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Humidity Discussion - VPD vs RH

This is a discussion on Humidity Discussion - VPD vs RH within the Cigar Accessory Questions forums, part of the Cigar Accessory Discussion category; [SIZE="1"]First, this thread is not being posted as some sort of scientific discovery that no one has thought of before. ...

  
  1. #1

    Young Puffer Fish RLembke's Avatar


     

    Post Humidity Discussion - VPD vs RH

    [SIZE="1"]First, this thread is not being posted as some sort of scientific discovery that no one has thought of before. This thread is not to proclaim a new religion in the method of maintaining cigars. This thread IS posted as a discussion that I hope involves many of the FOGs on this forum and results in some good dialog, both scientific and practical. If you're only going to state something to the effect of "that's how it's always been done" or "this is a myth, use the search", please don't post here. Simply roll your eyes and click "back". However, if you'd like to participate in a scientific discussion and have something worthwhile to contribute, please do![/SIZE]

    This discussion is a split-off from the discussion that began in my 65% Beads - 62F/80%RH Thread here - http://www.clubstogie.com/vb/showthread.php?t=69488 With help from several great BOTL, especially Scimmia, a nice discussion took place regarding what the proper way to maintain cigar moisture content within a wider range of temperature is.

    Historically, relative humidity (RH) has been the tried and true, proven method for monitoring humidity within humidors. FAQs such as http://www.cigargroup.com/faq/#6.1 have been referenced time and again. These FAQs have said that you should hold a set RH according to your tastes steady regardless of temperature.

    At times, BOTL have posted charts showing the relationship between absolute humidity (AH) and temperature. This chart is generally regarded as bad information and this can be proven to be the improper way to store cigars as I can promise you a cigar held at 60F and 80% ain't worth a THIS THREAD IS NOT ABOUT ABSOLUTE HUMIDITY VS RELATIVE HUMIDITY.

    During the course of the discussions in the aforementioned thread, an additional method of monitoring and controlling humidity was discovered - Vapor Pressure Deficit (VPD). As far as I can tell, this is the first time this specific topic has been discussed on this forum. If it is not, I apologize.

    I cannot state what VPD is, how it differs from RH and why it may be better for us more succinctly than Michael A. Dixon, PhD, University of Guelph (Ontario, CA) does in a section of his paper, "Humidity Control Algorithm in the Greenhouse" - http://www.microcool.com/cooling/bro...ityControl.pdf

    -----------------------------
    Before I continue with a description of some of our experiments, it seems appropriate to add a
    brief discussion of the terminology associated with the study of humidity. To a certain degree, many of the
    misconceptions and inaccuracies related to the interaction between humidity and plants have been fostered
    by the way we measure and talk about humidity. The average individual uses the term “relative humidity”
    with confidence and ease. Humans have a general “feel” for this assessment of atmospheric moisture that
    makes it relevant to their relationship and interaction with the aerial environment. It seems natural,
    therefore, to continue to apply this measure of humidity in research associated with greenhouse humidity
    control or in attempting to outline a management strategy of humidity control in the greenhouse.

    Relative humidity, as we are all reasonably well aware, is a measure, in percent, of the amount of
    water vapor in the air compared to the total amount of water vapor that could be in the air at a given
    temperature. Of course, as the air temperature rises, its capacity to hold water vapor also increases and,
    conversely, as air temperature falls it will hold less and less water vapor. So, arriving at this common
    measurement of aerial moisture requires knowledge of the relationship between water vapor concentration
    (or partial pressure) and air temperature. This allows us to predict the absolute maximum quantity of
    moisture that the air will hold at a given temperature which we then divide into the actual quantity of
    moisture present at the time of our measurement and multiply by 100 to arrive at “percent relative
    humidity”. Sadly, mental arithmetic is a lost art in this age of the computer so the trusty hand held
    calculator (or its equivalent in the circuitry of most solid state sensors) helps us to the final answer.
    Unfortunately, even the most sophisticated, genetically manipulated plant species has yet to master the
    simple calculator or even simpler mental arithmetic to achieve this evaluation of humidity.

