Re: [Phys-l] thermo question

[fizix29@aol.com]

Can we assume the box is not very tall and the gravitational
field is not very huge, so that the gravitational pressure 
head is negligible on the scale of things?

This is described as a "gas" problem.  Can we assume there is
no liquid water present (and no liquid oxygen or nitrogen)?

Given those assumptions, the question is, what is varying?
We have a fixed amount of gas, a fixed volume, a fixed 
temperature, Dalton's law of partial pressures, ideal gas
law ... it seems that there are no remaining variables of
any kind.

What is there to know that we don't already know?

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.

As with most forums, certain questions are asked by newer members several times a year (the dreaded "you didn't use the search function").? So far no one seems to really know how these products maintain air in a closed space at a desired RH regardless of the temperature of the air.? I am curious myself and would love to be able to make positive contributions to the conversation.

Some have proposed that the beads work by simply being stored in a particular environment, say air at 65%RH, then being shipped in a tightly sealed bag.? According to that theory there is no difference between the "65% beads" and the "70% beads" other than the conditions under which they were stored.? Others say they were manufactured differently.? I have not found any information on the retailers website.

So I am learning about adsorbtion and dessicants to try to understand.

Hopefully this clarifies where I was headed with this.


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: Sat, 1 Mar 2008 11:01 am
Subject: Re: [Phys-l] thermo question



On 03/01/2008 07:56 AM, FIZIX29@aol.com wrote:
> Consider a fixed amount of gas (moist air) in a container of fixed  volume.  
> The container is in an environment of fixed temperature.   When equilibrium is 

> reached, what quantity or quantities will be constant  throughout the volume 
> of gas?
>  
> This question comes from my desire to understand from first  principles how 
> to maintain a constant relative humidity in a fixed amount  of gas.

Can we assume the box is not very tall and the gravitational
field is not very huge, so that the gravitational pressure 
head is negligible on the scale of things?

This is described as a "gas" problem.  Can we assume there is
no liquid water present (and no liquid oxygen or nitrogen)?

Given those assumptions, the question is, what is varying?
We have a fixed amount of gas, a fixed volume, a fixed 
temperature, Dalton's law of partial pressures, ideal gas
law ... it seems that there are no remaining variables of
any kind.

What is there to know that we don't already know?

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