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.
When we measure moisture loss in different periods of the incubation process, we see that the moisture loss is slightly higher at the end of the incubation period than at the beginning. This suggests that the conductance of the egg shell is changing during incubation. This doesn’t sound unlogical as the embryo is taking up calcium from the egg shell, so we could expect that the shell would get thinner and could become more “conductive”. In infertile eggs we do not see the change in moisture loss over time, which support this idea. However, the reason for the increase in weight loss towards the end of incubation is not because of a change in conductance but a change in temperature. Because the embryo is producing heat the internal egg temperature is going up and with it the “driving force” for moisture loss is changing. This driving force is called the water vapor pressure deficit, and is the result of the difference between water vapor pressure inside and outside of the egg. Water vapor pressure is the result of a combination of temperature and relative humidity and can be calculated with the Mollier diagram. Nowadays there 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.
Let’s demonstrate this with an example. We place eggs in an incubator at 37 oC and 55% RH. We weigh the eggs at setting and after 7 days we measure them again and find that they have lost 5% of weight. At 12 and 19 days we measure them again and determine the weight loss again. But due to the increased heat production of the embryos, in that period between 12 and 19 days we have to lower the air temperature to 36oC, and the with that temperature the internal egg temperature goes up to 38oC. How would this influence the water vapor pressure deficit and with it the moisture loss of the eggs in those two periods.
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 (for the simplicity of the calculation we assume that the eggs and machine will be immediately on temperature, which will of course in reality will not be totally true) and again at the end of the 7 days, and we notice that the weight loss is 5%. In the incubator we have a temperature of 37 oC and 55% relative humidity. As the eggs are still relatively young in the incubation process, we do not have to worry yet about a temperature difference between egg and air due to the heat production of the developing embryo.
In the first period of 7 days we have a temperature in the eggs of 37oC and 100% relative humidity as inside there is just water. The 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.
In the second period of 7 days we lower the air temperature to 36oC and the internal egg temperature increases to 38oC. We assume that the relative humidity in the machine is still 55%, and of course in the inside of the egg shell there is still 100% RH
If we use the same program we can calculate the water vapor pressures in this situation:
Inside of the eggs (38oC, 100% RH) there is a vapor pressure of 66.2 mbar or 6620 Pa
Outside of the eggs (36oC, 60% RH) there is a vapor pressure of 32.7 mbar or 3270 Pa
So the difference (deficit) is 6620 – 3270 Pa = 3350 Pa.
The conductance of the eggs let them lose 0.01% per 1000 Pa per hour, so with a water vapor pressure deficit of 3350 Pa the eggs will lose (3350/1000) x 0.01% x 7 x 24 = 5,63%. So because of the difference in temperatures inside and outside of the egg, the moisture loss in that period of 7 days will go up from 5% in the first 7 days to 5.63% in the second period of 7 days. So the increase will be measurable and noticable, but is not related to a difference in conductance but to a difference in water vapor pressure deficit.