    Plants cannot respond to or detect moisture that is present in the air (until it condenses or falls on
    them) but instead respond to atmospheric demand for moisture, which is another way of defining the
    moisture that is not present in the air.
    That is the vapor pressure deficit and is the difference between
    the actual concentration or partial pressure of water vapor in the air and the maximum possible
    concentration at that temperature. It doesn’t require that the plant have any knowledge of arithmetic or
    predictive relationships. The plant simply opens its stomatal pores as the sun rises and permits the
    evaporation of water from the leaves in response to the atmospheric demand established by prevailing
    ambient conditions. If the atmosphere becomes too demanding, plants exercise a variety of mechanisms to
    cope and still maintain physiological functions. Viewing humidity from the plant’s perspective was the
    first step in approaching a rational strategy of humidity control.
    Therefore, all of our measurements of humidity which were to be correlated with plant
    physiological and growth responses were evaluated as vapor pressure deficit (VPD) rather than relative
    humidity (RH).
    Our greenhouse environment control responded to VPD feedback and our set points were
    established in units of millibars (mb) which is a pressure unit. Concentration units (eg. g/m&#179 are also
    common. To put the units in perspective, a VPD of 10 mb at a temperature of 20 C represents a relative
    humidity of about 56%, 10 mb at 30 C is closer to 76% RH. TO make the point about atmospheric demand
    more clearly, 80% RH at 20 C represents a VPD of about 4 mb. Raising the temperature to 30 C but
    maintaining the same 80 % RH results in a VPD of 8 mb. Thus, even though the relative humidity
    remained unchanged, the atmospheric demand for moisture felt by plants was doubled.

    -------------------------------------

    Along with Dixon's paper, Ohio State has an article discussing VPD and it's use in greenhouses - http://ohioline.osu.edu/aex-fact/pdf/0804.pdf

    and a paper titled "The Control of Atmospheric Humidity in a Closed System" by B. D. Bolas of the Imperial College of Science and Technology of London also touches on VPDs use instead of RH - http://www.blackwell-synergy.com/doi...1926.tb06686.x

    As for whether this method of humidity measurement is better for cigars or now has one very large caveat - do you have temperature issues? Are you able to maintain right at or around 70F? If so, RH is perfectly fine. Are you like me who is forced to store you precise cargo in a hidden place within the homestead and you may have less than ideal temperatures during the year? If so, VPD may be the better way for you to judge what RH you should be holding your humidor at in order to give you that pleasant 70F/65% smoke after your stogies have been down at 60F during the winter or God forbid 80F during the summer.

    Another issue regarding the use of VPD as a humidity measurement is let's say in my case, VPD says I should be holding my sticks at 50% RH at 60F. What method of humidification is going to help me hold 50%RH? I've already begun designing a system that may do just that.

    All that aside - what does everything think about Vapor Pressure Deficit vs Relative Humidity? Scientific "Kool-Aid"? Applies to living plants but not 'dead' plants such as ours?

    In the words of Hillary - "Let's Talk....Let's Chat"
    Last edited by RLembke; 03-12-2007 at 11:23 AM. Reason: fix links...dang it

  2. #2

    Unknown Specimen Scimmia's Avatar


     

    Re: Humidity Discussion - VPD vs RH

    It has been long established that Equilibrium Moisture Content (EMC; the moisture level where evaporation is equal to condensation) in wood varies with both temperature and RH, not just RH; so I'm convinced that temperature does come into play. It's not a specific RH at any temperature to keep a constant moisture content.

    Link pertaining to EMC in wood:
    http://www.woodweb.com/knowledge_bas...p/fplrn268.pdf

    I can't find any information about the relationship between VPD and EMC, but just looking at the graphs and charts available, it's looking to me like VPD may be the best measure we have short of EMC itself.

