Relative Humidity / Humidity Management

Discussion in 'Incubating & Hatching Eggs' started by twigg, Apr 8, 2008.

  1. twigg

    twigg Cooped up

    Mar 2, 2008
    Of all the subjects that surround incubation, the least understood, in my experience, is that of Relative Humidity.

    The raising and lowering of humidity is a relatively simple process, but one bound up in rumour and raised almost to a *Black Art*.

    Relative Humidity (RH), is simply the amount of water vapor in the air, compared to the amount that could be in the air, for a given temperature and pressure. This is important. The actual, or absolute amount of water vapour in the air is not relevant for our purposes, neither is it necessary to understand how RH relates to dewpoint. We can also dispense with pressure, and assume that we all live at, or close to, sea level. Above about 5000 feet you might have to make adjustments.

    Just a quickie about dewpoint. The dewpoint is the temperature at which the amount of water vapour in the air reaches a RH of 100%. Any further lowering of the temp and the water vapour must condense and form droplets. This is what causes fog. In an incubator, moisture forming on the glass could indicate 100% RH in the incubator. But be careful, it might be that the glass is simply cooling the air next to it to a point that some vapour has to condense, and it does so on the glass (bathroom windows steam up for the same reason). When you are watching the weather forcast … if the projected overnight low is close to, or below, the projected dewpoint, then your drive to work could well be foggy!

    In this article I simply intend to show how little you really need to understand, to get control of humidity, and how simple the process is.

    I do not intend to debate this, or argue with those who will not see, although I will answer any questions that seek clarification.

    Why do we need to consider humidity at all?

    Simply because eggs need to lose weight during the incubation process, and they do this via evaporation. An eggshell is porous, and water molecules can, and do, travel though the shell relatively easily. Eggshells are not uniform. Some have waxy coatings, some are different thicknesses, but the eggs of each species are generally uniform enough for our purposes. There can sometimes be issues when one attempts to incubate chicken and duck eggs together, for example, ideal conditions cannot be provided for both at the same time, but we can usually get close enough.

    Most eggs need to lose around 12% of their setting weight by day 18 (chickens). As the temperature is fixed, this rate is governed solely by the RH in the incubator, and the quality of the shell. We will assume the shells are uniform for now.

    All we need to know about humidity, for most purposes, is that a RH of 50% for the first 18 days, and 70% thereafter, will be enough. These figures are not Tablets of Stone, but guidelines established by generations of incubations that went before us. They did the work, we gained the knowledge.

    We know that this level of RH is appropriate for most eggs. What we don’t know is whether or not it is the correct level for the all of the eggs we happen to be incubating. There are two main ways to check. The first is by candling. An experience eye can tell whether or not the air space is developing properly. This is a skill that needs to be learned and practised. There is another way. We know the amount of weight an egg needs to lose. So we can weigh them. We can either weigh the batch as a whole and get an average, or we can weigh individual eggs. For rare or valuable eggs I would weigh them individually. As shells vary, eggs within a batch may lose more or less weight than their neighbours.

    If the weight loss is on target, then the humidity level is correct. It’s that simple.

    But what if it isn’t? What if we need to make adjustments in either direction. How can we accomplish that effectively, and is there any simple theory that will help us find a solution?

    To answer this we need to look at how the adding or removing of water from the different types of incubators affects the RH, and what are the constraints.

    Water enters the air by evaporation. The air in the incubator will contain some water vapour anyway. That amount is determined by the ambient humidity. We have warmed this air, so the RH in the incubator is generally going to be rather lower than the ambient. In very humid climates, however, it is entirely possible that no additional water is required, indeed some may need removing 

    It is important to understand evaporation. Water molecules will leave the surface of the water, and enter the air all the while the RH is less than 100%. Once it reaches 100%, then no more evaporation will happen unless the temperature rises. To understand this more easily we can think about how humans lose excess heat. We do it by sweating …. Water on the skin evaporates (latent heat of evaporation), and in doing so removes the energy required to evaporate it from the body. In very humid conditions, temperatures as low as 80F can be uncomfortable, because the sweat cannot evaporate. Conversely, 120F in the desert can be tolerable because of the extremely low RH.

