For an optimal incubation result, it is important that the total moisture loss of the eggs at the moment of hatching is high enough to create a sufficient air cell, to allow the embryo to start adequate lung respiration at the moment of internal pipping. An optimal moisture loss at the moment of internal pipping is approximately 12-14% of the initial egg weight, which relates to approximately 11-13% weight loss at transfer.
Moisture loss of eggs is detemined by the conductance of the egg shell (the "resistance" against gas exchange) and the difference in water vapour pressure inside and outside of the egg. This water vapour pressure is determined by temperature and relative humidity. Inside the eggs the relative humidity can be estimated at 100%, where outside it will depend on the conditions of the machine.
During incubation, the conductance of the eggs doesn't change. Although the embryo uses some of the calcium of the shell, this doesnt change the form and function of the pores in the egg shell, and therefore the resistance against gas exchange across the egg shell remains the same. This means that the conductance of the egg shell is not influenced by the presence of a life embryo, fertile and infertile eggs will remain to have the same conductance throughout the incubation period.
However, the water vapour pressure inside the egg depends on the relative humidity and the temperature of the egg. And although the relative humidity inside the egg doesnt change (it remains 100%) the temperature of a fertile egg increases throughout the incubation period, due to the heat production of the embryo.
This means that theoretically a fertile egg will lose more water than an infertile egg, because the water vapour pressure inside the egg is higher due to the increased temperature.
When we know the difference in temperature between fertile and infertile eggs, we can calculate the water vapour pressure deficit (the driving force: the difference between water vapour pressure inside and outside of the egg) of fertile and infertile eggs.
If we we estimate that fertile eggs will have an increase in egg shell temperature from day 7 onwards, and we estimate that the difference in egg shell temperature will increase lineair towards a difference of 1,6oC (3oF) at 18 days, we can calculate that the total difference in moisture loss during the setter period (18 days) will be approximately 3%. This means that if the total water loss until 18 days is estimated at 10% of the initial egg weight, fertile eggs will theoretically lose approximately 0,3% more water than infertile eggs, due to the heat production.
In our calculations, we estimated that the infertile eggs will have the temperature of the air. In reality this is not the case, as the infertile eggs will pick up heat from the surrounding fertile eggs, unless there is a very high level of infertility. But in a normal situation the infertile eggs will be less than the estimated 1.6oC (3oF) cooler than the fertile eggs at 18 days. Therefore, a more practical estimation of the difference between fertile and infertile eggs will be an approximately 0.2% higher moisture loss for fertile eggs at 18 days of age.
The consequence of all this is that when we determine the weight loss of eggs in a batch with a high percentage of infertile eggs, we are actually slightly underestimating the weight loss of the fertile eggs in the batch. However, as the differences are relatively small and it is more important to have at least the minimum required weight loss than to be concerned about having too much weight loss, the difference in weight loss between fertile and infertile eggs is relatively unimportant.
The actual level of water vapour pressure deficit in fertile eggs will depend on the egg shell temperature of the egg. As air velocity has a big influence on the cooling of the eggs and therefore on the resulting egg shell temperature, the level of air velocity will influence the moisture loss of the eggs as well. A higher air velocity will result in a relative lower temperature of the egg, and therefor a slightly lower water vapour pressure deficit and weight loss of the eggs. However, these differences will be marginal and hardly detectable in a practical situation.