INSTRUMENTATION by Ralph H. Müller
A.C. Circuits for Wheatstone Bridges, Acoustical Spectrometry RECENT DEVELOPMENT by InstruA- merits and Communications, Inc.,
of 33 Danbury Road. Wilton, Connecticut, results from the adaptation of solid state electronic techniques to the Wheatstone bridge having a null readout. For years, null detectors, including vacuum tube and spotlight galvanometers, have been used with d.c. bridge circuits in many laboratory instruments including cryoscopes, molecular weight apparatus, fluoromcters, osmometers, etc. The new device is a transistorized phase-locked a.c. amplifier system which allows the a.c. (rather than d.c.) operation of many laboratory bridges, and provides an output suitable for null readout, recording, on-off or proportional control. A phase-locked amplifier works as an extremely narrow band detector operating at a particular constant frequency at which the signal information has been made to appear. It has long been known that a.c. operation of a bridge is often highly desirable, but previously there have been few suitable a.c. detector circuits available. The key to the new system is high frequency (approx. 1500 c.p.s.) providing very high sensitivity and virtually drift-free performance with complete independence from 60-cycle effects. Practically all thermal and transient problems are eliminated. The low power requirements of silicon transistors allow battery operation, which ensures further freedom from external disturbances and permits the system to be floated above or below ground. The a.c. amplifier feeds a.c. into the bridge and provides the means of reading the bridge differential. The unit consists of a phase shift oscillator, a d.c. coupled broad band amplifier and a phase-locked, matched diode bridge demodulator. Construction is modular circuit board. Instruments and Communications, Inc., invites inquiries from analytical chemists having special Wheatstone or other bridge circuitry problems. Initial applications of the lock-in amplifier have demonstrated that it can easily be used with existing laboratory instruments or built into special apparatus. Tt can tailor the circuit for a particular application because circuit construction is completely modular and amplifier modules can be cascaded for additional gain if required.
Acoustical Spectrometry
Clarence Zener's prediction in 1948 is fast coming to pass. In "Elasticity and Anelasticity of Metals," University of Chicago Press (1948), he said " . . . it seems likely that the acoustic spectrum will come to occupy a position in solid-state studies comparable to that possessed by the optical spectrum in the theory of atomic and molecular structure." According to information received from the Nametre Company of 1246 Highway 27, Highland Park, New Jersey, their Acoustic Spectrometer, in addition to supplying important information of physical and engineering value, shows great potential value as an analytical tool. Inasmuch as we do not yet have the latest information on the Model IV Nametre Acoustic Spectrometer, we confine these remarks to some of the things which can be accomplished with acoustical spectrometry. With an acoustical spectrometer the absorption of acoustic energy in solid materials can be measured as a function of frequency and temperature. For a quarter of a century investigators have been carefully identifying acoustic absorption spectra of many different solids in terms of micro and atomic structural changes The composition of many solids has been correlated with the acoustic absorption spectrum (or internal friction). Some examples are the concentration of metals in alloys, gases in metals, oxides in glasses, polymers in plastics, and moisture in hardened portland cement paste. Solid-state reactions invariably cause a characteristic absorption. From measurements at two or more frequencies, the heats of reaction can be determined. Typical reactions which have been studied by this technique include solids with gases, phase changes, recrystallization, and order-disorder transformations. The technique is especially valuable in following nucleation and precipitation phenomena in solids. The rates of diffusion of carbon, oxygen, and nitrogen atoms in iron, tantalum, and many other metals have been measured quantitatively in terms of internal friction over extensive temperature ranges. Activation energies have also been determined. The diffusion of
alkali ions in glasses likewise has been studied. Other phenomena which can be studied by acoustical spectrometry and which may or may not be of analytical importance are grain size, degree of polycrystallinity, viscous effects, thermal history, elastic modulus, viscous flow, and a variety of engineering properties of materials. The experimental approach involves the introduction of an audio frequency signal into a specimen of definite dimensions. This is accomplished with a transducer (piezo-electric or magnetic) whereby acoustical energy of known frequency is introduced into one end of the specimen. A transducer at the other end of the specimen converts the transmitted acoustical energy back into an electrical signal. For a given specimen there will be a resonant frequency and at this frequency, the amplitude is inversely proportional to the acoustic absorption or internal friction. Obviously the spectrum, so obtained, can be studied as a function of sample temperature as well as frequency. We shall describe the Model IV spectrometer in this column as soon as detailed information becomes available. In its predecessor, resonance amplification curves are plotted automatically with an x-y recorder. The complete resonance curve is traced on the same chart, but as a function of frequency at periodic intervals. Then an accurate relationship can be established between resonance amplification and interval friction. Once more we see the two important trends which contemporary analytical research is taking. As so ably demonstrated by Howard Malmstadt in his recent Sargent Award address, automated methods, even though based upon well known physicochemical principles, provide us with analyses which are more rapid, precise, and reliable, in addition to which they afford a continuous indication of "what is going on." The other and complementary trend is the re-examination of entirely foreign techniques, heretofore the exclusive tools of the engineer or physicist, for their possible analytical significance. For a long time the resources of instrumentation have been more than adequate. I t is a question of developing an entirely new mode of analysis. VOL. 35, NO. 7, JUNE 1963 ·
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