INSTRUMENTATION by Ralph H. Müller
Simple Electromanometer, Solid State Relays, Thermistor Developments, and Beryllium Analyzer Described \
SIMPLE ELECTROMANOMETEK
Utlliz-
•£*- ing the CdS photoconductive cell has been developed by three Japanese investigators, [Azuma, T., Kanno, T., Hirota, T., J. Sci. Instr. 38, 413 (1961)]. Although the device was intended for physiological studies, it would seem to be useful in many other applications. The investigators used any of four slender CdS cells of 2.5-, 5-, 10-, and 15-cm. length placed parallel to and behind a mercury manometer which was illuminated with an ordinary incandescent bulb such as is used for microscope illumination. AVhen hydraulic manometers are employed, the authors use machine oil, deeply colored with a suitable dye—e.g., Sudan Black—with the oil layered on top of the water column for effective screening of light. A typical cell is monocrystal of CdS having an electrical resistance of more than 10a milliohms in darkness. The photoelectric current is proportional to both intensity of illumination and to the applied d. c. voltage. For one cell, the photocurrent was 3 ma. at an applied potential of 6 volts and illumination of 100 Lux, while at 500 Lux and 12 volts, the current was 20 ma. The dark current is negligible under 50 volts applied potential. [American investigators contemplating such applications should note that commercially available photoconductive cells vary widely in dimensions, electrical characteristics, and spectra! response. Some examples: from one manufacturer alone are the RCA types 6694-A, 6957, 7163, 7421, and 7536. The first of these has an extremely sharp and selective response at a wavelength of 500 nifi; the other four, a much broader response centering at about 580 ταμ.'] In the usual temperature range (0° to 40° C.) the rate of increase in cell re sistance is about 7% per degree C. [This is an oversimplification, because the temperature coefficient varies enor mously with the level of illumination,
being relatively small at high levels of illumination.] The authors obtained greater sensi tivity by connecting the cell in a Wheatstonc bridge circuit. Speed of pressure recording is limited by inertia of fluid in the manometer column. They succeeded in measuring carotid arterial pressure of a toad resulting from a single electric shock given to the left vago-sympathetic nerve and to re petitive stimulation of the splanchnic nerve. Faithful recording of phenom ena of high frequencies is limited by inertia—for example, it is not possible to note quick changes in intraventricu lar pressure during one cardiac cycle. The advantages of this device are low cost, simplicity, and no need for amplification. By proper choice of fluid in the manometer and length of the CdS cell a wide range of pressures can be handled (1 to 300 mm.) and either large or very small changes in pressure (a few millimeters of FLO).
circuit preceded by one stage of pentode amplification. The modern equivalent of this tech nique employs solid-state devices with resultant economy in size and weight and the other attendant advantages of transistorized circuitry. Ferdinand iSibler [Rev. Sci. Instr. 32, 1143 (1961)] uses a Zener diode driven near the Zener point. Since there is a rather abrupt change in current near this point an essentially triggering ac tion is obtained. Although the author believes it is possible to use a Zener diode alone in a series with a relay, he has preferred to use a two-stage tran sistor amplifier ahead of the Zener di ode and a single following stage to drive the relay. Using a 1000-ohm, 10-ma. relay, positive control was ob tained with an input signal of about 0.5 mv. The specific application was the control of a laboratory-built ther mostat using a thermistor as the tem perature-sensitive element in a Wheatstone bridge circuit.
Relay Operation
Improving relay operation is a per ennial problem. In mam· scientific and technical problems wherever an electromagnetic relay is used, it is de sirable to diminish the "dead-zone" as much as possible—that is, to make the difference between the "pull-up" and "drop-out" currents as small as pos sible. Good design of the relay can ac complish much in this direction but, as a rule, much more sensitive and posi tive control is desired, particularly if either the upper or lower control level of the relay is approached very gradu ally. For many years various trigger circuits have been used for this pur pose. Their advantage resides in their property of passing current to the re lay in an "all or nothing" manner at microsecond switching speeds. An early application of this principle in automatic potentiometric and photo metric titrations [Muller, R. H., Lingane,
J.
