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Chemical Instrumentation Edited by GALEN W. EWING, Setan Hall University, So. Orange, N. J. 07079
These articles are intended to serve the readers O ~ T H I SJOURNAL by calling attention to new developments in the theoy, design, or availability of chemical laboratory instrumentation, or by presenting useful insights a n d aplanations of topics that are of practical importance to those who use, or teach the use of, modern instrumentation and instrumental techniques. The editor invites correspondence from prospective contributors.
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maximum speed a t which the spot of light can move relative to the paper. This can vary from perhaps 1 0 m/sec to greater than 10 km/see. As an example, suppose it is required to record faithfully a sine wave a t a frequency of 1kHz and an amplitude (peak-to-peak) of 100 mm, with each cycle occupying 1 mm along the time axis. The length of trace per cycle approximates 200 mm, and this excursion must take dace in see..~ for a witine" speed 01'200m ser: the recording paper muil n w e nr n rate of I m sec 1%)give the requisire resolution. Figure 13shows an example of alaboratory oscillograph, and Figure 14 a portion of a mass spectrum recorded with it. The four traces are from galvanometers connected to the same signal through preamplifiers with the relative sensitivities shown in the righthand margin. (Continued on page A408) ~
XC. Laboratory ~ecorders*
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Galen W. Ewing OSCILLOGRAPHS Analog recorders designed to accept signals that change more rapidly than a servosystem can handle are known as oscillographs. The name is analogous to the eathode-ray oscilloscope, which is also suited for high-speed signals. Most oscillographsfall into one of two classes: those that use photographic recording, and those that rely on pen-and-ink, electrical, or thermal witing methods. Figure 11 shows a representative style of multichannel oscillograph.
nent magnetic field, the mirror deflects a beam of light across the width of a moving strip of photosensitive paper (Fig. 12). After suitable photographic development, the paper shows a trace representing the timevariation of the signal.
Figure 12. Light-beam ascillograph, two channels shown.
Light-beam recorders are often made in multiple-channel models with a separate galvanometer for each channel. The galvanometer movements are replaceable and interchangeable, fitting into slots between the pole pieces of a single magnet. In most models the traces from the several channels each cover theentire effective width of the paper, and hence some orovinion must be made for dirtinvuiqhing one trace trum another. Thrr ran IN ac;omphshed bv meunr of nn mter. rupter,r~rhtvmcrhaniral ra lypr ofshutter) or electrical, to convert a continuous line t o a series of dashes. This type of recorder has the least mechanical inertia and friction of any instrument for direct recording on paper, and therefore can respond to signals of high frequency. Light-heam recorders have been designed that will reproduce a 15-kHz signal, but few commercial models exceed about 5 ~~
Figure 11. A multichannel oscillographic recorder (Beckman Instruments).
Light-Beam Oscillographs
Figure 13. A multichannel light-beam oscillograph (Bell & HowelllDuPont).
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kHz. As the galvanometer coil turns in a permaAbstracted from "The Laboratory Recorder," by G. W. Ewing and H. A. Ashworth, Plenum Press, New York, 1974, by permissian of the copyright holder.
The maximum frequency is determined by such factors as the sensitivity of the photopaper, the intensity of the light source and its ultraviolet V'actinic") content, and the optical magnification. These parameters combine to establish the writing speed, the
Figure 14. A mass spectrogram recorded with the oscillograph of Figure 13. Volume 53, Number 10, October 1976 1 A407
The recording paper most often used in oscillographs is of the type called direct printout paper (POP). The intense ultraviolet light from a mercury lamp produces many latent image centers in the silver halide grains of the paper, and these are caused to grow into larger silver particles by exposure to ordinary room light. If greater sensitivity is required, a low-intensity ultraviolet illuminator, called a lotensifier, can he installed t o provide the needed postexpasure to the chart as it passes out of the recorder. Several companies make X-Y oscillographic recorders using a doubly deflected light beam. Figure 15 shows the optical diagram for one of these. The heam of radiation is first deflected in the X-direction bv,a .. ealvawmeter responding to on? vnrlahle, rhrn by a wwnd gnlvanometer uperaling in the )'-drrertmn in response to a second variable. Photosensitive paper in separate sheets is held against a transparent screen, then developed in the usual way.
an electrostatic charge to he deposited on the coated paper, which then passes through s "toner." The hlack articles of the toner are picked up by the charged areas to produce a clear and permanent record. This is the well-known printing method of xerography.
PAPER DRIVES The mnj~rrityof analog rrrorderv utilize eil herstriprhnrt~orrirn~lnrrhans.'rhestrip charti. supplied in n,lla, hnvr rhe prrat advantage that they can operate continuously for long periods without need of replacement. Circular charts, on the other hand, must be replaced periodically, and hence are used mostly far recording slowly changing variables, a single chart to last,perhaps, oneday or one week. K d I c n d n i are inconvenient from the poinc uf view of storare and retrieval of id'urmation. If only short segments are of permanent interest, as is often the case in connection with laboratory experiments, the strip must he cut up, and the useful portions filed far reference. The paper tends toretain its curl, which adds to its inconvenience. To overcome this failing, fanfold (or "Z-fold") paper has been introduced. In this form the supply of paper is stacked flat, is pulled through the feed mechanism, then stacks itself flat again (Fig. 16).The fanfold paper actually consists of a series of seements corresoondine to individual shett\:separawd by ioldi. The p ~ n can eariiy pair over the srwcd i ~ l d s , a n dthe strip can be separated readily intosheets by tearing along the scores. Fanfold paper can be substituted directly far roll paper only in certain models of recorders. In general, recorders using rolls can be more compact than those using fanfold paper. ~
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Figure 15. The writing mechanism tw an optical X-Y oscillograph.
Moving Pen or Stylus Oscillographs Recorders in this class are of the direct deflection type optimized far high-speed operation, with maximum paper speeds of the order of 200-500 mmlsec and signal frequencies up to 150 Hz. A mechanical deflection system t o respond to such frequencies must he specially designed to minimize the mass and inertia of the movine svstem while keeping it rigid. Any lack ofr&idity would result in distortion due to whipping of the pointer. The maximum amplitude of deflection is usually about 50 mm, so it is convenient t o arrange multiple channels in a side-hy-side arrav . (Fie. - 11).rather than overlapping .. .asis usual with optical types. Some oscillographs use capillary pens, but it is difficult t o ensure proper operation a t high writing speeds, so many manufacturers use thermal or electrical writing methods instead. Varian manufactures a series of oscillographic recorders under the trade name Stotos, which achieve high writing speeds by eliminating entirely all moving parts other than the paper transport mechanism. The recording head contains 100 fixed styli spread across the 100-mm effective paper width. An electrical signal applied to any stylus causes A408 / Journal of Chemical Education
Figure 16. A multiple-pen recorder using fanfold paper (Soitec Company). An alternative particularly adapted to a recorder built in as oart of a lareer instrument, such ns a specrrr.~~h~rrnmrr~r, roniists uf a series uf printed sheeta nmnr
