Relative Humidity

[Thecatch83]

So if you place your digital hygrometer on the top tray, or down on the bottom near the humidifcation source.....wheat reading would be more correct, higher or lower in RH?

 

[OB-Left]

So if you place your digital hygrometer on the top tray, or down on the bottom near the humidification source.....wheat reading would be more correct, higher or lower in RH?

Assuming the the hygrometers are correctly calibrated they are both correct. They are correctly reading the humidity at their location. See Ron's post (top of the page) up a few about temp. differences and the resulting humidity differences. That is why is is helpful to have a fan to stir or mix the air.

 

[Jwrussell]

Ok, just to give you something to ponder with your next cigar. :thumbsup: Assume that the cigars are slightly under-humidified and the box starts with completely dry air, and some properly hydrated beads. We know that the beads will release moisture to the air which in turn is absorbed by the sticks. With this scenario, the USA Today quote predicts total failure. As no water would be able to go into the air, and not then be available for the sticks.

I can't argue with that other than to say that it would be just about impossible to actually test this. First of all, Avogadro's law deals with "ideal gases". Second, it would be just about impossible to introduce beads into completely dry air and seal the system before any moisture is released. I think we are at an impasse. Avogadro's law is certainly accepted as true, as is the one you reference. I'll have to see if I can find some more info that might clear things up.

 

[Jwrussell]

Randy,

Just found this equation on a Physic's discussion site (which by the way referenced the USA Today article as well!):

The density of a mixture of dry air molecules and water vapor molecules is:

D =\frac{P_d}{R_dT} + \frac{P_v}{R_vT}

D = density, kg/m3
Pd = pressure of dry air, Pascals
Pv = pressure of water vapor, Pascals
Rd = gas constant for dry air, J/(kg*degK) = 287.05
Rv = gas constant for water vapor, J/(kg*degK) = 461.495
T = temperature, degK = deg C + 273.15

As you can see, the largest value for D will result when Pv is zero.

We can argue over my use of the word "light" and "heavy" as those are really not the best terms when talking about density, but I can't find anything that refutes my statement that humid air is lighter (less dense) than dry air.


Note "Dry air is more dense tha[n] humid air" halfway down the page

Microsoft document, see bottom of page

 

[stroke]

You getting all this, JNT? The test will count for 30% of your grade...

:rofl:

All I know is OB couldn't have chosen a better avatar.

All that aside, I'd say the simple answer is go with beads and an Oust fan. I did so in my cooli and I maintain a steady 65% on the center and top shelf (I move it once in a while depending on what tray I happen to grab). Here's a link to the thread I created about my cooli: http://www.botl.org/community/forums/showthread.php?t=43016

I was blessed enough to be gifted a Magic Chef wine cooler that doesn't cool at the moment by Ish and I'm in the process of fixing it. Although it would work just as well now, I'd rather wait 'till I get it fixed before moving the cigars in there. It will provide a MUCH better seal than the cooli does.

Long story short, stick with 65% beads and the fan and you're good to go! :thumbsup:

 

[OB-Left]

Randy,
We can argue over my use of the word "light" and "heavy" as those are really not the best terms when talking about density, but I can't find anything that refutes my statement that humid air is lighter (less dense) than dry air.

Jason,

There you go using that word "argue" again. I'm beginning to think you aren't enjoying this. :stretchgr Check my original post. I have never said that you were wrong about humid air being less dense. It is, I agree. I only said your reason was slightly off. You only said lighter, didn't mention density. The quote from USA Today about molecule displacement is however wrong. As my travel box example illustrates.

:rofl:

All I know is OB couldn't have chosen a better avatar.

Thanks stroke !

 

[Jwrussell]

Randy, don't worry about my choice of words, bro'. One of my passions is debating so the word "argue" just kind of slips in when I'm "discussin'" things. I'm enjoying the hell out of this disucssion.

I'm still having trouble understanding how you can say that the USA Today article is "wrong", though. I don't believe that your travel humidor example illustrated that.

Some further research:

Link

One of the important predictions made by Avogadro is that the identity of a gas is unimportant in determining the P-V-T properties of the gas. This behavior means that a gas mixture behaves in exactly the same fashion as a pure gas.

Isn't this exactly the opposite of what you've said earlier, or am I misunderstanding?

By the way, this is the guy who wrote the USA Today piece: [ame="http://www.amazon.com/Jack-Williams/e/B001K7X8HE/ref=ntt_athr_dp_pel_1"]Amazon.com: Jack Williams: Books, Biography, Blog, Audiobooks, Kindle@@AMEPARAM@@http://ecx.images-amazon.com/images/I/01Px86uLUkL.@@AMEPARAM@@01Px86uLUkL[/ame]


Like you, I'm not a huge fan of Wikipedia, but as that cat is out of the bag wink, I'll post this:

