Hi

For mesuring humidty, I have two digitals thermometers. One is mesuring
the ambient air temperature and the second mesures the temperature of a
wet cotton (which is in contact with the ambient air).

In addition, I have a digital pressure sensor.

But my problem is that I don't know the formula to calculate the
relative humidity from dry temperature, wet temperature and pressure.

Someone know this formula ?

Thank you very much

Alain

"Elekaj" wrote in message
...
Hi

For mesuring humidty, I have two digitals thermometers. One is mesuring
the ambient air temperature and the second mesures the temperature of a
wet cotton (which is in contact with the ambient air).

In addition, I have a digital pressure sensor.

But my problem is that I don't know the formula to calculate the relative
humidity from dry temperature, wet temperature and pressure.

Someone know this formula ?

Thank you very much

Alain

Do a google search on the Clausius-Clapeyron Equation and Relative Humidity.

Dave

David Moran wrote:
"Elekaj" wrote in message
...

Hi

For mesuring humidty, I have two digitals thermometers. One is mesuring
the ambient air temperature and the second mesures the temperature of a
wet cotton (which is in contact with the ambient air).

In addition, I have a digital pressure sensor.

But my problem is that I don't know the formula to calculate the relative
humidity from dry temperature, wet temperature and pressure.

Someone know this formula ?

Thank you very much

Alain


Do a google search on the Clausius-Clapeyron Equation and Relative Humidity.

Dave



Does Clausius-Clapeyron work with wetbulb? I've only
ever used it with dewpoint.

Back when I took obs at the PSU Wx Tower, we slung the
psychrometer to get wetbulb and then used "The Magic
Wheel" to figure out RH given the dry and wetbulbs.


Scott

"Scott" wrote in message
...
David Moran wrote:
"Elekaj" wrote in message
...

Hi

For mesuring humidty, I have two digitals thermometers. One is mesuring
the ambient air temperature and the second mesures the temperature of a
wet cotton (which is in contact with the ambient air).

In addition, I have a digital pressure sensor.

But my problem is that I don't know the formula to calculate the relative
humidity from dry temperature, wet temperature and pressure.

Someone know this formula ?

Thank you very much

Alain


Do a google search on the Clausius-Clapeyron Equation and Relative
Humidity.

Dave

Does Clausius-Clapeyron work with wetbulb? I've only
ever used it with dewpoint.

Back when I took obs at the PSU Wx Tower, we slung the
psychrometer to get wetbulb and then used "The Magic
Wheel" to figure out RH given the dry and wetbulbs.


Scott

I thought there was a way to do it, but I don't remember how. I remember
seeing a problem like this on a test back in school.

Dave

Elekaj wrote:

For mesuring humidty, I have two digitals thermometers. One is mesuring
the ambient air temperature and the second mesures the temperature of a
wet cotton (which is in contact with the ambient air).

In addition, I have a digital pressure sensor.

But my problem is that I don't know the formula to calculate the
relative humidity from dry temperature, wet temperature and pressure.

Someone know this formula ?

There is no empirical formula to calculate relative humidity from
temperature and wet bulb depression. You'd need to measure dew point
to enable a calculation..

AFAIK the humidity tables based on wet bulb depression have been
derived entirely from observation.

--
John H

Would a psychometric chart help you?
John_H wrote:
Elekaj wrote:

For mesuring humidty, I have two digitals thermometers. One is mesuring
the ambient air temperature and the second mesures the temperature of a
wet cotton (which is in contact with the ambient air).

In addition, I have a digital pressure sensor.

But my problem is that I don't know the formula to calculate the
relative humidity from dry temperature, wet temperature and pressure.

Someone know this formula ?

There is no empirical formula to calculate relative humidity from
temperature and wet bulb depression. You'd need to measure dew point
to enable a calculation..

AFAIK the humidity tables based on wet bulb depression have been
derived entirely from observation.

--
John H

If you know the air temperature and the wet bulb temperature, you first want
to calculate the actual mixing ratio of the air(W) using the following
formula.

(12) W=[(Tc-Twb)(Cp)-Lv(Eswb/P)]/[-(Tc-Twb)(Cpv)-Lv]

W=actual mixing ratio of air

Cp=specific heat of dry air at constant pressure(J/g)~1.005 J/g

Cpv= specific heat of water vapor at constant pressure(J/g)~4.186 J/g

Lv=Latent heat of vaporization(J/g)~2500 J/g

Tc=air temperature in degrees Celsius

Twb=wet bulb temperature in degrees Celsius

Eswb=saturation vapor pressure at the wet bulb temperature(mb)

P=atmospheric pressure at surface~1013 mb at sea-level

Once you have the actual vapor pressure, you can use the following formula
to calculate the saturation mixing ratio for the air.

(13) Ws=Es/P

Once you have the actual mixing ratio and the saturation mixing ratio, you
can use the following formula to calculate relative humidity.

(14) Relative Humidity(RH) in percent=(W/Ws)*100

Note: The latent heat of vaporization(Lv) varies slightly with temperature.
The value given above is an approximate value for the standard atmosphere at
0 degrees Celsius.

Note: Due to the large numbers of approximations using these formulas, your
final answer may vary by as much as 10 percent.

Long ago, I created an excel spreadsheet add-in with formulas for
calculating humidity etc. its at:
http://us.f1.yahoofs.com/bc/42902169...8zvmDBX4BH7Jvf

"Elekaj" wrote in message
...
Hi

For mesuring humidty, I have two digitals thermometers. One is mesuring
the ambient air temperature and the second mesures the temperature of a
wet cotton (which is in contact with the ambient air).

In addition, I have a digital pressure sensor.

But my problem is that I don't know the formula to calculate the relative
humidity from dry temperature, wet temperature and pressure.

Someone know this formula ?

Thank you very much

Alain

The El Paso Area NWS has a Weather Calculator
http://www.srh.noaa.gov/elp/wxcalc/wxcalc.shtml which you might find
useful; they also give the formulae you are looking for there at
http://www.srh.noaa.gov/elp/wxcalc/f...omWetBulb.html You
should be aware, however, that most tables/formulas are based on
observations made with a wet-bulb that is ventilated, either by whirling
or a fan. Strong ventilation is absolutely necessary to accuracy.

Bob

Elekaj wrote:

Hi

For mesuring humidty, I have two digitals thermometers. One is
mesuring the ambient air temperature and the second mesures the
temperature of a wet cotton (which is in contact with the ambient air).

In addition, I have a digital pressure sensor.

But my problem is that I don't know the formula to calculate the
relative humidity from dry temperature, wet temperature and pressure.

Someone know this formula ?

Thank you very much

Alain