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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