Jeff Bourne, from Northfield High School in John's Creek, GA, writes in:
I took your AP Physics workshop at Oglethorpe U in the Atlanta area several years ago. I was looking over your unit on fluids, and I wondered how do you use the vernier gas pressure sensor to measure the water pressure at a specific depth in a graduated cylinder? I can't find a lab manual that outlines how to do that. I would appreciate any help you can give me.
Hey, Jeff... good to hear from you. I attach some hollow plastic tubing to the probe, and put a small rubber stopper on the other end of the tubing. The tubing is maybe 1.5-2 feet long -- the gas pressure sensor comes with some of this tubing and a stopper, as you can see in the picture.
First, I read the atmospheric pressure, which can range from 97 to 105 kPa depending on altitude, weather conditions, and the probe's internal calibration. Then I measure the depth of water in the graduated cylinder. I do the calculation with the class of what the pressure should be at the bottom using P = Po + rgh. For a typical graduated cylender I use, I get that the pressure at the bottom will be 2-3 kPa above atmospheric pressure, which is easily measurable by this probe.
Finally, I start the data collection (taking about 20 points per second). I feed the tubing into the cylinder until the stopper hits the bottom. Voila, the pressure jumps from, say, 99 kPa to 102 kPa.