Relative humdity in incubators needs to be controlled to control the moisture loss (weight loss) of the eggs. Especially if the weight loss is too low, the air cell in the egg during internal pipping is too small to allow the embryo to effectively start lung ventilation, and hatchability will be influenced. It doesnt really matter when the moisture is lost from the egg, as long as the total weight loss is sufficient (11-12% at day 18) when internal pipping starts.
To control relative humidity during incubation, machines are equipped with humidifiers. Humidification increases the relative humidity, but it also creates local cold spots as evaporation of water costs energy, energy which is provided by the eggs in the area of the humidifier. Extensive use of humidifiers usually creates more egg temperature differences in the machine, and it is therefore better to limit the humidifier if the cooling capacity of the machine allows it.
To limit the use of the humidifier at the end of the incubation proces, we close the damper in the start of incubation, to avoid excessive moisture loss as a total. This creates a non-lineair moisture loss (less weight loss at the start, more at the end) and a more uniform egg shell temperature.
But the question is if we can do completely without the humidifiers inside the machine. This depends first of all on the cooling capacity of the machine (no humidifcation means no evaporative cooling) but also on the conditions of the incoming air and the minimum level of relative humidity and carbon dioxide we want to have in the machine. A quick calculation can help to make this more clear.
If we have a machine of 100.000 eggs (with 100% fertility) and we want to keep the carbon dioxide level at a maximum of 4000 ppm with incoming air of 500 ppm, we need to ventilate approximately 550 m3/hr. To keep the level at 3000 ppm, we need to ventilate 750 m3/hr.
If the eggs weigh 70 grams and we need 11% weight loss in 18 days, they lose 770 kg of water in 18 days, so 1,8 kg or liter per hour. As we will have a non-lineair moisture loss with more moisture loss at the end, lets assume that the moisture loss at the end (day 18) is 2,5 liter or 2500 g per hour. This water needs to be taken out by the ventilation, which means that every m3 of the 550 m3/hr that we ventilate takes out 4,5 g of water.
Single stage setters have at the end of the incubation process a temperature of approximately 37oC. If the only water in the machine would come from the eggs (completely dry incoming air and no humidication of the air in the machine) the machine would have air of 37oC and 4,5 g of water per m3. The Mollier diagram learns us that that air will have a relative humidity of 10-11%, extremely dry.
Fortunatedly, there is always some water in the air, but when the outside conditions are very cold and there is no additional humidification, it will be only a few grams. If the temperature outside is around 0oC and the relative humidity is 90%, there is less than 5 grams of water in the incoming air. Together with the 4,5 g of water per m3 that is provided by the eggs, the total will be 9,5 g of water per m3, which at a temperature of 37oC gives a relative humidity of 22%.
We actually don't know what this dry air does with the embryo. We know that the embryo will direct water from the allantois to the amnion when the moisture loss is high, so it seems mother nature is prepared to deal with very dry conditions. As long as we keep the total weight loss in the optimum range by closing the dampers in the beginning, the negative effects will probably be limited. However, a hatchery manager will feel very uncomfortable to have machines running at that low humidity.
If we would try to keep the carbon dioxide at 3000 ppm which means we would ventilate 750 m3/hr, the relative humidity would even be lower, because the 2500 gram of water provided by the eggs would now been divided over 750 m3 in stead of 550 m3 and would only add 3,3 g to each m3 of air. Together with the 5 g in the incoming air, that would result in 8,3 g/m3, which at a temperature of 37oC is 19% RH.
If we want to keep the RH in the machine to a minimum level of 35%, we need 15,3 g of moisture in the air. That means that the incoming air needs approximately 10 g of water if we want to keep the machine on 4000 ppm, and 12 g if we want to maintain 3000 ppm. 13oC outside temperature with 90% humidity means 10 g of water in the air, 16 oC outside with 90% gives 12 g of water. If there is less moisture in the air or we want to ventilate more, we need to humidify in the machine or in the incoming air, if we dont want the machine to drop under 35% RH.
We know that hatch results are negatively influenced by too high moisture loss, but the effect of very low relative humidity (below 30-35%) without total excessive moisture loss is not that well known. The first indication is that with an equal total moisture loss, relative humidity can be low at the end, but we dont know how low. Both the water flux in the egg and the fact that the embryo is protected in the watery environment of the amnion suggest that eggs can handle a lot, but most hatchery managers will have sleepless nights if they know there machines are running at 20-25% RH. That means that in colder climates, air handling units are needed if we want to avoid the use of humidifiers in the setters.