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11 January 2013

Mail Time: Where's that pressure sensor for the static column experiment?

Tennesseeeeeeeean Wendy Stallings, who attended one of my workshops last summer, writes in:

I'm having trouble finding a pressure sensor to immerse in liquid.  Unless I'm mistaken, I can't find the one you used in the static fluids demo last summer even though I've scoured the Vernier catalogue.  Can you point me in the right direction?

Certainly.  Wendy is referring to a quantitative demonstration I do:  I fill up a tall graduated cylinder with water.  We use the equation P = Po + ρgh to calculate the pressure at the bottom of the cylinder, knowing the pressure sensor reading of atmospheric pressure.  Then, I measure the pressure at the bottom; voila, the pressure is just as we predicted.  Physics works.

The sensor is the standard issue Vernier gas pressure sensor, as pictured, and as available on the Vernier website for about ninety bucks.  (Ask around your school, first -- biology teachers use these, and you may be able to borrow one.)  Thing is, I don't actually submerge the sensor.  

Look at the picture, at the right-hand edge of the sensor.  That's an attachment for a tube, one somewhere in the neighborhood of 5-10 mm diameter.  A bit of this tubing, with various connectors, comes with the sensor, at least according to the Vernier website.

I connect the 18" tube to the sensor, and submerge the tube.  Sure, the tube gets wet, but the sensor doesn't.  The sensor reads the pressure at the end of the tube, giving me the experimental verification for the P = Po + ρgh prediction.  I don't know whether or not Vernier recommends such an experiment; I know I've done this one for years with no adverse effect on my sensor.

And yes, this is the same pressure probe I use for the heat engine and gas law experiments.  I love equipment with multiple uses.


1 comment:

  1. For people wanting to roll their own, I've found the Freescale Semiconductor integrated pressure sensors easy to work with. The ones with the integrated amplifier are very easy to hook up to an Arduino, though—the high school students I had in a robotics club last year hooked one up to an Arduino for use as a depth gauge.


    I'm using a different Freescale pressure sensor for my circuits class, because I want them to design the instrumentation amplifier themselves, rather than having an integrated amplifier. It is a nice example of a strain-gauge bridge.