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March 2013

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Subject:
From:
Bob Landman <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Bob Landman <[log in to unmask]>
Date:
Fri, 8 Mar 2013 15:43:23 -0500
Content-Type:
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text/plain (215 lines)
Steve,

I have used saturated salt solutions to calibrate - not very accurate but good enough for your needs I would think?

Bob L.

P.S.  when we moved here (NH) from CA we asked a contractor to install humidifiers on our A/C systems.  I discovered he had installed the humidistat on the return air duct.  That might have been ok (I called the mfg who said it would work) except for one tiny detail.  The contractor bolted them to the outside of the duct so one sampled the cellar RH and the attic unit the attic RH!

Needless to say, he was immediately fired!

Science education in the USA....

Bob L.

http://www.conservationphysics.org/satslt/satsol.php

Bob L

Sent from my iPhone

On Mar 8, 2013, at 2:51 PM, Steven Creswick <[log in to unmask]> wrote:

> Bob L,
> 
> It is interesting that you bring it up.  At this moment I am working on
> setting up a ucontroller for a smokehouse [aka - smoked meats] application.
> It is using the psychrometric method, with wet/dry bulb sensors to
> monitor/control humidity in the chamber.  You can stick your desk in this
> one, provided you place the desk on end.  
> 
> A great deal certainly depends on where the wet/dry bulb sensors are located
> in the chamber, and air flow over them.
> 
> Are the numbers I am seeing real???  Probably to a certain degree - after
> the heaters stabilize.  Thanks for the article
> 
> Hmmmm.  'calibration', that will be another issue altogether, but
> fortunately 'close' will likely be 'good enough' in this application. 
> 
> Steve C
> 
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Bob Landman
> Sent: Friday, March 08, 2013 2:35 PM
> To: [log in to unmask]
> Subject: Re: [TN] Humidity Measurements: Relative or Absolute
> 
> Bob K,
> 
> Here's an interesting collection of data on water vapor pressure, etc..
> 
> http://hyperphysics.phy-astr.gsu.edu/%E2%80%8Chbase/kinetic/watvap.html#c1
> 
> I didn't think that water obeys the ideal gas law (PV = mRT) so I did a bit
> of research and found this discussion
> 
> http://www.physicsforums.com/showthread.php?t=531580 where I found....
> 
> "You need to use something like this:
> 
> http://www.engineeringtoolbox.com/wa...ure-d_599.html
> 
> As you can see, the dependence is non-linear and, therefore, inconsistent
> with the ideal gas law."
> 
> Bob L
> 
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
> Sent: Friday, March 08, 2013 2:16 PM
> To: [log in to unmask]
> Subject: Re: [TN] Humidity Measurements: Relative or Absolute
> 
> Hi,
> 
> The problem I see (or at least my confusion) is that during reflow I think
> the idea gas law holds and the force per unit of package surface area will
> be a function of vapor content of the plastic and temperature. That is the
> absolute vapor content. (Me thinks?)
> 
> Now there is nothing sacred about water vapor so I would imagine any air (or
> nitrogen or helium or any other gas) inside a package will produce forces to
> the same degree as water vapor. (Ideal Gas Law)
> 
> Bob K.
> 
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Steven Creswick
> Sent: Friday, March 08, 2013 1:33 PM
> To: [log in to unmask]
> Subject: Re: [TN] Humidity Measurements: Relative or Absolute
> 
> Bob,
> 
> You are correct.  I was partially asleep.  Where is Dewey when you need him
> to say that 'everything is relative'?
> 
> Going to be pretty tough to measure absolute humidity in a quick and easy
> manner.  
> 
>     One item I found suggested avoiding the term "absolute humidity". 
> 
> Steve C
> 
> -----Original Message-----
> From: Robert Kondner [mailto:[log in to unmask]]
> Sent: Friday, March 08, 2013 12:37 PM
> To: 'TechNet E-Mail Forum'; 'Steven Creswick'
> Subject: RE: [TN] Humidity Measurements: Relative or Absolute
> 
> Steve,
> 
>  Relative Humidity:  http://en.wikipedia.org/wiki/Relative_humidity
> http://www.youtube.com/watch?v=CL5cgXwKUXc
> 
>  If the absolute humidity is high (even 30%) but the temperature is also
> high, I think the humidity indicators will show low humidity exposure.
> 
> I think  that relative humidity is crucial to component water absorption or
> drying. 
> 
> But I wonder if package cracking doing to expansion of gases entrapped in
> the package material (plastic) is a function of the absolute vapor content
> in that material. 
> 
> Bob K.
> 
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Steven Creswick
> Sent: Friday, March 08, 2013 11:35 AM
> To: [log in to unmask]
> Subject: Re: [TN] Humidity Measurements: Relative or Absolute
> 
> Bob,
> 
> I have always considered them to be absolute.  If relative - relative to
> what?
> 
> Do the cardboard indicators provide a temp range for which they are valid?
> 
> Steve C
> 
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
> Sent: Friday, March 08, 2013 11:24 AM
> To: [log in to unmask]
> Subject: [TN] Humidity Measurements: Relative or Absolute
> 
> Hi,
> 
> 
> 
> As I start looking at humidity measurement devices I started to wonder if
> IPC humidity numbers are Relative or Absolute? Does anyone know off hand. I
> need to go buy some specs.
> 
> 
> 
> I would have thought Absolute but the cardboard indicators are temperature
> sensitive as are the desiccant drying systems in dry cabinets. 
> 
> 
> 
> If the temperature is high, say > 100C then a part will "Bake Out"
> regardless of the absolut humidity, is that right?
> 
> 
> 
> Bob K.  
> 
> 
> 
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