T H E JOUR.NAL OF I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
626
I
Vol.
LABORATORY AND PLANT AN ELECTRICAL CONDUCTIVITY RECORDER FOR SALINITY MEASUREMENTS' By
E. E. WEIBSL AND A . I,. THURAS Received May 3, 1918
The electrical conductivity of brines and other salt solutions varies with t h e degree of concentration. A recording instrument might, therefore, be of value where a continuous record of t h e density or concentration of a solution is desired. Recently such an instrument2 has been designed a t the Bureau of Standards for obtaining a continuous record of sea water salinity t o a high degree of accuracy. A brief description of t h e method is given with the hope t h a t it will be of value in its application t o other solutions. DESCRIPTION O F METHOD
The method consists in measuring t h e ratio of t h e resistance in two equal or nearly equal electrolytic cells, A and B, Fig. I. One cell, A, is sealed and con-
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The new and important feature of t h e method is the use of two cells containing liquids of nearly t h e same properties which make i t possible t o compensate almost entirely for t h e large temperature coefficient of t h e solution. The two cells are placed in a uniform temperature bath and the only error introduced is t h a t due t o the small differential temperature coefficient of t h e two solutions. For very dilute solutions which may vary greatly in resistance, as for instance distilled water containing traces of salt, t h e replacing of resistances C and D by an open and closed cell, respectively, is recommended. Then no matter how much the resistance of t h e solution changes in t h e two open cells B and C, the current through t h e two branches of the bridge will be equal, and consequently if all of t h e cells are geometrically equal t h e bridge will be completely compensated for polarization. With high resistance cells and high voltages the use of direct current on such a bridge is suggested. This modification has also the advantage of doubling the sensitivity of t h e bridge. M E A S U R E X E N T S OF S E A W A T E R
I
A
FIG. I-THEORETICAL DIAGRAM
tains an average sample of the solution t o be measured, whose salinity or concentration is known. The other cell, B, is open and has flowing through it t h e solution t o be measured. This ratio is obtained by a Wheatstone bridge using alternating current t o eliminate polarization effects in the cells. A calibration of the apparatus can be made at any time by using solutions of known salinity in the open cell or b y carefully measuring the solution flowing through t h e open cell by some accurate method. A record of t h e resistance ratio of t h e two cells is made by a recorder similar t o those now in use for measuring temperature, but some changes will have t o be made t o adapt this recorder t o t h e use of alternating current. 1 Published by permission of the Director of the Bureau of Standards. This publication is made without obtaining the consent of the senior author, who recently lost his life a t the front in France. The junior author has added a paragraph of new matter relating t o solutions varying greatly in resistance (Para. 4) and has suggested the application of the method t o a wider range of solutions than sea water. 2 Weibel and Thuras, J. W a s h . Acad. Sci., 8 , No. 6.
Preliminary experiments using sea water of different concentrations showed t h a t : I-Good balances can be obtained with a simple Wheatstone bridge circuit containing the two electrolytic cells, using either a telephone a t 500 cycles per second or an alternating current galvanometer a t 6 0 cycles per second. 2-The temperature compensation is sufficient. For the maximum difference in salinity in the two cells, which is about 0 . j per cent, the lack of compensation did not exceed 0 . 0 3 in salinity (0.03 g. of solids per kg. of water) for a change of 10' C. 3-So appreciable change in balance due t o the flow of the sea water through the open cell was obtained. 4-To obtain a continuous record of salinity an alternating current galvanometer similar t o the usual direct current galvanometer is needed t o operate t h e recorder. This galvanometer was constructed of t h e electromagnetic moving coil type1 and has a sensitivity and other operating constants as good as those of the direct current galvanometers now used. ' After these preliminary experiments had shown t h e feasibility of the method, a more careful study was made of certain sources of error in order t o obtain data upon which t o base the design of proper cells. These effects are: I-Heating produced by the current in the cells. a-Temperature lag of the sealed cell when the sea water temperature in the bath suddenly changes. 3-Time necessary for the resistance ratio t o reach its true value when the sea water passing through the open cell changes in salinity. I E. E. Weibel, Bureau of Standards, Scienti5c Paper, 297 (1917), 23.
Aug., 1918
T H E J O U R N A L OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
Consideration of these effects leads t o t h e design of specially constructed cells.
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the
T U B E CELLS
Each of the two cells, Fig. 11, contains 6 parallel glass tubes 14 cm. long a n d I cm. in diameter. These tubes are joined a t each end t o bulbs containing annular shaped platinum electrodes. Each electrode has a n area of 5 . 3 sq. cm. and is held rigidly in place by 4 platinum pins which are welded t o the electrode a n d sealed into t h e glass wall of t h e cell. T h e cells are designed so t h a t there are no pockets in which air can collect and the sea water is admitted in such a manner as t o sweep off any bubbles which might collect on t h e electrodes. The inlet a n d outlet tubes are sufficiently large t o respond t o the maximum change in salinity. RECORDER
I n order t o secure a continuous record of salinity or concentration t h e Wheatstone bridge and galvanometer must be embodied i n a recorder mechanism such as t h a t developed b y t h e Leeds and Korthrup Company. The electrical connections are as shown in Fig. 1. The most important changes in their present recorder are due t o t h e use of alternating current. This current may be obtained from t h e usual 60-cycle supply, but if only direct current is available then t h e small direct current motor used for driving the recorder mechanism can be equipped with slip rings a n d be operated as a converter. T h e recorder paper should be ruled so t h a t salinities can be read directly.
End view
r % Plan view 9 d e view
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sea water 1 intQke
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End view
Plan view
FIG.111-CONNECTIONS OF BATHAND CELLS
cell in order t o eliminate t h e resistance error due t o shunting the cell. S U M 11A R Y
An apparatus is described t o give a continuous record of the salinity or density of a solution by t h e measurement of its electrical conductivity. A pair of electrolytic cells is described which, when used with a suitable alternating current galvanometer, will give satisfactory operation in connection with a recorder. The temperature compensation is obtained by placing both cells, which are in t h e two arms of a Wheatstone bridge, in a uniform temperature bath or directly in t h e solution which is t o be measured. The application of this method, with such modifications in details of construction and arrangement as are necessary t o meet the needs of a particular case, is suggested for t h e measurement of t h e salinity or concentration of brines and other salt solutions and also many other substances whose composition is constant throughout changes in concentration. BUREAUOF STANDARDS WASHINGTON, D. C.
FIG. 11-ELECTROLYTIC CELL IKSTALLATION AND O P E R A T l O N
The recorder should be properly mounted in some convenient place and with insulated wires leading from it t o t h e cells. The bells, Fig. 111,should be placed close together in a bath through which water direct from t h e solution continuously flows, or t h e cells may be immersed directly in the solution if convenient. This will insure a uniform temperature throughout t h e bath. A flow of water, also taken directly from t h e solution, should be maintained through the open cell. This flow must be broken as i t leaves the open
AN ALINEMENT CHART FOR THE EVALUATION OF COAL B y A. F. BLAKE Received April 16, 1918
Some time ago t h e writer published a description of “A Graphic Chart for the Evaluation of Coa1,”l which, t o judge from the inquiries received regarding i t , has proved of value t o a number of chemists a n d engineers. As a result of a recent study of nomography it has become evident t h a t the method of charting can be very much improved by the substitution of alinement principles for those of ordinary 1
THISJOURNAL, 8 (1916), 1140.
