NaCl produces 75% RH, this is table salt. MgCl produces 33% RH, this is
commonly used as deicer. LiCL will produce 11% RH. Little harder to find
but usually available on ebay for not much. These 3 reference points will
calibrate your humidity sensor to be fairly accurate. Cost of the salts is
next to nothing.
On Fri, Mar 8, 2013 at 12:43 PM, Bob Landman <[log in to unmask]>wrote:
> 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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--
Brian Stumm
ETS Energy Technology Systems, Inc.
http://EnergyTechnologySystems.com
http://www.TrioTek.net Curing Ovens
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