A basic physics question

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ecolechio
ecolechio's picture
A basic physics question

This is not exactly a homework question. I am trying to set up an air-jet for somatosensory stimulation. The air travels from a tank, past a pressure gauge, to a manifold with electric valves. When a valve is open, the air travels through another tube that is directed toward the animal. For regular experiments, the pressure at the gauge is 25 psi, and the tube from the manifold to the animal measures about 15." However, for fMRI experiments, I need to make the tube between the manifold and the animal much longer - about 228." But I want the stimuli with the 2 lengths of tubing to be comparable in terms of the air pressure delivered to the animal. I broke this problem down into 2 steps.
1) How can I calculate the pressure at the end of the 15" tube (with gauge at 25 psi)?
2) Using the same equation(s), calculate what the pressure at the gauge must be in order for the pressure at the end of the 228" tube to equal the pressure at the end of the 15" tube.  (I have 2 manifolds which can be independently controlled.)

g a
g a's picture
Hi Ecolechio

Hi Ecolechio
Well Im not capable to answer your question.
But Its good to see that people from other science disciplines are also joining in. I dont know whether we have a physics scientist in the forum as well but I surely believe that it will be a key for starting up a common venture between all disciplines where physics and chemistry will merge and biology will feel blessed by the union.
 

Jon Moulton
Jon Moulton's picture
Why calculate the pressure? 

Why calculate the pressure?  You should be able to measure the gas velocity at the end of the tube directly, using (for instance) a rotating vane and tachometer, or a rotometer.  Tune your pressures with the regulator so that you have the same gas velocity output for each of your tube lengths, the short rig and the fMRI rig.

ecolechio
ecolechio's picture
Well, I want to have

Well, I want to have flexibility with the tube length because 228" is the minimum tubing length; it will be longer depending on where in the room I can put the equipment. So I would ideally like to have a calculation on hand so that I know how to adjust gauge pressure depending how long the tube actually has to be when I get into the magnet room. Also I don't know if the lab has a rotometer or tachometer.

Jon Moulton
Jon Moulton's picture
Someone around the department

Someone around the department will have a rotometer.  Use a few tube lengths and measure the flow, setting the regulator pressure so you get the flow you want.  Plot a calibration curve then and you can change the tube length anytime and refer to the curve.   However, I wouldn't go to all that work.  You can just change the tube length, stick the rotometer on the end and adjust the regulator so you get your desired flow.  Getting your hands on a rotometer you can use for a few weeks will save lots of time (since if you have the instrument on hand, you won't need to make the calibration curve).
Here are a few links to companies that sell rotometers.  I'm not recommending these necessarily, this is just to show you what I'm talking about.  You can probably find one in a dead instrument and salvage & use it.  Ask at your institution's machine shop, they likely have one lying around.
http://www.coleparmer.com/catalog/product_index.asp?gcid=s18582x018&cls=1687&request=category&keyword=rotometer&gclid=cocszudikzscfqk_agodvguxbg&wt.srch=1
http://aalborg.lambda-med.hu/katalogus/Aalborg-Rotameter.pdf
 

ecolechio
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Cool, thanks, I will check

Cool, thanks, I will check around.