Gas
Cell for Beckman Quartz Spectrophotometer NORMAN D. COGGESHALL, Gulf
Research & Development Company, Pittsburgh, Pa.
A
double-compartment garatmrption cell for use with the Beckman quartz spectrophotometer is described. The assembly conrirtr of a machined brass block, quartz windows, and packless valves connected to standard ground-glass joints. SECTION A-A
B
N THE course of applying a Beckman quarts spectrophotom-
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eter to analytical work, it was found desirable to analyze for gaseous absorbers such as butadiene. Since most of the demands for analyses were for liquidsamples and the need for the analyses of gas srtmples was not continuous, B gas cell that could be used in the same manner as the liquid cells without any special conversion was needed. The cell described below has been very successful for this type of need, and this description is being published that others may take advantage of the author's experience.
Jmh fi PLAN VIEW
r
A
L---J
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ELEVATION
stopcocks and are needle va1ve"s equipped with metal bellows for mechanical movement.. The distance between the centem of the two gas compartments is the same a8 the corresponding distance for two adjacent cam: Dartments of the liauid cell holder furnished with the Beckman
BRASS PLUGS SILVER SOLDERED
SECTION 8 - 8
dmensions, the two gas oompahments are located in the same positions as the two center liquid cell compartments and this allows light transmission to be obtained when the cell positioning rod is in position 2 or 3. The packless valves are connected to the brass block by means of sections of 0.25-inch outside diameter metal tubing approxi-
Figure 1.
END VIEW
Figure
P.
Structural Details of Bras Block Forming M a i n Body of Gar-Absorption Cell
mately 1inch long. Attached to the ends of the valves opposite the metal cells are short sections of Kovar tubing onto each of which has been made a glass-to-metal seal. To this is sealed the inner part of a Pyrex ground-glass joint using B graded sed.
Figure 3.
Double-Compartment GarAbrorption Cell
Gas-Absorption Cell i n Working Position
Llsht4bht c o w k brhlnd tlikonb011iw knob
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INDUSTRIAL AND ENGINEERING CHEMISTRY
For evacuating a cell or filling it with a sample it is connected by means of the ground-glass joint to a vacuum and gas-handling system. The windows on the two gas cells were made by cementing two quartz plates onto the machined faces of the brass. These quartz plates were obtained from The Thermal Syndicate, Ltd., 12 East 46th St., New York 17, N. Y., and were polished enough to give them good transparency. General Electric Glyptol No. 1201-red was used to cement them on. In Figure 2 may be seen the construction details for the brass block, which is machined from a solid piece of metal by standard shop procedures. The metal tubes from the valves are softYoldered into the holes entering the top of the block. The holes drilled from the ends to allow communication with the gas compartments are sealed by means of brass plugs set in silver solder. The extended lightproof cover built onto the instrument to allow head room for the metal valves and the ground-glass joints can be seen in Figure 3. A metal flange is built around the sample compartment, and fastened down by screws to the metal parts that house the filter slide and sample mover. Onto this flange is slipped the rectangular cover seen resting on the instrument just beyond the slit-controlling knob. This arrangement provides a light-tight cover that allows adequate head room for
vol. 17, No. 8
the movement of the complete gas cell assembly. The flange does not interfere with the placing of the liquid cell holder in the instrument, and the same cover is used for both gas and liquid work with no changes. One compartment is used as comparison cell. Thus if the optical density of one cell is known, using the other as a standard, the optical density of the sample is corrected for the difference of cell transmission by a simple addition or subtraction. The cell correction can be found with the two cells either evacuated or filled with air. For the author’s case, one cell had a transmission of about 98.5% of that of the other over the useful range of wave lengths. The fact that this gas cell can be used a t any time with no instrument modifications is a convenience, since gas and liquid samples may be run in sequence or an occasional gas sample may be run in the midst of a group of liquid analyses with no appreciable interruption of the analytical work. LITERATURE CITED
(1)
Topanelian, E,, Jr., and Coggeshall, N. D.,Rw. Sci. Instruments, in press.
