<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META http-equiv=Content-Type content="text/html; charset=iso-8859-1"> <META content="MSHTML 6.00.2900.2802" name=GENERATOR> <STYLE></STYLE> </HEAD> <BODY> <DIV> <DIV> <DIV><FONT face=Arial size=2>----- Original Message ----- </FONT></DIV> <DIV><FONT face=Arial size=2>> You have failed to understand the example, and changed what I said. Sorry I<BR>> was not clearer that all factors *except* the number of molecules in the<BR>> container were the same.<BR>> <BR>> Regards,<BR>> Don Rose, B.Mus., A.M.U.S., A.MUS., R.P.T.</FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial>I don't think I changed anything you stated, but rather asked for clarification, which is still needed to understand what it was you said. </FONT></DIV> <DIV><FONT face=Arial></FONT> </DIV> <DIV><FONT face=Arial>However......</FONT></DIV> <DIV><FONT face=Arial></FONT> </DIV> <DIV><FONT face=Arial>If you have a container of air that has some moisture in it of let's say 50% RH, and remove some air from the container, I think the RH will remain unchanged at 50% (or there abouts - I don't know - does atmospheric pressure affect RH some small amount?). I guess the question is whether atmospheric pressure affects the saturation vapor pressure. RH is defined as: The ratio of the actual vapor pressure of the air to the saturation vapor pressure. So if you remove air (oxygen, nitrogen, etc. and water vapor) from a container, you lower the atmospheric pressure in the container, which would presumably lower the saturation vapor pressure, but by removing some water vapor, you would also lower the water vapor pressure - and I would think, proportionately the same amount. So, I'll admit that I'm not 100% sure, but I think RH would stay about the same, if not exactly the same.</FONT></DIV> <DIV><FONT face=Arial></FONT> </DIV> <DIV><FONT face=Arial>Maybe someone can weigh in on that - I thought we had a resident chemist?</FONT></DIV> <DIV><FONT face=Arial></FONT> </DIV> <DIV><FONT face=Arial>Now it is true that if you remove some air (and water vapor) from the container, the absolute humidity would be reduced, as absolute humidity is defined as: The ratio of the mass of water vapor </FONT><FONT face=Arial>to the volume occupied by a mixture of water vapor and dry air, or the mass of water contained in a unit volume of moist air.</FONT></DIV> <DIV><FONT face=Arial></FONT> </DIV> <DIV><FONT face=Arial>Terry Farrell</FONT></DIV></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2><EM>Bob Davis wrote:</EM></FONT></DIV> <DIV><FONT face=Arial size=2><EM>Relative humidity is the amount of water vapor in the air, versus the amount it can hold, at a given temperature. If the temperature is, say, 70 degrees, and the air is holding all the moisture it possibly can, the RH is 100 percent. Since warm air holds more water than cold air, if the temperature then rises, the RH would go down, since the warmer air would be capable of holding more water.</EM></FONT></DIV> <DIV><FONT face=Arial size=2><EM></EM></FONT> </DIV> <DIV><FONT face=Arial size=2><EM>Ambient humidity is just the relative humidity at a particular location, like in the living room where the piano sits.</EM></FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2>Exactly, Bob.</FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2><EM>Don Rose, B.Mus., A.M.U.S., A.MUS., R.P.T. wrote:</EM></FONT></DIV> <DIV><FONT face=Arial size=2><EM>Suppose you have 1 liter of air and it is at 20 degrees Celsius. Suppose<BR>the measured humidity inside the container is 30%.</EM></FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2>Absolute humidity or relative humidity?</FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2><EM>If one increases the<BR>temperature without allowing any air to escape the humidity will remain at<BR>30%.</EM></FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2>The absolute humidity would remain constant, but the relative humidity would decrease in the scenario above.</FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2><EM>If on the other hand one allows air to escape--then there are fewer<BR>molecules of air--and fewer molecules of water so the "relative" humidity<BR>will be lower.</EM></FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2>Impossible to predict without additional information. Are we still increasing the temperature? Are we allowing the environment in the one liter vessel to equilibrate with the surrounding atmosphere? Theoretically, if one opens the vessel, and if the humidity outside was 100%, then both absolute humidity and relative humidity could rise in the vessel.</FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2>By itself, relative humidity has nothing to do with "letting air escape".</FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2>Terry Farrell</FONT></DIV></DIV></BODY></HTML>