Inexpensive Gas Law Apparatus D. D. Garrett, M. C. Banta', and B. E. Arney Sam Howton State University. Huntsville, TX 77341
The demonstration of Boyle's law is commonly performed using one of two general types of apparatus: a manometerlike setup is used where mercury acts as the piston and pressure source, or a syringe is used to measure volume with a spring scale or heavy objects as a means of applying pressure. The apparatus described here (Fig. 1) avoids the problems associated with mercury as well as eliminating the friction problems usually encountered with the disposable syringes having rubber plunger tips. The differential pressure transd u ~ e rwhich , ~ is attached to the end of a syringe using a short length of Tygon tubing, is a relatively inexpensive (about $20) solid state device. I t is reported to be linear with a f15psi differential and have a relatively high sensitivity (10 mV/ psi using a 10-V dc supply). The internal circuitry of the transducer is configured as a Wheatstone bridge. The transducer has four output pins. Two are used for the power supply; the other two for output.
Slope = 3.85
...
Intercept = '40
Flgure2. Callbratlon data for me 16PC15OFpressuretransducer uslnga 1 2 4 Storage banery as a power source.
Pressure-volume data were obtained by first settin i he syringe plunger a t 30 cc, then attaching the transducer i .he tip, and finally taking millivolt readings a t various voiu~nes in the neighborhood of 30 cc. Some data obtained usin:. the apparatus are shown in Figure 3. The linear graph of F vs. 1/ V (R2 = 0.99456) clearly demonstrates Boyle's law. From the slope of the line (21000 torr cc) one calculates the numl~erof This compares moles of air in the apparatus to be 1.1X well with the number of moles in 30 cc a t 747 torr and 295 K calculated using the ideal ga.. law. +
Slope = 21,000 Pinouts /Honeywell
IEPC05DF
Figure 1. Apparatus usedto demonsnam Boyle's law. Typical calibration data obtained using a mercury manometer and a 12-Vstorage battery for the power source3are shown in Figure 2. The correlation on the line is R2 = 0.99922, and the transducer sensitivity (3.93 mvltorr) is more than adequate.
' To whom wrrespondence should be addressed. 'Model 16PClSDF avallable from Mlcro Switch, a Honeywell DlvC slon, Freeport. IL 61032. The sensitivity of the transducer is dependent on the power supply voltage.
Flgwe 3. byle's law data taken uslng the apparatus shown In Flgure Volume 88 Number 8 August 1991
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