Does Atmospheric Pressure Affect Humidity?
Friday May 12, 2006
Does atmospheric pressure affect relative humidity? My inclination would be to say 'yes'. The answer, as you will see, might not be so simple. Following is an e-mail I received. After reading it, I was able to convince myself of both sides of the argument, which doesn't help the person asking the question at all. Therefore, I'm opening it up for discussion. Please feel free to add your own comments and observations.
"I have a question for you that concerns the preservation of millions of dollars of art and priceless archives in many libraries and museums in the Utah, Colorado, Arizona, Idaho and Texas. Because of the nature of the question, I ask that you please answer in complete thoughts, sentences and paragraphs. Although I have requested for completeness, it has been difficult for me to apply the same rules and be concise.
Problem: understanding the definition of relative humidity, its measurement, and finally the calibration of RH sensors. Below are two opinions, which is correct and why?
My opinions:
1) Relative Humidity is defined as a ratio of mole fraction of actual water vapor, to a mole fraction of water vapor that can be saturated in dry air, where the two values are obtained at the same temperature and pressure. Key word to focus is ... Pressure.
2) Mole fraction values are obtained from water density values.
3) Water density values vary with atmospheric pressure.
4) Atmospheric pressure varies with altitude.
5) The temperature boiling point of water varies with atmospheric pressure (or altitude).
6) Saturated Water Vapor pressure value is dependent on the boiling point of water (such that the values of the boiling point of water is lower at higher altitudes).
7) Humidity in any form is the relationship between the saturated water vapor pressure, and the sample-air's partial water vapor pressure. Partial water vapor pressure values are dependent on pressure and temperature.
8) Since both saturated water vapor property values and partial water pressure values are observed to non-linearly change with atmospheric pressure and temperature, then the absolute value of atmospheric pressure is required to accurately calculate the water vapor relationship as it applies to the perfect ideal gas law (PV = nRT).
9) To accurately measure humidity and use the principles of the perfect gas law, one must obtain the absolute atmospheric pressure value as a fundamental requirement for calculating relative humidity values at higher altitudes.
10) Since the majority of the RH sensors do not have built-in pressure sensor, they are inaccurate above sea level, unless a conversion equation is used with a local atm. pressure instrument.
Other people's opinions as they were stated to me:
A) Nearly all humidity related processes are independent of total air pressure, because water vapor in air does not interact with oxygen and nitrogen in any way, as first demonstrated by John Dalton early in the nineteenth century.
B) The only RH sensor type that is sensitive to air pressure is the psychrometer, because air is the carrier of heat to the wet sensor and the remover of evaporated water vapor from it. The psychrometric constant is quoted in tables of physical constants as a function of total air pressure. All other RH sensors should not need adjustment for altitude. However, the psychrometer is often used as a convenient calibration device for hvac installations, so if it is used with the constant for the wrong pressure to check a sensor that is in fact correct, it will indicate a sensor error."
Worked Pressure Conversion Problems | Ideal Gas Law Problems
"I have a question for you that concerns the preservation of millions of dollars of art and priceless archives in many libraries and museums in the Utah, Colorado, Arizona, Idaho and Texas. Because of the nature of the question, I ask that you please answer in complete thoughts, sentences and paragraphs. Although I have requested for completeness, it has been difficult for me to apply the same rules and be concise.
Problem: understanding the definition of relative humidity, its measurement, and finally the calibration of RH sensors. Below are two opinions, which is correct and why?
My opinions:
1) Relative Humidity is defined as a ratio of mole fraction of actual water vapor, to a mole fraction of water vapor that can be saturated in dry air, where the two values are obtained at the same temperature and pressure. Key word to focus is ... Pressure.
2) Mole fraction values are obtained from water density values.
3) Water density values vary with atmospheric pressure.
4) Atmospheric pressure varies with altitude.
5) The temperature boiling point of water varies with atmospheric pressure (or altitude).
6) Saturated Water Vapor pressure value is dependent on the boiling point of water (such that the values of the boiling point of water is lower at higher altitudes).
7) Humidity in any form is the relationship between the saturated water vapor pressure, and the sample-air's partial water vapor pressure. Partial water vapor pressure values are dependent on pressure and temperature.
8) Since both saturated water vapor property values and partial water pressure values are observed to non-linearly change with atmospheric pressure and temperature, then the absolute value of atmospheric pressure is required to accurately calculate the water vapor relationship as it applies to the perfect ideal gas law (PV = nRT).
9) To accurately measure humidity and use the principles of the perfect gas law, one must obtain the absolute atmospheric pressure value as a fundamental requirement for calculating relative humidity values at higher altitudes.
10) Since the majority of the RH sensors do not have built-in pressure sensor, they are inaccurate above sea level, unless a conversion equation is used with a local atm. pressure instrument.
Other people's opinions as they were stated to me:
A) Nearly all humidity related processes are independent of total air pressure, because water vapor in air does not interact with oxygen and nitrogen in any way, as first demonstrated by John Dalton early in the nineteenth century.
B) The only RH sensor type that is sensitive to air pressure is the psychrometer, because air is the carrier of heat to the wet sensor and the remover of evaporated water vapor from it. The psychrometric constant is quoted in tables of physical constants as a function of total air pressure. All other RH sensors should not need adjustment for altitude. However, the psychrometer is often used as a convenient calibration device for hvac installations, so if it is used with the constant for the wrong pressure to check a sensor that is in fact correct, it will indicate a sensor error."
Worked Pressure Conversion Problems | Ideal Gas Law Problems
Comments
I to am working to better understand this subject and unfortunately have little to accurately add on the subject.
However something I have learned on the subject is that science can often be quite certain about various parameters before and after an event and is quite good at stating as fact what occurs in the interveening period, despite not exactly knowing what is happening provided before and after situation is repeatable. Why this is of relevance to your text, I am unable to completely describe (due to my time constraints, and lack of your contact details) however Thermodynamics is often quite happy to to perfrom the above ‘trick’ because it works in many engineering application, applications that are closed system with measurable inputs and outputs. The real world, outside the closed systems of engineering, is what we live in and are surrounded by, and is prehaps less easily defined accurately, due to its variblity and various scales of time and space. I’d be happy to converse further on this subject more specifically if I had your contact details, I am at yahoo as ‘napincat’ but get lots of junk mail, so inclusion of the word ‘humidity’ in the subject may well help, cheers.
In needing to calculate a ‘virtual’ wet bulb temperature inside a vacuum chamber using an absolute pressure sensor and a relative humidity sensor, I spent a decent amount of time researching this. I eventually crafted a formula which approximated the wet bulb temperature, and a component of it was the atmospheric pressure, although it had a relatively small impact of the final value.
You can see this for the type of source material I used: http://www.4wx.com/wxcalc/formulas/rhTdFromWetBulb.php
I basically had to reverse what everyone else is doing, most are taking a wet bulb value and calculating the RH, I am taking an RH and calculating the wet bulb…
OMG!!! thank you so much!!!! i’m working on a science fair project, and i needed that answer! once again
THANKS!!!!! =)