    I'll be interested to get more people involved, I've been told we scared some people off.

    Edit: I'll be VERY interested to hear your results once you get a way to keep your RH that low. Regardless of the science, most people are going to be very leery with their collection until someone else does it.
    Last edited by Scimmia; 03-12-2007 at 11:51 AM.

  3. #3

    Young Puffer Fish RLembke's Avatar


     

    Re: Humidity Discussion - VPD vs RH

    I agree completely.

    Depending on the outcome of this discussion, I would offer to store several sets of sticks at various temp/RH settings according to the VPD charts and have a few FOGs do a blind test of those sticks and see if they can tell a difference.

    Of course, by the time I get an automated system that will adjust RH according to temperature, it'll be summer and I'll no longer have a basement at 60F......grrr

  4. #4

    Silverback gvarsity's Avatar


     

    Re: Humidity Discussion - VPD vs RH

    I guess I'll just have to build a walk in wine cellar/humidor that is temperature and humidity controlled with seperate micro-climates for the different items.

    Sigh.....sacrifices will have to be made, gods will have to be appeased but it can be done.



    Interesting articles. A little over my head but I love a well documented post. Well done.
    Hanlon's Razor: Never attribute to malice that which can be adaquately explained by stupidity.


  5. #5

    SvilleKid
    Guest


     

    Re: Humidity Discussion - VPD vs RH

    While I'm not in the same league scientifically as you, I did understand where you are coming from with the VPD discussion. My main thought, which Rlempke put forth, and which I have NO knowledge to use as a basis of an informed answer, is what the difference in humidity release would be between living tobacco plants, and dead, dried, fermented tobacco leaves.

    Logic tells me that the humidity released from living plants could be much greater, but could also be plant controlled to be much less, when in the best interest of the plant. It is obvious that the dead leaves in tobacco DO have the ability to take in, as well as give out humidity. I would guess that cigars would have a slower reaction time, and would also have less of an ability to "self-regulate" compared to living plants.

    WFIW, any idea where to buy my new VPD electronic measurer? And will it be significantly more accurate than the analog version

  6. #6

    Big Country Flavor Marlboro Cigars's Avatar


     

    Re: Humidity Discussion - VPD vs RH

    Quote Originally Posted by SvilleKid View Post
    While I'm not in the same league scientifically as you, I did understand where you are coming from with the VPD discussion. My main thought, which Rlempke put forth, and which I have NO knowledge to use as a basis of an informed answer, is what the difference in humidity release would be between living tobacco plants, and dead, dried, fermented tobacco leaves.

    Logic tells me that the humidity released from living plants could be much greater, but could also be plant controlled to be much less, when in the best interest of the plant. It is obvious that the dead leaves in tobacco DO have the ability to take in, as well as give out humidity. I would guess that cigars would have a slower reaction time, and would also have less of an ability to "self-regulate" compared to living plants.
    From the plant's perspective,
    Dead plants don't 'react' more than any other inanimate objects.

    when you put cello inside your humi and it slowly raises to 65%,
    is it really 'reacting' similarly to the tobacco
    or is it just absorbing moisture like anything else would?

    The basis behind this argument is that the plant senses humidity and temperature differences and opens the stoma accordingly allowing for transpiration (exchange of gases). This exchange occurs through osmotic diffusion - where air pressure and humidity will slowly equalize through a permeable layer.

    This is not completely true for cigars. I do not think rolled tobacco leaves inside a humidor (without a glass top) have any way to sense daylight and regulate opening of its stoma. Gas exchange, in such a case, is solely dependent on osmotic diffusion. Stomatal openings dilate with air pressure changes. Although this may sound like the plant is actually "doing" something, diffusion is just the equalization of air pressures and humidty contained within that air.

    The air moves to where it will be equal through the stoma still,
    (the only opening tobacco leaves have to absorb through)
    but the plant isn't really doing anything anymore beyond reacting to the environment.

