High Temperature Thermal Conductivity Cell Herman R. Felton and Adolph A. Buehler, Jackson Laboratory, E. I. du Pont de Nemours & Co., Inc., Wilmington, Del.
MEW, compact, and versatile high temperature thermal conductivity cell which may be of particular interest to workers in high temperature gas chromatography has been developed in this laboratory. Whereas the conventional cells are limited to about 300” C. for units containing thermistors (Victory Engineering Co., Catalog 1956) and to 250’ C. for filament-type cells (GowMac Instrument Co., Bull. H-TC7-56), this new cell performs satisfactorily above 400’ C. Model airplane “glow plugs” are used as sensing elements in the unit rather than thermistors or filaments. The most satisfactory plug found for general use was the Champion model VG-2. A typical design of a cell used for analysis of gases by thermal conductivity measurement is shown in Figure 1. Both elements are of the “diffusion” type. “Direct pass” cells have been made by pulling the &-irecoil out of the plug so that it may be placed directly in the gas stream and suitably modifying the holes drilled in the cell block. As one side of the mire heating coil is shorted to the body of the glow plug, the measuring bridge into which the plugs are incorporated is grounded a t this point. The circuit of such a bridge design is shown in Figure 2. Full wave rectified current is used as a heating source for the elements. A Variac may be used in the 110-volt alternating current line to control bridge current. Operation has been found both stable and sensitive a t 1.5 volts across the bridge. The higher the voltage, the greater the sensitivity. A 10-mv. re-
SIDE VIEW DRILLED AND
GLOW PLUG
1/8” HOLE
STEEL BLOCK D
END VIEW
Figure 2. plug cell
-11. R1,R2. R3. R, R. P , P. G, G .
Transformer 10 v o h , 4 amperes, CT Rectifier UICISIG (radio receptor) Electrolytic condenser 500 mmf. 50 volts Voltmeter (0-3 volts d.c.) Resistor, 47 7 Helipot 10-turn, 25 7 Jacks for recorder lead i Jacks for glow plug leads Champion glow plugs,
B.
Conductivity cell’block
2’. Figure 1. cell
Cross section of glow plug
corder 11-assatisfactory, although a millivoltmeter may be used directly. These units have been used both as detectors for gas chromatography and for gas analysis by thermal conductivity. Response time, sensitivity, shortterm drift, noise, and reproducibility were comparable to the characteristics of a conventional “hot wire” cell. The simplicity of design and circuitry makes for simple replacement and maintenance. A burned-out plug may be changed n-ith very little lost time and only slight zeroing adjustments are needed after the replacement. The parts cost of the entire unit, exclusive of the recorder, is less than $100.
Electrical circuit for glow
S. C.
VG-2
The great advantage of these units is in high temperature operation, where more conventional units fail. For example, gas chromatography units using this design have been operated a t 420’ C. The maximum operating temperature has not been determined. Further detailed work to evaluate the applications and limitations of the device is in progress.
Small-Volume Five-Centimeter Absorption Cell Gerald Goldberg, A. S. Meyer, Jr., and J. C. White, Oak Ridge National Laboratory, Oak Ridge, Tenn. SERIOUS limitation
to the use of ab-
A sorption cells with long light paths is
the large volume of solution required to fill them. Commercially available absorption cells with 5-em. light paths normally require about 15 ml. of solution. The total available volume of the chromogen in solution must be a t least 25 ml., if such cells are to be used. An absorption cell that would provide a maximum light path yet not restrict the volume of solution would have numerous applications in the anal-
ysis of radioactive solutions and in the case of a limited volume of solution for analysis. This work was undertaken to fabricate a cell that would (1) fit the cell carrier of the Beckman spectrophotometers, Models DU and B, (2) provide suitable reproducibility of measurement, (3) be inert to various acids and solvents, and (4) be easily filled, drained, and cleaned by the operator. The length of the light path was arbitrarily selected as 5 em. To use the cell in the cell carrier of the
Beckman spectrophotometers, the outside diameter of the cell must be equal to that of the 5-cm. glass cells. The minimum diameter is limited by the cross section of the light beam of the Beckman spectrophotometer. Therefore, the inside diameter is 0.391 inch and the capacity is approximately 4 ml. Because the cell must fit into a standard cell carrier and the spacing between the inner surfaces of the windows must be precisely reproducible, the structural material must be machinable to close tolerances and inert to the acids and solvents used in colorimetric analysis. VOL. 30, NO. 6, JUNE 1958
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