THE RUGOSIMETER A n Instrument for Measuring Surface Roughness of Calendered Sheet Rubber MELVIN MOONEY, General Laboratories, United States Rubber Company, Passaic, N. J. A new instrument has been designed for measuring the rugosity, or surface roughness, of calendered raw rubber sheet or similar samples. The property actually measured is the resistance to air Bow between the rough surface and 4 plane test surface resting on it. The apparatus consists essentially of the following elements i n series: a constant-pressure air valve, a large needle valve with a calibrated scale, a manometer, and an annular test plate which rests upon the surface under test. The needle v a k e is opened to the point at which the pressure on the manometer i s one half the pressure maintained b y the constant-pressure valve. The resistance of the needle valve to the air flow is then equal to and measures the resistance of the test plate on the sample. By a theoretical formula this air-flow resistance i s converted to “rugosity height”, which i s the height of the hills above the valleys in an idealized rough surface of sinusoidal profile.
I
F THE processing behavior of a raw rubber stock were perfect,
the stock would come through a calender or tuber with a perfectly smooth surface. However, even a close approach to such behavior is the exception rather than the rule in the rubber industry; and the surface irregularities associated with imperfect processing are of such general occurrence and of such a magnitude that they are of considerable practical importance. Heretofore, imperfections of this type could be graded only by visual inspection. The purpose of the instrument described in this paper is to measure surface roughness, and thus yield a quantitative, impersonal figure for this processing quality of raw stock. If the surface of a sample is rough but approximately planethat is, departs from a plane surface only by small distancescomplete characterization of the surface would require in general a double Fourier series. Obviously, then, any instrument which yields a single figure for “roughness” must carry out some arbitrary selecting or averaging process which reduces the infinite number of parameters of the Fourier series to the single roughness tigure. This means that samples of different surface types would be rated differently, in general, by different methods of roughness measurement; and ratings by any one method might not be correct for a particular application. However, calendered rubber surfaces are usually of a wavy type, characterized by rounded
ridges or valleys and approximate symmetry of the surface above and below the median plane. Then, to the extent that samples to be measured are similar in type, almost any measure of roughness would be .of practical value. The basic principle of the instrument described is the same as that of an instrument described by Nicolau (1) for testing metal surfaces. What is actually measured is the resistance to air flow between the rough surface and a plane surface resting on it. In conformity with Nicolau’s terminology, roughness measured by such a method is called rugosity; and the instrument used is called a rugosimeter. The instrument as described is designed primarily for measurements of calendered sheet, but auxiliary devices can be devised for similar measurements of other forms of sample. A clearer conception of the quality of the rough surfaces under discussion will be obtained by referring to the article by White, Ebers, and Shriver (8). The maximum deviation from the median plane in most samples is of the order of a few tenths of a millimeter or less. The present article is limited to a description and theoretical analysis of the rugosimeter. G E N E R A L FEATURES OF RUGOSIMETER
A photograph of the rugosimeter is shown in Figure 1, and a schematic drawing of the essential parts in Figure 2. Comressed air, fed through the inlet, I, flows through the water trap, the float valve, F, the graduated needle valve, V , and the flexible rubber tube, R , to the annular test plate, T. Here the air escapes through the spaces between the smooth bottom of the test plate and the rough surface of the sample, S. F maintains a constant pressure, PI,in the line between F and V . The pressure, P,,in the line between V and R is indicated on the tilted manometer, M . Obviously, we deal with pressure above atmospheric, not absolute pressure. The rougher the sample, the reater are the air spaces between it and T,and the more easily t%e air flows through these spaces. The function of the apparatus is to measure the flow conductance, or the reciprocal of the flow resistance, in terms of the graduated scale on valve V . If the test plate conductance is equal to that of V , the pressure drop between F and atmospheric pressure will be equally divided between V and T,the drop in the connecting lines being neglected. Standard practice is therefore to adjust V so that its conductance is equal to the conductance of T,the correct adjustment being indicated by the fact that the obserwd
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