Relative humidity and the science behind cigar storage

[Flatliner]

I recently got heavier into cigar smoking and want to start storing cigars at home in a humidor. Like most things that become a sort of hobby of mine, I research the hell out of my new fascination.

So while learning about humidors, I discovered that the cigar community is split on whether or not the relative humidity the humidor is kept at should change based on temperature. Below I’ll paste two differing opinions along with the rationales:

RH shouldn’t change with varying temps:

The modern myth of varying humidity with temperature is based on an entirely false premise. Cigars don't care about absolute moisture content. They need just enough moisture to keep the tobacco pliable, but not so much to swell it. This occurs at 70-73% REGARDLESS of temperature.

Tobacco should be kept at a relative humidity which just keeps the leaves pliable without swelling them. Since that pliability is based on the interaction of the cellular membranes with the air surrounding them, *relative humidity*, not absolute moisture content is what's important. This isn't chemistry, it's mechanics - on a cellular level. Ideally, the correct point is 70-73% regardless of temperature. At 70% relative humidity (a measure which by definition is independent of temperature) cigar leaves become pliable without swelling. at 75% or higher, the cells begin to swell. at 68% or lower, they can become brittle. These points are regardless of the ambient temperature or the absolute moisture content.

http://en.allexperts.com/q/Cigars-2143/2009/9/Absolute-vs-Relative-humidity.htm

RH should change with varying temps:

The most common rule of thumb for your cabinet humidor climate is commonly referred to as the 70/70 rule. Keeping a temperature of 70°F and a RH of 70%, means that you will have the right level of AH or water vapor pressure to store your cigars. Each temperature has a corresponding RH level that helps maintain the correct AH to keep your cigars aging nicely, as displayed in the chart below. If your temperature changes from 70°F to 72°F the RH should read 65% in order to keep an AH comparative to the standard of 70/70. There are exceptions to the rule, though. Once the temperature goes below 60°F, there is no corresponding RH level that will give you an appropriate AH. The air simply can not hold that much moisture.

http://vigilantinc.com/cigarhumidors/humidity-and-temperature.php

So, I come to the Physics Forums for the science smackdown. Who is right?

 

[klimatos]

"At 70% relative humidity (a measure which by definition is independent of temperature)"

Your source in incorrect. In humid air, of the common measures of humidity, only mixing ratio and specific humidity are independent of temperature. Absolute humidity, relative humidity and vapor pressure all vary with changes in air temperature.

 

[russ_watters]

Based on the above noted error alone (and then by the apparent lack of quality of the source), I'd be skeptical of that advice. A few other notes:
1. I'd be surprised if cell structure is intact enough to matter for dried plants.
2. Perhaps there isn't research on this for cigars (I'd be surprised if there isn't), but there certainly is for wood and baseballs. I suggest looking into it.
3. As yourself this: what parameters vary in the cigar as environmental conditions change? Ie: does temperature alone make a difference? If you store a cigar in a cold environment and then quickly warm it to room temperature, will it behave differently? How about if you smoke one on a cold day? For wood, what matters is the mass ratio of water in the wood, not the temperature.

 

[Flatliner]

"At 70% relative humidity (a measure which by definition is independent of temperature)"

Your source in incorrect. In humid air, of the common measures of humidity, only mixing ratio and specific humidity are independent of temperature. Absolute humidity, relative humidity and vapor pressure all vary with changes in air temperature.

I discussed your answer and the quote you are responding to in the first article with another source and this was the reply I received:

They're saying the same thing in different terms.

This "At 70% relative humidity (a measure which by definition is independent of temperature)" means 70% relative humidity is the desired RH for cigars, regardless of the humidor temperature (for reasonable ranges of temperature)."

Whereas this "... Absolute humidity, relative humidity and vapor pressure all vary with changes in air temperature" means given a fixed amount of moisture trapped in a sealed container, as the temperature goes up the relative humidity measurement will go down and vice versa.

The first statement is about cigars, the second about physics.

Combining them gives the practical result that:
1) You DO want to maintain a fairly even humidor temperature.
2) If the humidor gets much hotter or colder than normal, you DO want to add or subtract moisture to maintain 70% RH.
3) You do NOT want to target different RHs at different temperatures.