J., ANAL.
CHEM.
20, 795
(1948)] employed a Schmitt trigger
Thermistor Interest High
Interest in thermistors continues at an unremitting pace. For those who still doubt the reliability and precision of thermistors, a most convincing and elegant example is furnished in the paper by II. V. Larson, I. T. Myers, and W. II. LeBlanc, General Electric Co., Richland, Wash. [J. Sci. Instr. 38, 400 (1961)]. It is concerned with a method of linearizing thermistor ther mometer data in calonmetry. It is not, feasible to reproduce the straight forward but extensive mathematical treatment here, but the results are im portant and of interest to anyone in tending to make precise temperature measurements with thermistors. As the authors show, the correction for the nonlinearity of thermistor thermometers may either be experi mentally determined or calculated from the thermistor specifications. This sys tem of reducing thermistor data was applied to measurements over temperaVOL. 34, NO. 1, JANUARY 1962 ·
101 A
COLEMAN
GALV-O-METER makes easy work of precise measurement The Coleman Galv-O-Meter is a precision measuring instrument built around a taut-suspension galvanometer and a slide-wire potentiometer. Already of proven value in routine flame photometry and trace fluorimetry, it now is emerging as an important research tool. Its characteristics make critical analytical conditions
easy to establish and just as easy to duplicate. They are: GALVANOMETER SENSITIVITY — 1 micro-ampere full scale. POTENTIOMETER ACCURACY — better than 0.5%. The instrument's versatility is limited only by its user's ingenuity. Here are but a few of its major laboratory applications: • • • • • • •
indicating galvanometer precision potentiometer indicator in bridge circuits millivolt meter linear voltmeter microammeter photometer readout
Capitalize on this versatility to m a k e your own work easier. Write for Bulletin AB-261 COLEMAN I N S T R U M E N T S , INC. MAYWOOD, ILLINOIS Circle No. 73 on Readers' Service Card 102 A
·
ANALYTICAL CHEMISTRY
INSTRUMENTATION ture differences of several degrees in two different adiabatie calorimeters with a linear temperature resolution of 105 to 1. The treatment is essentially one of combining the resistance temperature characteristic of the thermistor with those of the adiabatie calorimeter and its power input. By rearrangement of the respective equations, an equation in the form of a straight line is obtained, the least squares solution of which permits a precise evaluation of the constants. In a second calorimeter, heat input to the calorimeter was furnished by a standard half-gram radium source, rather than electrically. The high precision attained in these studies as compared with ordinary methods for thermistor calibration arises from the elegant and precise means for measuring the power input to the adiabatie calorimeter. Beryllium Analysis
A standard procedure in nuclear physics has been reversed, so to speak, to provide a selective and very sensitive means for the analysis of beryllium. For some years it has been possible to obtain low-level as well as high-level neutron sources from Oak Ridge. These sources consist of a cylinder of antimony inserted in a beryllium capsule and sealed in an aluminum jacket. The neutron emission is produced by the gamma-neutron reaction produced in beryllium by the hard yravs emitted during the decay of Sb 124 . The half life of Sb 124 is 60 days and after it has decayed, arrangements can be made for its reactivation in a reactor. The Research Chemicals Division, Nuclear Corp. of America, Burbank, Calif., has brought out an instrument, the Model BEL 100B Berylometer. This consists of a 5-decade scaler, detector head, and a lead castle for shielding. The sample, metal, powdered rock ore, or beryllium-containing compound to be tested, is placed in a container and inserted into the sample slide. Gammarays from an antimony-124 source interact with the beryllium in the sample, giving off neutrons, which are detected by a scintillation counter. The count rates are compared with samples of known beryllium content. We have no definite information on the selectivity and sensitivity of this procedure, but they should be high. Presumably, new slugs of Sb 124 are available to replace those which have decayed beyond a certain useful level. The steady decay is of no importance in an actual determination, because an unknown is always compared with a standard sample.
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