The pores in an egg shell allow gas exchange, the uptake of oxygen and the release of carbon dioxide and moisture. The porosity of the egg shell is called the conductance. With a higher conductance an egg in the same conditions will be able to uptake more oxygen and release more carbon dioxide and moisture in the same time frame. This is important not only to achieve a proper weight loss during incubation (preferably 12 to 14% of the initial egg weight during the whole incubation process, or minimum 10% until transfer at 18 days) but also to allow the egg/embryo to uptake enough oxygen for the energy demand.
It is therefore important to know what the conductance of a set of eggs is. If the conductance is low, we need to lower the relative humidity to allow the eggs to lose sufficient weight, but we also might want to lower the incubation temperature to allow the eggs to metabolize sufficient energy for the development of the embryo.
To determine the conductance, we can use the weight loss of the eggs. Weight loss (moisture loss) is the result of the conductance (resistance of the shell against moisture loss) and the difference in water vapor pressure across the egg shell (the water vapor pressure deficit). If we know the water vapor pressure deficit over a period of time and we measure the resulting weight loss, we can calculate the weight loss per unit of water vapor pressure deficit, and in that way have a measurement of the conductance.
Water vapor pressure is the result of temperature and relative humidity, and is expressed in mbar or more commonly in Pascal (Pa). Inside of the egg the relative humidity is 100%, where outside the relative humidity depends on the temperature and the amount of moisture in the air. When we know the temperature of the egg and the air and we know the relative humidity, we can calculate the water vapor pressure inside and outside of the egg and the resulting difference, the water vapor pressure deficit or the “driving force” for moisture loss. The calculation of the water vapor pressure is a result of the Mollier diagram, but nowadays there are a lot of tools on the internet available that calculate it for us, for instance www.hvac-calculator.net, www.psych-chart.com or www.flycarpet.net, to name a few.
Lets take an example to see how this works out. Lets suppose we place eggs in an incubator for 7 days (7 x 24 hours = 168 hours), we weigh the batch at the start and again at the end, and we notice that the weight loss is 5%. In the incubator we have a temperature of 37 degrees and 55% relative humidity. As the eggs are still fresh, we do not have to worry about a temperature difference between egg and air due to the heat production of the developing embryo.
In the eggs we have a temperature of 37oC and 100% relative humidity, and outside temperature is 37oC and 55% relative humidity.
If we use one of the programs mentioned, we can calculate that
Inside of the eggs (37oC, 100% RH) there is a vapor pressure of 62.7 mbar or 6270 Pa
Outside of the eggs (37oC, 55% RH) there is a vapor pressure of 34.5 mbar or 3450 Pa
So the difference (deficit) is 6270 – 3450 = 2820 Pa.
The eggs lose 5% weight (moisture) in 168 hours, so 0.03% (0.02976% to be precise) per hour. This means that a difference of 2820 Pa results in a moisture loss of 0.03% per hour, or 0.03/2.820 Pa = 0.01% (0.01064% to be precise) per hour per 1000 Pa water vapor pressure deficit.
How much water would these eggs lose if we would put them in storage, let say at 16oC and 60% RH.
Inside of the eggs (16oC, 100% RH) there is a vapor pressure of 18.2 mbar or 1820 Pa
Outside of the eggs (16oC 60% RH) there is a vapor pressure of 10.9 mbar or 1090 Pa
So the difference (deficit) is 1820 – 1090 Pa = 730 Pa.
The conductance of the eggs let them lose 0.01% per 1000 Pa per hour, so with 730 Pa difference they will lose 0.0073 % per hour. So if we store those eggs for 5 days, they will lose 0.0073 x 5 x 24 = 0.88% of moisture.
If we would store them at 20oC and 60%, the calculation would be:
Inside of the eggs (20oC, 100% RH) there is a vapor pressure of 23.4 mbar or 2340 Pa
Outside of the eggs (20oC, 60% RH) there is a vapor pressure of 14.0 mbar or 1400 Pa
So the difference (deficit) is 2340 – 1400 Pa = 940 Pa.
As the conductance of these eggs let them lose 0.01% per 1000 Pa per hour, with 940 Pa water vapor pressure deficit the moisture loss in the same period of 5 days would be (0.01 x 0,940) x 5 x 24 = 1.13% weight (moisture).
This calculation shows that moisture loss is not only dependent on relative humidity, but also on temperature. Knowing what the conductance of the eggs is can help to control and predict the moisture loss. We can determine the conductance at any given time frame in any situation, for instance during the storage period or during (part of) the incubation period. Conductance doesnt change over time or in a different temperature or relative humidity condition, so wherever we have a period of time with constant conditions and we measure the weight loss, we can calculate the conductance and use it for other situations. Of course the longer the period for weight loss will be, the more accurate the weight loss can be determined. If we only put the eggs in a specific condition for a few hours the weight loss will be limited and more difficult to calculate precise.
We can even use the weight loss to calculate average relative humidity over time. If we have established the conductance of an egg and we place it in an evironment with a constant (and known) temperature, the weight loss will indicate what the relative humidity will have been.