the computer bulletin board Measuring Pressures Through the Apple Ile Game Port James P. Blrk, Davld Walters, and Evert Frunman Arizona Stale University Tempe, AZ 85287
An inexpensive pressure measurement device can he constructed with a nressure sensor and some simole electronic parts. The sensbr is interfaced to an Apple i1e computer throueh the eame port. Modified Proiect SERAPHIM3software [s used-to collect game-port paddle readings and convert them to pressure values. Input to the Apple IIe game port can he in the form of either resistance changes or current changes. We have designed a device that can measure pressures. This device uses the SCX15DN pressure sensor with an SCX-EB universal evaluation board, available for about $50 from SenSy~n.~ All other parts were obtained from Radio Shack for a total of less than $10. The pressure sensor produced a 0-90-mV output over a O15-psi pressure range relative to a reference pressure. In all the experiments we have done with this pressure sensor, we used atmospheric pressure as the reference pressure by leaving one port open to the atmosphere. Other reference pressures could be used to shift the range of pressures that could he measured. One port measures pressures lower than the reference pressure,- while the other port measures higher pressures. The evaluation hoard converts the sensor output voltage to a 0-5-V output that is linear with pressure. More detailed characteristics of the pressure sensor and a circuit diagram for the evaluation board may be found elsewhere (21). For use with the Apple game port, we had to add electronics to convert the variahle output voltage from the pressure sensor and evaluation hoard to a variable current or a variahle resistance. A device, diagrammed in Figure 3, was de-
from
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CA3130
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To GND
F l g m 3. Circundlagramfwa device lo mventhe pressure sensor output to
a variable current.
3Disk AP 1203, Project SERAPHIM. Department of Chemlstry. Unlverslty of Wisconsin. Madlson, WI 53706 SenSym Presswe Sensor Handbook SenSym: 1244 Reamwoad Ave.. Sunnyvale. CA 94089.
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A224
Journal of Chemical Education
signed to carry out this conversion. In this device, the 100-0 and 5 0 0 4 resistors take a portion of the output voltage from the pressure sensor and use it to drive an op amp, which supplies a current through the 100-kR resistor to the Apple IIe game port. The output current varies as a function of oressure. The inherent stabilitv of the OD amo eives this hevice excellent long term stability. Measurements of a known pressure gave identical readings over periods of months. The pressure device was calibrated by collecting Apple Ire game-port paddle readings as a function of pressure. Pressures were applied to the pressure sensor port by connecting it to an open-end manometer. Adjusting the height of the open end provided a variable pressure difference, whose vhue was measured from the difference in mercury levels. A of 0.999, is obgood fit, with a correlation coefficient (RZ) tained these ~ - when ~ ~ data ~ are ~ fit to a fourth-order oolvnomial. readings change between aboui 255 and 50 Although over a pressure range of 300 torr, we limited pressure changes to 150 torr to avoid regions of the curve where paddle readings did not change substantially with pressure. Software was developed (22),based on Project SERAPHIM disk AP 1203, to read the game-port paddle, to calibrate the pressure device, to convert the g&e port paddle readings to pressures, and to display data in tabular or graph&al form. This software usesa matrix inversion routine to fit paddle readings to a fourth-order polynomial equation as a function of pressure (relative to atmospheric
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pressure). This polynomial equation is then used, with a aleorithm. to convert individual successive aooroximations .. paddle readings to pressures in torr. A variety of experiments has been developed with the pressure-sensor device. For example, rates of production of hydrogen gas by the reaction of magnesium ribbon with hydrochloric acid were measured easily. From these data, the complete rate law for this reaction could be determined. ~easurdmentsof the final pressure in these reactions were used to determine the stoichiometry of the reaction. Results were in eood aereement with the theoretical stoichiometrv. The Gess&-sensor device provided an excellent m e a k of ex~lorinethe eas laws. For examole. measurements of the of"sami1es of air as a function of temperature provided data for Charles' law. Plots of pressure against temperature were quite linear and could he extrapolated back to zero pressure to calculate a value of ahsolute zero. With three-different air samples, each having a different amount of gas and thus, a different starting pressure, values of absolute zero of -264, -272, and -270 OC were obtained. By attaching a syringe to the pressure sensor, the dependence of pressure on the volume of the gas could be examined. A Boyle's law plot of pressure against the inverse of volume was ouite linear. ~ i e s are e but a few of the experiments that can he carried out readily with the pressure sensor device. Details of these
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(Continued on page A226)
Volume 66
Number 9 September 1991
A225
the computer and other experiments will be reported separately. In the interim, we hope that this device can be used as an effective tool for teaching avariety of chemical and physical concepts.
How to Save Time with Current Contents on Diskette and Interlibrary Loan Requests Ernest C. McGoran DBpartrnent 01 Cheminry/Blochemistry Eastern Washington University Cheney. WA 99004 The time needed for keeping abreast of chemical literature can be sienificantlv saved throueh comouterized methods that g r e h y improve efficiencfin locating articles of interest. Searchine databases is facilitated bv -com~uterized . on-line services such as STN5 and Dialogqhat allow for broad exolorations of the scientific literature retroactivelv and help t o maintain current awareness. A popular way to k e e ~informed of recent oublications is through Current contents, a service of the-institute for scientific Information (ISI).7 The weekly issues reproduce the content pages fromthousands of scientific publications in both a s o f t cover, printed format and, since 1989, a disk format for both IBM-compatible and Macintosh PC's. Current Contents has different editions t o match the interests of the reader; the "Phvsical. Chemical and Earth Sciences". "Life Sciences", a i d ' ' ~ E n ~ i n e e r iTechnology n~ and ~ p p l i e dScience" versions are the ones widely used by chemists. Advantages of using Current Contents are access to uncommon scientific nublications and timeliness. eenerallv a maximum of two to ;our weeks lag between a journal's publication date and its appearance in Current Contents (23). The time required for studentss or other users to obtain a copy of a desired article can vary significantly. Users can employ ISI's "Genuine Article" service or can send reprint requests directly to article authors. The interlibrary loan service provided by most university libraries is a convenient alternate. Generally there isa form to be filledout giving the standard bibliographic information required for proceusing. A straightforward procedure for generating computerprinted interlibrary loan requests uses Current Contents on Diskette and Mcrosoft Word. The bibliographic information can be directlv transferred from Current Contents on Diakelle many srandard interlibrary loan requevt form.The method discussed aoolies to the Macintosh but can be readily adapted to the s a k e software on IBM compatibles. The process makes use of the Print Merge command of Microsoft Word for form letters and an exported Personal Interest Checklists (PIC) file generated from within Current Contents on Diskette. ~~
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STN International, c/o Chemical Abstracts Service, 2540 Olentangy River Road, P.O. Box 3012, Columbus. OH 43210. B~ialog,3460 Hlllvlew Avenue, Palo Alto, CA 94304. Institute for Scientific Information. 3501 Market Street, Phiiadetphia, PA 19104. We introduce our students to Current Contents on Diskefie in a chemistry course on research and literature methods. Although our students also use STN, Current Contents provides a more economical alternate for independent student searching of the computerized literature, c.f., Miller, J. M. J. Chem. Educ. I989 86, 24. A226
Journal of Chemlcal Education