    Though many plant tissues have evolved for the function of water retention,
    any other kind of water permeable tissue (human) would have a similar reaction.

    If you died underwater, you would absorb the water and become all bloated.
    If you died in the desert, you would dry out.

    This doesnt mean you still sweat.
    Last edited by Marlboro Cigars; 03-12-2007 at 03:14 PM.

  7. #7

    Unknown Specimen Scimmia's Avatar


     

    Re: Humidity Discussion - VPD vs RH

    great post Marlboro Man, what you're saying is that what is being described is an active process and therefor not applicable? Wouldn't this be the same situation as if the stoma is open all of the time, so we just don't have the fail-safe that the plant has of being able to close it to protect itself? And in that case, is the VPD still applicable? (I believe so from that article)

    It sounds like you have some specific knowledge on this subject, are we at least on the right path?

  8. #8

    Young Puffer Fish RLembke's Avatar


     

    Re: Humidity Discussion - VPD vs RH

    Marlboro Man -

    Nice post...thanks for the info. I may be mistaken but are you saying that, yes, you agree that cigars would absorb and give off moisture to it's surroundings in the same way a living plant would? And that VPD is an accurate measurement pertaining to cigars?

  9. #9

    Not Bob bhudson57's Avatar


     

    Re: Humidity Discussion - VPD vs RH

    uh, yeah. what he said...

    You guys are way over my head (not that that is too hard).
    Bill
    Check out the Newbie Sampler Trade:clicky

  10. #10

    Leading Puffer Fish Dr. Stogie Fresh's Avatar


     

    Re: Humidity Discussion - VPD vs RH

    Thanks for starting this discussion and for defining some terms. I don't have any answers, but I am full of questions. I had the same questions as SvilleKid: what are the differences between tobacco leaves used in our cigars and live plants in a greenhouse?

    Is the tobacco in our cigars is still permeable to gasses? Most tobacco growers will tell you that the tobacco leaves will "soak up" many substances, which is why they recommend carefully handling your cigars, washing hands before touching, etc. That is to say, they are permeable and active (or, at least, reactive).

    Do the finshed cigars interact with the environment inside the humidor and if so, how? If you have opened up a humidor or a box of cigars and smelled the ammonia, you will know that some kind of reaction is taking place inside the cigars. There is still some amount of fermentation (the question of if this is the same kind of fermentation process as in the curing and fermentation barns is another good question) going on that is emptying by-products of fermentation into the environment within the humidor. Further, the oleoresins in the cigars continue to migrate, probably from areas of higher pressure/concentration to that of lower pressure/concentration and either by direct contact with other cigars (conduction?) or with the atmosphere (evaporation?). So, as Marlboro has stated: even dead plants "react" with their environment.

    Besides adding to the concentration of gasses in the humidor, does this fermentation also affect heat production? It does in the curing and fermentation barns, why not in a humidor (though the total effect would be less apparent)?

    The measurement of conditions with a humidor, whether temperature, RH or VPD is only important to the extent that they can accurately relate something useful about the internal condition of the cigar. For truly, this is where the "rubber meets the road." It is the condition of the tobacco leaf that will determine whether we have a tasty smoke. All measurements are really only predictive, at best, of the condition of the leaf; and this only because we feel that there is some relationship between certain measurements and said leaf condition. In other words, these measurements are not direct measurements of the leaf condition, but rather by using them we feel we can make some assumptions about the condition of the tobacco in our cigars. But first we must prove these assumptions of predictability are valid in cigars, which is different than in greenhouse plants and other organic materials.

    I have found through experience that after we get outside a reasonable range (say, 62-78%RH and about the same temp), all inferences about how temperature and relative humidity speak to the condition of the tobacco seem to blur.

    I happen to believe that AH is an important factor in determining the direction of moisture within cigars, but only up to a point. I think the mistake that has been made is to assume that this relationship is linear, when it may, in fact, be non-linear and "tail" up or down as we move away from the "temperate zone."

