Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] thermo question




Thanks!



Next question:? Silica works as a good buffer, presumably, because it has such a large surface area to volume ratio and can adsorb a lot of water.? How does the propylene glycol/water mixture work?? The retailers claim it will maintain 70% RH humidity regardless of temperature and the *exact* ratio of water to PG.? Is this true?



Silica gel has poor buffer behavior at 45% RH and above, so
what you're buying is probably something else. Either that or
you're being swindled.




Justin Parke
Oakland Mills High School
Columbia, MD


-----Original Message-----
From: John Denker <jsd@av8n.com>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
Sent: Mon, 3 Mar 2008 12:44 pm
Subject: Re: [Phys-l] thermo question



On 03/03/2008 07:04 AM, fizix29@aol.com wrote:

Let me cut to the chase:

I participate in a cigar smoker's forum and one of the topics of
discussion is the proper storage of cigars.? Most folks adhere to the
70/70 rule, that is, 70 degrees F and 70% RH, although some claim 65%
is better.? Several products exist to meet these requirements, two of
the most popular being a mixture of distilled water and propylene
glycol soaked up in wet florists foam and silica gel beads that are
supposedly "preprogrammed" to maintain a desired RH.? These can be
purchased in the 65% or the 70% variety.
........
Hopefully this clarifies where I was headed with this.

That helps a lot.

This is mostly an engineering question, not a deeply theoretical
question.

1) Let's assume that the temperature variable is controlled first.
a) That's easy to do, and
b) That simplifies the rest of the problem. In particular,
controlling relative humidity is now equivalent to controlling
the /absolute/ humidity. In turn that is equivalent to
controlling the partial pressure of H2O. I will use all
three of those ideas interchangeably.

2) The thing that is changing here is the total amount of water.
It changes due to new cigars coming in, new air coming it, et
cetera. The goal is to maintain constant RH even as water
flows in and out of the chamber.

3) Therefore you need something that will buffer a lot of H2O
at the desired humidity.

4) Silica gel has poor buffer behavior at 45% RH and above, so
what you're buying is probably something else. Either that or
you're being swindled.
http://www.apsnyc.com/html/humidity_control_FAQ.html
http://www.apsnyc.com/pdf/silica_gel_reconditioning.pdf

5a) Due to the phase change, a puddle of water has essentially
infinite buffering capacity at 100% RH. There is a flat spot
in the phase diagram.

5b) In contrast, there is no comparable flat spot in the phase
diagram of desiccant gels. As they get more loaded with water,
their vapor pressure goes up. The best you can hope for is
to have enough mass of desiccant that you can push a sufficient
amount of H2O in or out without moving the operating point very
much. It helps if you also have a feedback loop as described
below.

6) If this were something I cared about, I'd want to have an
indicator. Some gels come with built-in indicator chemicals,
or you could just buy a separate indicator (chemical or
otherwise).

7) The reference
http://www.apsnyc.com/pdf/silica_gel_reconditioning.pdf
mentions drying gel by heating it. I'll bet it works much
better to combine heating with _vacuum_. You don't need a
hard vacuum; a mechanical pump will work fine.

Think about the physics: If you can get the total pressure
below the desired partial pressure of H2O, it makes a huge
difference.

This allows you to use much gentler heating (and less time).

8) In the winter you will be wanting to add humidity (not
remove it). Adding H2O is the easiest thing in the world
... but you need an indicator to tell you how much to add.

9) One fun thing to think about is that the SVP of water is
a strong function of temperature. As a relevant example,
70% RH at 70 F corresponds to a dewpoint of 59.76 F as you
can verify at
http://einstein.atmos.colostate.edu/~mcnoldy/Humidity.html

Therefore it seems to me you could have essentially infinite
buffer capacity if you are in equilibrium with a puddle of
water at 59.76 F. This could either be used directly, or
used to recondition gel. You should be able to get a small
thermoelectric cooler for less than 50 bucks. As a proof
of existence, see:
http://www.coolerstores.com/


_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l