Assuming I have a weather report (temperature, pressure, and relative humidity), how can I determine the mass of water vapor in some volume of air? - Gabe

Surely somebody on FF knows this! - Gabe

You need to place your request in the form of "DO ANYBODY NO" to get a response. - Louis Gray

Hah! I'm not sure, but you could try asking Chris Baskind. He might be able to hook you up with someone who knows. - Rochelle

http://efdl.cims.nyu.edu/project... http://www.gorhamschaffler.com/humidit... http://adsabs.harvard.edu/abs... hope that answers your question - chaz2b

chaz2b: Thanks, but neither of them seem to have what I want. The efdl link has variables labeled rh, pa, and Ta. I can infer that they mean rh, pressure, and temp, but it doesn't say what units they're supposed to be. The gorham link tells how to get absolute humidity, not specific humidity. The adsabs link seems to have differential equations, but nothing I can use. - Gabe

Does this help? http://www.engineeringtoolbox.com/specifi... - Greg Guitarbuster

I can't see what chaz2b provided. Here's a chart that might be of use: http://www.tis-gdv.de/tis_e... - SuezanneC Baskerville

Greg: that link requires knowing the density of water vapor and air. All I have is a weather report. - Gabe

SuezanneC: your link has absolute humidity (g/m^3), not specific humidity (g/kg). - Gabe

You say that as if that means it doesn't answer your your question, but I"m not sure that is the case. You ask for the mass of water vapor in a volume of air, which seems more closely related to grams per cubic meter than it does to grams per kilogram. Here's another link. http://forum.onlineconversion.com/showthr... . Also, when people take the time to try to solve your problems for zero dollars per hour, a thank you would be appropriate. - SuezanneC Baskerville

Also you might try searching usenet. - SuezanneC Baskerville

try this http://www.cactus2000.de/uk... - jamar78

Thanks, jamar78, that's basically what I was looking for. - Gabe

SuezanneC: thanks for the help, but I wasn't looking for somebody to do research for me. Google was supplying a bunch of links that didn't answer my question so I just figured that there would be some engineer who would know the answer off the top of their head. - Gabe

If you have a psycometric chart handy, you can read it off that: http://en.wikipedia.org/wiki... - Clare Dibble

Clare, I was hoping you would just give me an equation. It's hard to write a computer program to read things off a chart. - Gabe

http://www.gorhamschaffler.com/humidit... - J. Abdul-Qahhar

The problem is that you need to know the total amount of water that air will hold at a given temperature and pressure. After you have that, you only need multiply that by the relative humidity to get the specific humidity. Water and air is a very common system, so the saturation pressure and temperature are compiled for a very wide range. It's hard to fit an equation to such a wide range, so people usually look it up in a table or fit an equation to their desired range of temperatures and pressures. - Clare Dibble

Update: if you are near ambient temperatures and pressures, you can assume the ideal gas law and use Antoine's equation to calculate saturation. The coefficients are available here: http://webbook.nist.gov/cgi... - Clare Dibble

You guys are geeks - Becca

Thanks, Clare. I just figured that there was some common equation to tell me specific humidity. I didn't expect that it was so uncommon that it would require derivation and table lookups. You see, I am trying to figure out the heat rate for a gas turbine to compute what it would cost to run. The heat rate is specified at a specific temperature, pressure, and humidity, so I wanted to compensate for variations in those. I assumed there would be some engineer out there who does combustion with atmospheric intake air who would have to do similar heat rate compensation regularly and would know how to do it. - Gabe