    I think this is an interesting discussion. I hope you will be able to follow through on your tests.

    Doc
    Dr. Stogie Fresh
    Visit Stogie Fresh at: http://www.stogiefresh.com


  11. #11

    Young Puffer Fish RLembke's Avatar


     

    Re: Humidity Discussion - VPD vs RH

    Amazing post, Doc. All well said.

    I guess I should have posted this in the first place....I made up a spreadsheet that uses the VPD formula to simulate a given environment (say, 70F/65%RH) at any temperature. Decide which environment you want to simulate, identify the temperature you are currently at and it gives you an RH that you should be at.

    In this case, you wouldn't necessarily need to have a VPD-meter on your humidor...you would just have to be aware of how your RH should be adjusted as the temperature changes.

    I offered this up in the other thread as well....if anyone would like a copy of the spreadsheet, PM me your email addy and I'll send it over. I'll throw it up on my web server tonight so it's available to all without solicitation.
    Attached Images Attached Images  

  12. #12

    Leading Puffer Fish Dr. Stogie Fresh's Avatar


     

    Re: Humidity Discussion - VPD vs RH

    Quote Originally Posted by RLembke View Post
    Amazing post, Doc. All well said.

    I guess I should have posted this in the first place....I made up a spreadsheet that uses the VPD formula to simulate a given environment (say, 70F/65%RH) at any temperature. Decide which environment you want to simulate, identify the temperature you are currently at and it gives you an RH that you should be at.

    In this case, you wouldn't necessarily need to have a VPD-meter on your humidor...you would just have to be aware of how your RH should be adjusted as the temperature changes.

    I offered this up in the other thread as well....if anyone would like a copy of the spreadsheet, PM me your email addy and I'll send it over. I'll throw it up on my web server tonight so it's available to all without solicitation.
    Yes, I did see that chart from the other thread and found it very interesting. Of course, there is still the problem that it uses as its foundation for accuracy, the assumption that both temperature and RH% are known and accurate, which leans heavily on the accuracy of those readings. And, the assumption that temperature, RH and VPD make a valid contribution to predicting the condition of the tobacco. Nevertheless, I find this discussion very stimulating and worthwhile. You and Scimmia have done a wonderful job of carrying on this discussion.

    By the way, I would like to have a copy of the spreadsheet. PM sent.

    Doc
    Dr. Stogie Fresh
    Visit Stogie Fresh at: http://www.stogiefresh.com


  13. #13

    Unknown Specimen Scimmia's Avatar


     

    Re: Humidity Discussion - VPD vs RH

    You're right, EVERYTHING here depends on having accurate readings, which shouldn't be taken for granted.

    I guess one basic question that you touch on that we've been missing is simply knowning WHAT condition of the leaf are we trying to maintain? I think we've been assuming that it's only moisture content, but maybe there's more to it than that.

  14. #14

    Young Puffer Fish RLembke's Avatar


     

    Re: Humidity Discussion - VPD vs RH

    Quote Originally Posted by Scimmia View Post
    I guess one basic question that you touch on that we've been missing is simply knowning WHAT condition of the leaf are we trying to maintain? I think we've been assuming that it's only moisture content, but maybe there's more to it than that.
    Very true! As far as I know, no one has ever quantified the dependent variable(s) in this equation - we have only been told what independent variables to set and TADA...perfect cigars.

  15. #15

    Leading Puffer Fish Dr. Stogie Fresh's Avatar


     

    Re: Humidity Discussion - VPD vs RH

    Yes, the only dependent variable that I am aware of is the moisture content of the tobacco in a finished cigar (between 14-17%). And this is, in part, dependent on sufficient curing and fermentation of the tobacco. I have no idea how the cigar industry determines moisture content. That is a question that I will have to ask.

    As Scimmia has questioned: are there other criteria (besides moisture content) that are used to determine the condition of the tobacco?

    Doc
    Dr. Stogie Fresh
    Visit Stogie Fresh at: http://www.stogiefresh.com


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