<div class="gmail_quote">On Wed, Mar 2, 2011 at 14:11, Toby Seiler <span dir="ltr"><<a href="mailto:seilertechco@yahoo.com">seilertechco@yahoo.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<table cellspacing="0" cellpadding="0" border="0"><tbody><tr><td valign="top" style="font:inherit"><div>Mark, Daniel C., list,</div>
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<div>OK I admit that I'm lost when I get to "mole" in the ideal gas law. (A mole is a furry creature my dog brings up on the porch to chew up.)</div></td></tr></tbody></table></blockquote><div><br></div><div>
Yeah moles can throw one for a loop, I think it is the Chemists' way of keeping all the fun stuff to themselves.</div><div><br></div><div>In engineering applications the equation for the ideal gas law is expressed a bit differently. Even though it means exactly the same thing, the quantities that it uses are more relevant to engineering approaches:</div>
<div><br></div><div>P = rho * R * T</div><div><br></div><div><meta http-equiv="content-type" content="text/html; charset=utf-8"><div>where P is the pressure, rho is the density of the gas, R is the gas constant <i>of that gas</i> (not the Universal Gas Constant), and T is the temperature (in an absolute temp. scale i.e Kelvin or Rankine).</div>
</div><div><br></div><div>These quantities are chosen because for many engineering problems it is more convenient to work with the density of a gas than with the volume of the gas and the number of moles of gas in that volume (we don't ordinarily control or measure the number of moles)</div>
<div> </div><div>This equation is valid in any consistent set of units. While I have nothing against Imperial or US Customary units and in fact use them quite freely for many things, there is a big trap waiting for the unwary (such as me!) with the difference between "pounds-mass" and "pounds-force". This is the reason that you often see "w/g" or "w/32" factors in equations, the "g" or "32ft/sec/sec", it is the gravitation acceleration term used to convert pounds-mass to pounds-force. Frankly since I didn't learn things that way and am worried about making a mistake I prefer to switch into SI units for my physics or engineering calculations and then perhaps switch back to Imperial for building things.</div>
<div><br></div><div>In SI units we have:</div><div><br></div><div>P = pressure, in Pascals (which is Newtons per square metre). FYI standard sea level atmospheric pressure is 101,325Pa which is conveniently close to 100,000Pa that that is usually used. If you are more comfortable thinking in PSI (like I am for some things) you can convert PSI to Pascals by multiplying by 101325/14.69. If you prefer approximations there are about 7000 Pascals per PSI. And when dealing with lower pressures such as from fans and blowers, one inch of water column is very close to 250 Pascals</div>
<div><br></div><div>rho = density, in kilograms per cubic metre. Air has a density of about 1.22kg/m3 at a temperature of 15C. I find it very convenient since I can visualize how big a cubic metre is and I can also visualize how much a kilogram is.</div>
<div><br></div><div>R = individual gas constant. For air it is 286.9 Joules/kg/K. BTW I didn't know that I had to look it up - see <a href="http://www.engineeringtoolbox.com/individual-universal-gas-constant-d_588.html">http://www.engineeringtoolbox.com/individual-universal-gas-constant-d_588.html</a> for more info. Gases of interest to us are mixtures of water vapour, oxygen, nitrogen, carbon monoxide, carbon dioxide and hydrogen. The gas constant of mixtures of gases is quite straightforward, you use multiply "volume percent" of each gas by its individual gas constant, and then sum all of these contributions.</div>
<div><br></div><div>T = temperature in Kelvin. Freezing (0C, 32F) is 273 K, "standard temp" (15C , 59F) is 288K. Typical warm air temps are conveniently about 300K (which is 27C or 80F).</div><div><br></div><div>
<br></div></div>-- <br>- Daniel<br>Fredericton, NB Canada<br>