Greetings,

Please explain if water vapor diffusion occurs by a differential in
vapor pressure, concentration or both. My problem is this, we
frequently measure the relative humidity beneath interior concrete
slabs-on-ground to be 100% RH even when the water table is many feet
below the slab and the moisture content by weight in the fill beneath
the slab is low.  A temperature gradient exists beneath the slab. The
temperature just beneath the slab is warmest and gets progressively
cooler as we go deeper into the ground.  If I plot the saturated vapor
pressure beneath the slab from top to bottom, the vapor pressure is
higher just beneath the slab (ex: 70F 100% RH = 0.363 psi)  than it is
just above a water table that is say 15 feet down.  Ruling out
capillary action, how does the entire zone from beneath the slab to
the water table reach 100% RH when the vapor pressures are higher at
the top than at the botton of the zone?

I also do not understand the published statement that "Vapor Pressure
depends only on temperature"  If the saturated vapor pressure of air
at 73.4 F is 0.41 psi is not the vapor pressure at 73.4F and 50 % RH 
0.205 psi?
(ex: 55 F , 100 % RH + = 214 psi.

I appreciate you help.

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Request for Question Clarification by hedgie-ga on 28 Dec 2003 02:30 PST

I can explain diffusion of vapor and partial pressure in gas,
 but I would need to understand the structure of site better.

  The vapor pressure in gas phase is related to, but different
  from moisture content of the soil. Are you measuring RH in the air pockets
  or are you talking only about the soil only, or both?

---

Clarification of Question by cc100-ga on 28 Dec 2003 04:25 PST

We measure the relative humidity in the soil not the moisture content.
The RH is typically 100% even when we find the moisture content of the
soil <5% by weight.

We have always believed that water vapor in the soil is acting as a
gas moving through the interstitial voids in the soil structure.

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Clarification of Question by cc100-ga on 28 Dec 2003 04:32 PST

Sorry, I should have said we measure both RH% and moisture % by
weight.  What we are trying to understand is how the entire zone
beneath the slab reaches 100% RH even when the vapor pressures seem to
war against the upward movement of water vapor.

Example:

Just below slab  70F  100% RH = .363 psi
3  Feet down     65F  100% RH = .306 psi
6  Feet down     60F  100% RH = .256 psi
10 Feet down     55F  100% RH = .214 psi

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Clarification of Question by cc100-ga on 28 Dec 2003 04:35 PST

Please wait until Monday to complete any reasearch and responses.

Thank you.

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Request for Question Clarification by hedgie-ga on 28 Dec 2003 07:25 PST

We may need some definitions here, perhaps a pointer to reference


Common meaning of HR refers to gas
http://dictionary.reference.com/search?q=relative+humidity&r=67

Water content describes how much water is in soil.
http://jan.ucc.nau.edu/~doetqp-p/courses/env320/lec9/Lec9.html

which definions are you using

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Clarification of Question by cc100-ga on 29 Dec 2003 06:17 PST

Thank you for the linl on soil moisture.

Again I am not taking about soil moisture but the relative humidity in the soil.
We have been testing in-situ relative humidity of concrete and soils for years.
For both we drill a hole into or through the concrete into the base material.
What we have found is that the relative humidity in the fill beneath
the slab is most often 100%. This is observed even when we remove and
oven dry the fill and find the moisture content to be less than 5% by
weight. We also know that a temperature gradient exists in the ground
beneath the slab, It is warmest just beneath the slab and gets
progressively cooler until we reach the water table.
If I use the conventional vapor pressure charts the vapor pressure in
the air space between soil particles works out to be higher just
beneath the slab than it is just above the water table. My question is
how does the entire zone beneath the slab reach 100% RH if the
diffusion of water vapor occurs by pressure differential only. Or can
diffusion take place against pressure?

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Clarification of Question by cc100-ga on 29 Dec 2003 06:48 PST

As I continue my research into this issue I also question how water
vapor moves against air pressure.

According to the vapor pressure tables I have even at 100F, 100% RH
vapor pressure is les than 1.0 psi (.948). If the air pressure at sea
level is approximately 17 psi how does water vapor diffuse against air
pressure?

I know I am missing something and hope you can help me set things straight.

Hi
    I understand the temperature profile, and I gather when you say
   HR you indeed mean humidity in the gas phase, in the pockets of air or pores.

  Let's address the diffusion of vapor first:

     1) Gases do not fight for space (as they are mostly an empty space)

So a volume filled with gas A, will get filled also with gas B
even when (partial) pressure of B is less then  (partial)  pressure of A.

In the end, (when equilibrium is reached) the pressure of A and B will add.
 see
http://members.aol.com/profchm/dalton.html  
 or   
 http://www.chm.davidson.edu/ChemistryApplets/GasLaws/DaltonsLaw.html   

 and in general:                         
SEARCH TERM: Dalton Law
          ://www.google.com/search?hl=en&ie=ISO-8859-1&q=Dalton+law   

2)  Space under the slab is not necessarily (and probably is not) in equilibrium  
    with atmosphere.

  Meaning: If you take a piece of adobe and put in the open, fresh air, 
 (on sunny summer day ...) it will dry out.  Same piece may retain water
  under a slab of concrete. The tables of vapor pressure (usually)
assume equilibrium.

3) In interstices the situation is different then on the flat
interface.    Surface tension
   of the material  (soil) will influence the equilibrium:   

  Consider hydrophilic vs hydrophobic gel in a closed space with some
water. One will
 soak the water in, other will ignore it.       

