INSTRUMENTATION by Ralph H. Müller
Many Off-the-Shelf Electronic Instruments Have Sufficient Precision and Stability to Use for Analytical Purposes SEVERAL COMMERCIALLY AVAILABLE ELECTRONIC INSTRUMENTS have
now attained such high levels of precision and long-term stability that analysts and other chemists may do well to re-examine some of their techniques in order to take advantage of these developments. E P U T Meter. One of these is the E P U T (events per unit time) meter. It is a means of counting or of measuring frequency in a direct reading with a precision of one part in one hundred million (1 in 10 8 ). Of the several types available we happen to have a HewlettPackard instrument. For the precise measurement of frequency this device counts individual cycles. The time interval during which they are counted and stored is determined by selecting a start and stop gate. The latter is controlled by a temperature-regulated quartz crystal oscillator. The cycles thus counted, per unit time interval, are presented in bold illuminated digits. The presentation time can be as short as operator recognition will permit or the incoming frequency can be sampled at longer intervals and thus present periodic values of the frequency. Between readings the counting system is automatically cleared to receive the next sample. In measuring rotational speeds or linear translations, photoelectric tachometers or detectors can supply the pulses while the built-in quartz crystal is measuring elapsed time. A simple attachment to the EPUT meter permits all readings to be fed to a
high speed printer. Although frequency has long been one of the easiest variables to measure with high precision, some of the methods were elaborate and quite complicated. The newer instruments are by no means simple, but they are completely automatic and give an almost instant direct indication. These features are so attractive, aside from the high precision, that it is worth while to go to the trouble of converting other variables to a frequency or change thereof. An obvious example is in the measurement of dielectric constants. Heretofore such measurements have been made by using a capacitor containing the dielectric to change the frequency of a local oscillator and beating the latter against a crystal-controlled reference oscillator. By then tuning a precision variable capacitor connected in parallel to the measuring capacitor and once more establishing zero or reference beat, the necessary information can be deduced from the readings of the reference capacitor. Although elegant capacitors have been designed and built for this purpose, they can't hope to attain precision of the order of parts per million or hundred million. Voltage-Frequency Converter. An essential adjunct in this connection has appeared in the form of voltage-tofrequency converters. In these, small d.c. potentials are used to control the frequency of an oscillator. This can be done in several ways, only two of which will be mentioned. A type of
capacitor is available in which the application of a d.c. potential will change the capacitance over a fairly wide range. If this capacitor is connected in the tank circuit of an oscillator, the output frequency becomes a function of the applied d.c. potential. Several types of relaxation oscillators can have their output frequency controlled by a suitable d.c. bias voltage. One of the most recent applications is a radiation hazard indicator. In this vest-pocket-sized device, counts from a Geiger counter or other detector are converted to a d.c. potential by means of a conventional count rate meter type of circuit. The resulting potential is applied as a bias voltage to a transistor oscillator, the output of which drives a minute loud speaker of the hearing-aid type. For zero or tolerable radiation levels the oscillator is initially biased to produce a frequency below audibility and as hazardous conditions are attained a note of rising pitch results. Presumably when the pitch or frequency passes beyond the upper limit of audibility, it's time to scram. At this time, voltage-to-frequency converters have not attained the precision of an EPUT meter which would be used to measure their output. However, careful design will permit a precision of the order of a few hundredths of 1%. Digital Voltmeters. As we mentioned some time ago, digital voltmeters have been with us for some time and these provide means for the rapid and VOL. 33, NO. 9, AUGUST 1961 · 8 9 A
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ANALYTICAL
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INSTRUMENTATION precise accumulation of data. A typi cal instrument reads to five digits with automatic indication of decimal point and polarity. Most of them at present are available as wide-range instruments, say from 0.001 to 999.99 volts, using a range selector. A guaranteed accuracy of ± 0.01% is the rule. Practically all meters of this class have provision for printed readout as well as indication and in many cases this is almost man datory if one is utilizing the fastest sampling capability of the instrument. These elegant devices have been de veloped of necessity because in rocket and missile researches an enormous amount of accurate information must be accumulated in a short time with the added problem of telemetering it to the ground. Although this is primarily the reason for their development, there is no reason to believe that they will not be even more important and widely used in the research laboratory. Communications. From the very beginning, communication theory and practice · have set the pace for other electronic applications in science. Even now the gas optical maser shows im mediate application in broad band com munication, but as a means of produc ing intense narrow beams of coherent radiation it will undoubtedly revolu tionize several branches of optics. In the latter field the analyst has long had a heavy stake. With all the fantastic technical de velopments in communications, it's a serious question of whether man is able to keep up with them or even how to handle them intelligently. Recent ar guments about the quality of television programs and the comments of govern ment officials, the industry, and the public have evolved much heat and some light. From the point of good taste, perhaps it may be conceded that we have a new 4-H Club in America— horror, homicide, halitosis, and hemor rhoids. Whatever one's tastes and preferences may be, it still takes time to flip the station selector switch from clinical pathology to entertainment. It can be done electronically and almost instantly but that is the difference be tween technology and human attributes and desires. Advertising. We are currently hav ing trouble with the printed word. Whenever we drop a journal or tech nical magazine it no longer falls flat and loses our page for us; it reposes at a dihedral angle, supported by a hard paper insert which insists that "it pays to see John Doe" or "your shirts come out cleaner with Allgone." This is wildly amusing—the first time. It's a two-hand job to hold a modern journal
and if one hand strays to the ashtray to recover the cigarette—pop! back comes John Doe. In the old days these tear-out inserts were good for a nice set of dishes but costs have gone up and times have changed. Some of these inserts are perforated near the stitch ing and by tearing them out at the be ginning one can settle down to serious reading. Others are firmly embedded and even if deftly removed with a razor blade the little yellow, pink, or green stub has remarkable rigidity and stamina and keeps coming back like Marley's ghost to haunt the reader. We've tried placing journals on the desk, but one false move and the whole thing folds up, as if to say, "Brother, you may think it's early but it's really time to go to bed." We have tried weights on each side but the last time, engrossed in a well written paper, we hurriedly turned the page and tore off that important reference to the "Make shift fur Physik." The Madison Avenue people under stand these things perfectly, but to what lengths one can go to attract at tention without enraging the customer would seem to be worthy of serious con sideration. An intelligent conversa tionalist can present an argument quietly and convincingly ; he does not have to reach for a baseball bat. It is doubly encouraging and delight ful to encounter dignified, well written, and informative advertising. There is a lot of it even if it takes some search ing to find it. A notable example is the inspired page by Eastman Kodak and its subsidiaries. Many of their re leases are more interesting and scien tifically provocative than contributed papers. These highly informative notes are a real service to the research man. These people are actually apologetic about having something to sell and more often than not complain about the outrageous price which they are re quired to charge for brand new prod ucts which are available in small quan tities. The Sigma people in New England who make good relays and some very unusual ones, manage to convey a lot of information in their advertising. They seem to delight in tackling diffi cult jobs, particularly if they have goofed in initial attempts to solve them. In our opinion, inspired technical ad vertising has unlimited possibilities. For the alert research man such mate rial is often more stimulating and help ful than conventional scientific con tributions. After all, he is looking for new ideas and new techniques rather than repeated confirmation of well established principles.
