Temperature versus relative humidity issue.

[Hermit]

Originally Posted by Cletus

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All this science talk is making my head hurt.....

Don't worry about it.
If yer hygro says 65, then it's 65,
whatever the temperature is.

[scottwpreston]

Nice info, Thanks for posting this stuff

[e-man]

I think I understand what your saying. It goes against what we are typically "indoctrinated" with.

Let me summarize from your post:

Air is NOT hygroscopic. It is simply a solution...a mixture of gases. You could fill a vacuum (no air) with pure water vapor, just like any other gas. That would result in a theoretical "infinite" percentage of relative humidity. Gases have different properties than hygroscopic materials as the water is "free floating".

Cigars are hygroscopic. The water is absorbed, and becomes part of the mass of tobacco. The water is not occupying void space as a vapor. It is bonded to the solid. Heat can give the water molecules enough energy to break this bond.

When temperature changes, the air expands and contracts. You have a different mass of air depending upon temperature. So long as you have the same ratio of water in the air, you have the same vapor pressure.

Differences in vapor pressure will equalize. The moisture will push into a cigar or push out to reach equilibrium. So, the moisture contained in the cigar is the same at a given rH, regardless of temperature, since the vapor pressures don't change. If the humidity drops, then moisture will push out of the cigar until the air has enough vapor pressure sustain the moisture content of a hygroscopic material.

Am I right so far?




Here's where I have some breakdowns.

I know that atmospheric pressure affects the transition from liquid to gas. Water boils at 212 at sea level. Here in Colorado, it's a little above 200 depending on your altitude. If you pressurize hot water, it can be heated well above 212. A car's radiator and a pressure cooker allow the heat to exceed 212 because they hold pressure.

My chemistry teacher boiled water in a flask. He took it off the heat while it was still boiling, and inserted a stopper that had a thermometer running through it. He chilled the flask with Ice, which created a vacuum. The water continued to boil, well below 200 degrees. Even at that lower temperature the water molecules still had energy to overcome the force of the atmospheric pressure, to break free and become a gas.

You say that an increases in heat excites the water molecules in the cigar until they have enough energy to overcome the ambient vaopr pressure and break free. This seems to be the same process, but you say that atmospheric pressure doesn't affect the equation. Maybe the impact is very slight in real world applications? Maybe they are indirectly related.


Lets put everything together:

If you put a glass of water in a vacuum the laws that govern vapor pressures still apply. There is nothing in the "air" as it is, in theory, completely void of anything.

Based upon your statements, I'm guessing you would say that no moisture would leave the glass, as even one molecule of water in the "Air" would become an infinite rH, causing enough vapor pressure to prevent evaporation. That would suggest that you can't fill a void with water vapor as it would instantly condense, but it is a gas that will fill a void. Am I missing something?

We know that the water will boil at a lower temperature in a vacuum because it takes less heat to excite the molecules to break away. That would mean that, there is some temperature that the water want to evaporate. We know that water evaporates at much lower temps than boiling.

I know that under some conditions, water molecules will break free, which would conflict the argument that it would maintain. I don't mean to put words in your mouth. I'm just trying to piece this together. I guess I don't understand enough of the theories behind vapor pressure.

Your thoughts?

[e-man]

Originally Posted by Hermit

Don't worry about it.
If yer hygro says 65, then it's 65,
whatever the temperature is.

I think everyone is in agreement on this point.

The question being debated is whether the total moisture content of the cigar is different for 70% at 70 degrees, compared to 70% at 65 degrees.

[AspiringGent]

I used to research and think about this stuff quite a bit. Then I put my humidor in my wine cellar, bought some 65% beads, and temp and humidity are always perfect now.

[e-man]

After researching vapor pressures and re-reading the posts, I see the cause of the breakdown in the theoretical example I posted.

By definition, RH is the ratio between two PRESSURES at a prescribed temperature. The "partial pressure" of water vapor, and the saturated "vapor pressure" of water


Partial Pressure is the pressure a gas would have at a given temperature if it occupied the volume alone. This is what we commonly refer to as the "potential".