Consider desiccant
 http://www.ecompressedair.com/desiccant.shtml
 http://environment.about.com/cs/glossary/g/desiccant.htm
and     hydrophobic material:
  www.cnr.uidaho.edu/extforest/F5.pdf
   http://www.netc.net.au/enviro/fguide/waterepel.html   

 So, even in equilibrium with gas having a  high HR,
 hydrophobic soil will keep low water content.           
SEARCH TERM : HYDROPHOBIC SOIL    


                                   So, in conclusion
  answer to:
                 can diffusion take place against pressure?    

  is: Yes.
                       The rate of diffusion is affected by presence
of other molecules og gas,
 but in the long run (in equilibrium) the gas will diffuse into a space occupied by
other gas as much(same amount)  as if that space is be empty.

hedgie

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Request for Answer Clarification by cc100-ga on 29 Dec 2003 10:42 PST

Am I correct in understanding that as it relates to diffusion of
gasses each gas in a mixture will act independently of one another?

Am I also correct that if the saturated vapor pressure at 70F and 100%
RH is .362 psi, the vapor pressure at 70F and 50 % RH is .181 psi ?



I appreciate your help with this.

---

Clarification of Answer by hedgie-ga on 29 Dec 2003 22:37 PST

Diffusion of gases  - overview

  http://members.aol.com/profchm/graham.html     


question:   in   diffusion of gasses each gas in a mixture 
will act independently of one another ?   

  No.  
     Let's elaborate the previous answer:    
    "  The rate of diffusion is affected by presence
      of other molecules og gas,  but in the long run (in equilibrium) ..:


 We have to differentiate between equilibrim properties,
        such as pressure and non-equilibrium processes such as
       diffusion.     
       Diffusion constant depends on speed of a molecule and 
number of collisions (mean free path).
     Speed depends on T and mass of molecule, as described here
    http://www.rutchem.rutgers.edu/Courses_f03/ Chem161/evelecture10.pdf 
  
  and collision count on  size and concentration of other molecules.
    So presence of other gases will slow down the diffusion .

    Here is a complex math, describing this based on Boltzman eq.
     www.grc.nasa.gov/WWW/wind/class2002/Fluids.ppt   

  To put it simply:
 The more molecules of any kind,the  smaller the Diffusion coeficient 
 (Henry's law)   

 In particular, for water vapour in air:
          D( H20 in air)= 1.87E-10  T * T / P  
           (approximate see more exact below)
     http:// www.me.polyu.edu.hk/subject/me430(yzhou)/ notes/chapt10.pdf  

Here P= pressure, depends on all other gases, more molecules, more
total pressure, lower diffusion. E-10 is scientific notation, as on a
calculator.

  SEARCH TERMS:   diffusion mixture  gases 
                 Henry's law

   Bonus Sites (general info)
      A brief summary   of Gas  Laws
    http://www.pdh-odp.co.uk/GasLaws.htm   
 
                           
http://www.okonekpo.com/education/tutorials/gases/gases03.html
                      
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/henry.html
   question:
                 if   saturated vapor pressure at 70F and 100% RH is .362 psi, 
                               the vapor pressure at 70F and 50 % RH
is .181 psi ?

    Yes.  This is the Ideal Gas law, properly applied to partial pressure of vapor
             according to Dalton's law 
                 Tables and charts are here:
                                      
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/watvap.html

SEARCH TERMS:                 water vapor pressure    
    Bonus site                  All about water (excellent site)
                                                 
http://www.lsbu.ac.uk/water/phase.html
    
      If you are happy with the answer, please do rate it . It helps me 
     to improve my skills if I know how well,or how badly, I did answer.
     If you are not happy, please do ask for clarification.
      
         hedgie

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Request for Answer Clarification by cc100-ga on 30 Dec 2003 11:05 PST

We are getting closer.

Two of the links do not work properly.

They are:  www.me.polyu.edu

www.hk/subject/me430(yzhou)/notes/chapter10.pdf

Please check these links and resend corrected.

Thank you.

---

Clarification of Answer by hedgie-ga on 30 Dec 2003 23:18 PST

IS  
     http:// www.me.polyu.edu.hk/subject/me430(yzhou)/ notes/chapt10.pdf  

SHOULD BE
http://www.me.polyu.edu.hk/subject/me430(yzhou)/notes/chapt10.pdf

IS

http://www.rutchem.rutgers.edu/Courses_f03/ Chem161/evelecture10.pdf 

SHOULD BE
http://www.rutchem.rutgers.edu/Courses_f03/Chem161/evelecture10.pdf 

 Sorry about that. Sometimes extra blanks intrude into URLs, when
text is pasted in and long lines are folded to fit the box.
 In general, delelting the extra blanks will fix the URL.

---

Clarification of Answer by hedgie-ga on 30 Dec 2003 23:27 PST

Once more 

the following expression in  [ ]  is the  link

[  http://www.me.polyu.edu.hk/subject/me430(yzhou)/notes/chapt10.pdf  ]

is the URL. It may be necessary to paste it into 'location' field of  
the browser. Or try this
http://www.me.polyu.edu.hk/?http://www.me.polyu.edu.hk/subject/me430(yzhou)/notes/chapt10.pdf
or this:
 http://makeashorterlink.com/?Q5FF21CE6

The problem is due to the improper format of the link and way GA handles URL
text strings.

---

Clarification of Answer by hedgie-ga on 30 Dec 2003 23:33 PST

or 
http://makeashorterlink.com/?N20012DE6

This link i temprary, the expression in  [  ]  should work
permanently.