . Since the thermometer is in a relat ively immobile air medium, e r r o r s could a r i se due to the heating of the the rmometer by radiation f rom the flask wall [7]. This possibil i ty is eliminated by s u r - rounding the the rmometer r e se rvo i r by a polished ref lec tor 7 of thin sheet metal . The ref lec tor is open at the top and bottom, so it does not significantly hinder convection. The bottom of the flask is covered (on the outside) with asbes tos .

Valve 1 is used to equate the p re s su re in the flask to the a tmospheric p r e s s u r e (at the beginning of the experiment) . After the measurements , valve 1 and then valve 11 are opened, the hot a ir escapes f rom the flask, and the p r e s s u r e in the sys tem falls to a tmospher ic . It would also be possible to design the ap- paratus without valve 1; with this a r rangement , the air would be admitted to the f lask and allowed to escape f rom it by bubbling through the liquid in the null gauge.

The entire apparatus is placed in an a i r - f i l led thermal bath - a box 3 of sheet metal thermal ly in- sulted on the outside. The front and rea r walls are double glass panels. The exhaust windows 2 are used to rapidly cool the apparatus after an experiment . The oven must be a r ranged in a position such that the hot air uniformly heats the flask wall.

In our apparatus, the oven is based on a flask heater with a 300 W closed coil. Because of its high thermal inert ia, the tempera ture can be ra ised quite slowly (at 0.05 deg/min) at high tempera tures (region CD in Fig. 2). The time dependence of the tempera ture in this region is very near ly l inear . Under this condition the extent ~ by which the the rmomete r readings lag behind the tempera ture of the medium is near ly constant [8, 9]. For this reason in region CD the l ineari ty of Char les ' law can be followed with a tempera ture range of 2-3 ~ (Fig. 3) - a methodologically important considerat ion. We obtained the curve in Fig. 2 by turning the oven on for 15 rain at 220 V (region AB) and then at 100 V for the res t of the t ime.

F r o m [8, 9] we find an equation which we can use to calculate the tempera ture t of the medium f rom the the rmomete r indication 0 for the case of a constant heatIng rate r , i.e., in region CD (Fig.2):

t = o + ~r. (1)

Komi State Pedagogical Institute. Translated f rom Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 6, pp. 118-120, June, 1970. Original ar t ic le submitted June 10, 1969.

�9 1975 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00.

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9

Fig . 1.

F ig . 2.

Fig . 3.

Pump

Fig. I

!

/ ; f t 'c

5 $0

h

, ~ D 5a / 67 r

- .(6 /

2.D JO 40 ~;min J2 JJ Fig . 2 Fig . 3

I 34t'r

Construct ion of the gas t h e r m o m e t e r .

Approximate t ime dependence of the t h e r m o m e t e r indications.

Dependence of the height of the wa te r column in the gauge on the a i r tern- pe r a t u r e in the f lask .

We measu red the lag constant X for an ord inary chemica l t h e r m o m e t e r in a conical f lask, finding ~60 sec . Calculation on the bas i s of Eq. (1) showed the d i f ference between t and 0 to be about 0.05 ~ in region CD. Accordingly, the t h e r m o m e t e r essen t ia l ly shows the medium t e m p e r a t u r e when the t e m p e r a t u r e is read c o r r e c t to within 0.1 ~

The the rma l coefficient 7 of the p r e s s u r e can be found conveniently f rom

1 (2) ~ = p l t2 - - t l _ _ t l

P~ --Pl

where a number approx imate ly equal to 273 should be obtained in the denominator .

The p a r a m e t e r s Pt and t 1 of the initial s ta te a r e m e a s u r e d before the oven is turned on and with valves 1 and 11 open. Then t I is the teml~erature which is es tabl i shed in the f lask for the long t ime preceding the exper iment , and Pl is the a tmospher i c p r e s s u r e . The f ina l - s t a te p a r a m e t e r s P2 and t 2 a re found f r o m Fig . 3, where a point A has been se lec ted a r b i t r a r i l y on the ave rage l ine.

When an o rd inary chemical t h e r m o m e t e r is used, our appara tus r ep roduces the tabulated coefficient well , with a r e l a t ive deviation of about 2%. P r e c i s e m e a s u r e m e n t s a re poss ib le because the final t e m p e r a - ture is m e a s u r e d as it is being increased; this is an impor tant considerat ion in operat ion with a m e r c u r y - g lass t h e r m o m e t e r . In addition, the sensi t iv i ty of the wa te r gauge plays a ro le .

The author thanks M.E . Kuznetsov for in te res t in this study.

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2. 3. 4.

5.

6 .

7.

8 .

9.

L I T E R A T U R E C I T E D

Schiavini, Bull .Union Phys ic iens , 58, No.474, 473 (Dec. 1963-Jan. 1964). G. Harbeck , P r a x . Naturwiss , A13, No. 5, 114 (1964). N , A . L i s e n k o v , I z v . V U Z . F i z . , l~o. 1, 176 (1965). G. I. Dluzhnevskii , Abs t r ac t s of Repor t s to the XVI Scientific and Technological Conference of the Al l - Union Cor respondents Power Engineer ing Institute (1966). Yu. F . R e d ' k o , Abs t rac t s of Repor t s to the XXII Scientific Session. Section on Phys ica l and Mathemat i - cal Sciences , Chernovtsy Univers i ty (1966). L . A . Zhukova et al . , I z m e r i t e l ' . T e k h . , No. 5, 17 (1965). / E . A . Paperny i and I. L. E ide l ' sh te in , E r r o r s Involved in Contact T e m p e r a t u r e Measuremen t s [in R u s - sian], Moscow (1966). G. M.Kondra t ' ev , Regu la r T h e r m a l Conditions [in Russ ian] , Moscow (1954). A . N . G o r d o v , Industr ial Methods for Measur ing T e m p e r a t u r e [in Russ ian] , Moscow (1952).

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