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Instrumentntion Ultraviolet photometers are valuable for continuousanalysisap licationsin plants if the user understan s their limitationa
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be difficult to operate many modern continuous flow processes without instruments to give a dependable automatic analysis of the process streams at various critical points. It is of primary importance that instruments for such use be perfectly dependable because the yield and quality of the product and perhaps the lives of those who operate the plant and the safety of the plant itself depend on the data reported by these instruments. The processes most readily adapted to continuous automatic analysis instrumentation are those involving fluid reactants and/or products, especially gaseous ones. Solids or suspensions of solids in liquids or gases are usually hard to work with. In this oolumn for December 1948, page 65 A, we discussed some of the points to be considered in continuous analysis instrumentation, We listed briefly the properties of matter which could be used a basis for the development of continuous analysis instruments. Methods based on infrared and ultraviolet absorption are among the most selective and sensitive methods available. They are particularly well adapted for analysis of gaseous process strezlms. It is interesting to consider some of the problems involved in the design, selection, and use of a recording ultraviolet photometer. This instrument can be used for determining the concentration of ultraviolet absorbing materials such as aromatic hydrocarbons, conjugated diolefins, and chlorine with nonabsorbing gases such as saturated hydrocarbons, oxygen, nitrogen,
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Figure 1. Recording Ultraviolet Photometer
hydrogen, carbon dioxide, or carbon monoxide. Of course the operating wave length must be chosen to lie within the region where the component to be determined has high absorption and the remaining ones have negligible absorption. This will vary with the components present in the mixture. A simple schematic diagram of a recsrding ultraviolet photometer is shown in Figure 1. A light source, absorption cell, photocell, amplifier, and recorder are the essential parts of the instrument. If the light source emitted light of only one wave length a t a constant intensity, all the optical parts maintained constant transmission, and the photocell-amplifier-recorder combination maintained a constant zero and a constant sensitivity, such a device would be very satisfactory. However, these conditions are hard to attain. Power supply variation, ambient temperature variation, and deposits inside the bulb of the lamp and dirt on its outside change the intensity of the light source. Dirt from the sample stream as well as from the atmosphere outside decreases the transmission of the cell windows. Power supply variations, ambient temperature variational aging of components in the amplifier, and dirt deposited on the photocell window alter the zero of the photometer and its sensitivity. A device of the type shown in Figure 1can only be used if the eero and 100% transmission points on the scale are checked frequently. The zero can be checked by placing an opaque shutter in the light beam and the 100% end of the soale by displacing the sample with a gas having negligible absorption. If these checks are made often enough that changes in aero and 100% readings are negligible between checks, and if a cell length is chosen so that the sample transmission is not too near either end of the scale, the instrument can give useful results. Manual checks, as described, greatly decrease the value of an instrument. The frequency of these checks may be reduced by using voltage stabilizers for the lamp and amplifier and by using inverse feedback to stabilize the zero and gain of the amplifier. Circuits that control the intensity of the source by using an auxiliary amplifier and photocell to monitor and control its intensity can be uaeu. No combination of these or similar measures known to the writer can eliminate the need for standardizing an instrument at too frequent intervals. Another scheme is to use differential measurement. A single light source with two photocells placed equidistant from it i s used. The measuring cell is placed between the lamp and one photocell and a shorter compensating cell and adjustable shutter (ConCinud on page 88 A ) 61 A
Instrumentation are placed between the lamp and the other photocell. The sample passe8 through both cells in series. The difference in output of the two cells ia used to operate servomechanism which directs the shutter so that the light intensity reaching the two photocells is equal. At balance the shutter is cutting off as much light as a cell with a length equal to the difference between the two cell lengths filled with the sample. This arrangement eliminates variation of light intensity and amplifier sensitivity as factors. Since identical optical components are used to each beam, it partially eliminates the effect of dirt on cell windows. It is still subject to amplifier zero drift, however. The beat solution is to use the simple arrangement shown in Figure 1 with automatic standardiaation. The Beckman Model DUR recording ultraviolet photometer has automatic standardization of the 100% and is designed so that the zero is stable enough to require checking at not too frequent intervals. Automatic zero standardization would also be desirable. Such an arrangement would compensate for all factors mentioned. When an ultraviolet photometer is caIibrated by passing known mixtures through it and plotting optical density as a function of sample concentration, a straight line, as A in Figure 2, should result. The chances are that a curve more like B or C will be obtained because the light used is not sufficiently manochromatic If a curve such as cplci B or Cis obtained, it is likely that * the calibration of c z VI the instrument 0 W will change owing -1 to a wide variety uof factors. One can attempt to obtain monoCOfi#jENtRATlOW chromatic light by using a source Figure 2. Optical Density as Functlon of Sample Concentration which emits a continuum with a monochromator. A better solution, if a source that emits a line at a suitable wave length can be found, is to use a line source with a monochromator. Sometimes filters that will give sufficiently good isolation of a line from a line gource can be found. Filters used with a continuous spectrum source are rarely good enough. Spectral sensitivity of the photocell can be chosen so that it does not respond to unwanted radiation. This is very helpful. If samples are measured at atmospheric pressure, variations in barometric pressure and cell temperature will affect the amount of sample in the cell and therefore the calibration. Cells can be thermostated and pressure regulated at a fixed value where necessary. When the user realizes that these factors affect the results he obtains with an ultraviolet photometer on a gas analysis application, he can obtain excellent results. I
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