    A large part of this process is air movement. As the water vapour leaves the surface of the water, then the RH of the air adjacent to the water is very high, and this will slow the process down to a crawl. If this layer is being constantly disturbed, then the process is much faster.

    This has implications for still air incubators, and nicely explains why the control of RH is easier in a forced air incubator. It is imperative that still air incubators are disturbed as little as possible, especially during that last three days, as regaining the moisture lost is very slow. Air does move, even in a still air incubator, convection sees to that. But it moves very slowly. It is unfortunate that the cheaper incubators are manually turned, as this means the incubator has to be opened at least three times a day to perform this operation, and each time the RH is compromised. Forced air incubators do not suffer anything like as much from this. Quite apart from temperature stability, RH is actually the single most important reason why a fan upgrade is money well spent.

    A major factor on the rate of evaporation is surface area. Depth is irrelevant, as water evaporates from the surface. The larger it is, the more water will evaporate. So humidity pans are wide and shallow. If air is moving across the surface, it will be faster still. We control humidity by a combination of ventilation and surface area.

    Crucially, any proposed solution that compromises the surface area, or reduces the ability of moving air to remove the moist surface layer, will not work. It cannot work.

    There was recently a fierce discussion about the use of water filled eggs to increase humidity in an incubator. While I respect the ingenuity, and commitment to finding solutions, this is one solution that simply defies the rules. And it can never have more than a marginal effect on the RH.

    On the other hand, all the regular suggestions about the use of sponges, towels, any absorbent material, are founded well in the theory of evaporation. To some extent or another, they ALL will work. What these solutions all have in common is that they increase the surface area of the water, quite considerably, therefore they work … easy wasn’t it?

    It is worth pointing out that increasing RH becomes more difficult as the RH rises. So it might seem extreme, but a vast increase in surface area may be what is required to achieve 70% RH, even if 50% was easily achieved.

    If we are faced with trying to increase RH, and are wondering whether or not a proposal might be effective, then simply ask two questions … does this proposal increase the surface area of the water, and/or does it increase the air movement? Either will work. Although I haven’t actually seen proposals for this, moving the water would work too.

    What we do not want to do is reduce ventilation too much. The eggs need oxygen, and the incubator needs to lose carbon dioxide. This happens by ventilation, and is most important when the need for high RH is also most important.

    So there you have it. It’s not magic, nor need it be especially confusing. Just a couple of simple principles working together.

    If you are building your own incubator, or just struggling to get to grips with humidity management in one you bought, I hope there is something in this to help.

    Last edited: Apr 11, 2008
  2. shortcake1806

    shortcake1806 Sassy McSassington

    Sep 11, 2007
    Very informative, thanks!
  3. twigg

    twigg Cooped up

    Mar 2, 2008
    Quote:Thank you Shortcake, you are very welcome [​IMG]
  4. allen wranch

    allen wranch Crowing

    Jan 11, 2007
    San Marcos, TX
    Explains a lot to me...
  5. tiki244

    tiki244 Flock Mistress

    Jan 1, 2008
    Thanks for writing that it helps [​IMG]
  6. twigg

    twigg Cooped up

    Mar 2, 2008
    Quote:Explaining and helping ... two words that make the effort worthwhile [​IMG]
  7. twigg

    twigg Cooped up

    Mar 2, 2008
    Lot's of questions about Humidity, so let's see if this thread can help anyone else [​IMG]
  8. Vamp-A-Billy Princess

    Vamp-A-Billy Princess Songster

    Apr 13, 2008
    Maybe its just so humid here in Indiana that mine doesnt need water? I wondered if the climate made a difference...thanks for all the info.
  9. MaransGuy

    MaransGuy Songster

    Oct 25, 2007
    Greenfield, MA
    Quote:This is a great explanation that I think will help a lot of people. The point I quote above is also important. One can have the same challenge when trying to hatch LF and Bantam eggs at the same time. I do set that combo myself at times and always have to try to find the balancing point. The bantams will loose moisture more quickly than the LF will. I have also found that my marans loose very little moisture during incubation and tend to dry down slowly. I am only assuming that it has to do with the extra layer of pigment laid down on the surface of the egg that causes this. The result is that I must hatch them drier than I would normal eggs. I agree that the more experience one has the easier and successful your hatches will be.


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