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. ______________________________________________________________________ This email has been scanned by the Symantec Email Security.cloud service. For more information please contact helpdesk at x2960 or [log in to unmask] ______________________________________________________________________ ______________________________________________________________________ This email has been scanned by the Symantec Email Security.cloud service. For more information please contact helpdesk at x2960 or [log in to unmask] ______________________________________________________________________ ______________________________________________________________________ This email has been scanned by the Symantec Email Security.cloud service. For more information please contact helpdesk at x2960 or [log in to unmask] ______________________________________________________________________ ______________________________________________________________________ This email has been scanned by the Symantec Email Security.cloud service. For more information please contact helpdesk at x2960 or [log in to unmask] ______________________________________________________________________ ______________________________________________________________________ This email has been scanned by the Symantec Email Security.cloud service. For more information please contact helpdesk at x2960 or [log in to unmask] ______________________________________________________________________