I-VDCSTRIsL A S D ESGISEERISG CHEMISTRY
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Photoelectric Cells in Chemical Technology’ A. J. McMaster G-M LABORATORIES, INC., GRACEA N D R A V E S S W JAva J ~ ;,, CH:CAGD, ILL.
S IS other branches of engineering and scientific research, the application of light-sensitive devices t o chemical operations has been greatly enhanced by the development of alkali-metal photoelectric cells. Although the photoelectric effect has been known and studied for several decades, it is only within a comparatively few years that a product of sufficient sensitivity and uniformity has been developed to warrant its use to a considerable extent in research laboratory and industrial projects, for measurement or control purposes. Even now the characteristics of light-sensitive tubes are not all that is to be desired in a device of
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Figure 1-Schematic Drawi n g Illustrating Action of a Photoelectric Cell
this nature, but for some purposes their use can be depended upon to give satisfactory and consistent service. The possibilities of using electric currents activated or controlled by light are so large that a brief description of some of the applications to chemical technic are justified at this time. Photoelectric Cell Action
The photoelectric cell in conjunction with other electrical apparatus may be considered as an electrical instrument for performing any one of a number of electrical operations as the result of a variation in intensity or color of a beam of VISITRON CELLS-TYPES Z
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I n Figure 1 a schematic illustration of the action of a modern type of cell is shown. If light is allowed t o fall upon the interior sensitire surface, electrons are emitted from the surface and are attracted to the anode owing to the polarizing action of the battery in the external circuit. This flow of electrons from the sensitive surface to the anode and continuing through the external circuit comprises the photoelectric current. There are two general classes of alkali-metal or alkalimetal hydride cells. The vacuum type, as the name implies, contains no gas. The gas-filled cell, however, is filled at a low pressure with an inert gas such as helium, argon, or neon. I n this type of cell the photoelectrons ionize the inert gas in their passage from the sensitive surface to the anode. The resulting ionization current is added to the electron current, giving a larger total photoelectric current than in the yacuum type of cell. Photoelectric cells of maximum sensitivity are therefore of the gas-filled type. The magnitude of the current in the gas-filled cell depends upon the specific surface sensitivity, color of light, the volume, pressure, and nature of the inert gas, the cell voltage, and the total light flux entering the cell. I n the vacuum cell the current depends only upon the surface sensitivity, color, and amount of light, and to a certain extent upon the cell voltage. As there is no ionization in the vacuum cell, the sensitivity does not materially increase after the saturation voltage is reached, and the cell is not critical with respect to operating voltage. This characteristic is illustrated in Figure 2 (type AV). Where highest sensitivity is not required, the vacuum cell can be uzed to advantage. However, the sensitivity of modern vacuum cells is only one-fourth to one-fifth that of gas-filled cells, and the latter are used where sensitivity is of more importance than absolute constancy. The principal characteristics of typical cells are sho~vnin Figures 2 and 3. In Figure 2 the specific cell sensitivity in
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Figure 2-Voltage-Current
Characteristics for Vacuum and GasFilled Cells
light. The cell, which in some respects is similar to a radio tube, depends for its action upon the emission of electrons from a sensitive surface of an alkali metal or alkali-metal hydride. 1
Received July 10, 1930.
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LIGHT FLUX-LU~ENS
Figure 3-Current-Light Flux Characteristics of Vacuum and GasFilled Cells a t Various Operating Voltages
micro-amperes per lumen is plotted against cell voltage. As the voltage of the gas-filled cell (type A) is raised, the sensitivity of the cell increases rapidly until the curve rises almost vertically. If the voltage is raised above this critical point, the cell “spills over” into a state of cumulative ionization or
I X D U S T R I A L A X D ENGINEERING CHEMISTRY
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glowing. In this condition tlie cell is inoperative and is subject to serious damage. The niaxiniurri safe operating voltage of the gas-filled cell depends upon tlie ainrrunt of light energy enteriiig tlre cell. With greater light intensity the operating voltage must bc lower to prevent cumulative ionization.
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the photoelectric current itself is used to actuate a sensitive electrical instrument, such as a galvanometer; and (2) circuits in which the photoe1ect.ric current a,ctustes a radio-tube amplifier, thereby increasing the magnitude of the current which (:ail he used to control a meter, relay, or other electrical device. In the first classiticat,ioiithe circuits of Figures 5 and 6 can be used in the measurement of opticzl properties of various niatcrials. As an exrtmple, the arrangement of apparatus sliown in Figure 7 in conjunction with one of these circuits caii be used for the deterniination of light transmissive power, reflecting power, or absorption factor of metals, glass, tiles, other ceramic materials, paints, varnistirs, and various cellulose products. By using the proper color filters and accessory calibration api)aratiis, the color characteristics of sucli LAMP4 LENS SYSTEM
FiCure (--Four Sfandnrd Size8 of Visifron 'Type A and AV Phof~elecfricCells
Figure 3 sliows tlio linear light-fiur cell current cbaracteristics for different operating voltages. Here again tlie increase of cell sensitivity with cell voltage will he noted. The ratio of the micro-ampere output per lumen of light fliix at a defiiiite cell voltage is a measure of cell sensitivity at that voltage. To make possible roinparivons between different cells having diflerent voltage ratiiie the specific cell
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6XLYz4mM7ER Fiaure 5-Simple Phofoelecfrlc Photometer Circuit for Measuremenf of Liltlit from Various S"rCea
FiBure 7-Arrangement of Optical ApparBtUli for Measuring Re-
Recflng Power of Various Materiale Such a$ Metals, P a l n t ~ Var.
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inaterials ran also be studied. Figure 8 shows tlie response of the humsii eye as well as tlie photoelectric cell t,o various colors of light. In addition to tlre use of this photometer type of circuit for the determination of physical properties of materials, this arrangement has many applications as a light-measuring inst,rument in studying actual chemical processes. Tbus the elid point of various reactions in which the criterion is the disappearance or change of color, degec of t,ransparency, or change of colloidal properties ran he quantitatively detcimined by means of the photoelectric cell in place of the human eye. (Figure 9) Among ninny such applications is tlie ineasurement of liquid ingrcdicnt densities, proportions of liquid and solid ingredient mixtures, thickness of paper-rill stock, etc. Such apparatus, wlien properly adjusted and maintained, eliminates the depndence upon prsonal judgment or ahility of operators to determino such critica1 EAetors. By means of amplifier and relay circuits it is possible to provide automat,ic coutrol of titration and similar operations. The difference i n the cireuits of Figures 5 and 6 is mainly mi