Vapor Pressure describes the tendency of particles to escape from the liquid (or a solid). It IS a barometric pressure. A high vapor pressure for a liquid indicates a low boiling point. Like butane. It is a gas at room temp in our environment. If you add barometric pressure (ie. compress it) until it overcomes the vapor pressure threshold, it will force the gas back into a liquid. If you release pressure out (open the value) the vapor pressure of the liquid overcomes the barometric pressure and gas escapes from the liquid again.

Vapor pressure for a liquid IS direct linked to temp and altitude, however the partial pressure also changes with environment. Since the effects of atmosphere are present on boths sides of the ratio, they cancel each other out, just like the x's in this equation:

10x / 2x = 5 because x/x=1



So, placing a cigar in a vacuum at room temperature will cause the moisture to leave the cigar until the pressure in the container equalizes with the vapor pressure of the tobacco.


The fallacy in my interpretation of your post is that you can't have an infinite RH, as it does not need other gases present to calculate an RH.


The fact that there ARE other gases in air, means that these gases ALSO create pressure as they are in competition for space. That's why a bowl of distilled water in a humidor only reaches approx 85% RH in a constant environment. Because the combined pressure of all the gases overcomes the vapor pressure of water at 85%, preventing it from reaching it's full potential. To reach a higher rH, you would need to cool the 85% air.


So for the chart to say that you need to maintain 85% at 65F to have the same moisture content as 70% at 70F is definitely true for the AIR, but we still need to address what is happening in the hygroscopic materials.


From the PDF you referred to, there are large scale lumber operations who need to keep the total moisture content of their wood around 6%, even thought the relative humidity in their climate would cause it to sky rocket. They heat the sealed warehouse to lower the rH. The same amount of moisture is present in the air, but the additional temp lowers the rH, protecting the wood from absorbing more water.

If a major industry can control total moisture with this principle, then there is no doubt that a cigar's moisture content IS directly controlled by rH, regardless of temp, contrary to the write-ups and analysis readily available to common Joe Smoker.

Here's the link he posted since he's can't post links yet:

http://owic.oregonstate.edu/pubs/emc.pdf



Here's where I still need help understanding the mechanics:

I know that hygroscopic materials attract water. By definition, they typically have a low vapor pressure, which allows them to absorb moisture when other sources with a higher vapor pressure are willing to give it up.

A higher vapor pressure will boil/evaporate at a lower temperature (and vice versa).

Typically, hygroscopic materials (often called desiccants) need to be heated to get rid of the water, because they hold onto that moisture until the vapor pressures overcome the environment, allowing the molecules to break free.

The humidifier is hygroscopic (salt, PG, etc), as are the cigars. The air is the transport between the two objects.

I ASSUME that the humidifier has a higher vapor pressure than the cigar. The humidifier gives off moisture until the vapor pressure equalizes, let's say at 65%. The cigars is willing to take on that moisture, and would absorb MORE moisture if it were available. We know that if you put a 70% humidifier in, the cigar would gladly take on the extra moisture.

That would seem to imply that the cigar has a lower vapor pressure then the humidifier, but I could be wrong.


Now, lets say you put 2 humidifiers full of beads in a sealed container. One at 65%, and one at 70%. Lets say that both are half charged. The 70% has a higher vapor pressure and is willing to release moisture until the vapor pressure equalizes at 70%. The 65% beads will absorb moisture, as the 70% beads maintain the environment higher than the vapor pressure of the 65% beads.

The 65% beads will absorb/retain moisture until the 70% humidifier runs dry, essentially taking the moisture from the 70% beads. They will hold that moisture until the rH drops. Once the pressure falls below the equilibrium of the 65% beads, they will start releasing moisture.

We know the cigar will adjust its' humidity in proportion with the rH. Why doesn't the cigar lock onto the moisture like the 65% beads? They're both hygroscopic. In theory, they both have a lower vapor pressure than the 70% beads.


Your thoughts welcomed. I'll be doing some more reading.

[ToJo]

I didn't read this whole thread, but this line from the initial post made me cringe:

"Air holds more water at higher temperatures."

Please avoid thinking about it that way. The air is not "holding" anything.
Google "bad science" and "humidity" if you're interested.