Humid air is less dense than dry air because a molecule of water (M ≈ 18 u ) is less massive than either a molecule of nitrogen (M ≈ 28) or a molecule of oxygen (M ≈ 32). About 78% of the molecules in dry air are nitrogen (N2). Another 21% of the molecules in dry air are oxygen (O2). The final 1% of dry air is a mixture of other gases. For any gas, at a given temperature and pressure, the number of molecules present in a particular volume is constant - see ideal gas law. So when water molecules (vapor) are introduced into that volume of dry air, the number of air molecules in the volume must decrease by the same number, if the temperature and pressure remain constant. (The addition of water molecules, or any other molecules, to a gas, without removal of an equal number of other molecules, will necessarily require a change in temperature, pressure, or total volume; that is, a change in at least one of these three parameters. If temperature and pressure remain constant, the volume increases, and the dry air molecules that were displaced will initially move out into the additional volume, after which the mixture will eventually become uniform through diffusion.) Hence the mass per unit volume of the gas—its density—decreases. Isaac Newton discovered this phenomenon and wrote about it in his book Opticks.[2]

[ame="http://en.wikipedia.org/wiki/Humidity"]Humidity - Wikipedia, the free encyclopedia@@AMEPARAM@@/wiki/File:Cloud_forest_mount_kinabalu.jpg" class="image"><img alt="" src="http://upload.wikimedia.org/wikipedia/commons/thumb/8/83/Cloud_forest_mount_kinabalu.jpg/220px-Cloud_forest_mount_kinabalu.jpg"@@AMEPARAM@@commons/thumb/8/83/Cloud_forest_mount_kinabalu.jpg/220px-Cloud_forest_mount_kinabalu.jpg[/ame]


My guess is I'm just missing something obvious, but this stuff, at this level, is really pretty far over my head. Perhaps what I (we?) am missing in the "equation" of what we are discussing (especially as it relates to the travel humidor example) is a change in pressure that would need to take place. And that's where I get lost because I don't understand the concept of "Pressure" in this discussion at all.

 

[Jwrussell]

Huh, those links came up weird.

 

[OB-Left]

Jason,

Thank you for resolving our misunderstanding with your last post. You see it bothered me that USA Today would F&#% it up as badly as they did. With your last post you included that the author of the USA Today article was a “meteorologist”. That makes a huge difference, and at this point I have to eat a few of my words. From the perspective of a meteorologist, what was said in the USA Today article is correct. However, for everyone else, and especially cigar enthusiasts, what was said is still WRONG.

The basic law that we are dealing with is the Ideal Gas Law. (PV = nRT)

P = Pressure
V = Volume
R = Universal Gas Constant
T = Temperature
n = moles (basically an amount, chemistry term like a dozen, but it is 6.02 x 10^23, Avogadro’s number)

Rearranged slightly it takes the following form. PV/T = nR (Avogadro’s law) If pressure, volume, and temp remain constant then yes the number of molecules must remain constant.

However, in both situations we are trying to add water vapor to the air. We do this by changing water from a liquid to a gas. This action increases the right side of the above equation, more molecules are being added, n must increase. Correspondingly, that means that something on the left side of the equation must also change. (increase)

The meteorologist, works outside with the air above our heads, so what can change? Pressure (outside) is relatively constant in one place over a short time span. And temperature is determined by other factors, so the meteorologist lets volume increase.

So when water molecules (vapor) are introduced into that volume of dry air, the number of air molecules in the volume must decrease by the same number, if the temperature and pressure remain constant. (The addition of water molecules, or any other molecules, to a gas, without removal of an equal number of other molecules, will necessarily require a change in temperature, pressure, or total volume; that is, a change in at least one of these three parameters. If temperature and pressure remain constant, the volume increases, and the dry air molecules that were displaced will initially move out into the additional volume, after which the mixture will eventually become uniform through diffusion.)

For everyone else we work inside and gases are usually contained in containers, like our traveldor or humidors. With these containers the volume is fixed and can’t change and we hope that they don’t leak too badly. So the increase in moles from the addition of water must change something else. That becomes the pressure. Pressure is not hard to understand, just think of it as the mass of all the air above your head pushing down on you. At sea level this pressure supports 760 mmHg (millimeters of mercury). Your body and everything else must push back with an equal force, or collapse. From the Dalton’s law link you provided:

All molecules in the gas have access to the entire volume of the system, thus V is the same for both nitrogen and oxygen. Similarly, both compounds experience the same temperature.

P = P nitrogen + P oxygen

The above equation is called Dalton's Law of Partial Pressure, and it states that the pressure of a gas mixture is the sum of the partial pressures of the individual components of the gas mixture.

So for us pressure changes. Of the 760 mmHg of pressure 78% is due to nitrogen and 21% oxygen, so essentially 593 mmHg for N and 160 mmHg for O. The equilibrium water vapor pressure at a temp of 70 deg F is 47mmHg. This would be the reading for water at 100% RH. So we are shooting for 30.5 mmHg of water (65% RH) for our stogies. Not much of an increase in the overall pressure of the system inside our container.

Hopefully this explanation clears up our misunderstanding. Basically we were talking apples and oranges. If not, let me know where we are still having problems. Thanks brother.

 

[Jwrussell]

Hey, thank you, Randy. I still can't claim to understand the nitty gritty of what we are discussing, but I'm glad we have cleared things up. It just hit me last night while reading through things. I figured something else in the equation was changing that I wasn't paying attention to and didn't really understand. I also got the feeling that the concept and in the USA Today article that allowed things to move in and out was causing problems when compared to the travel humidor example.