I'm not sure why there is so much emphasis on numbers regarding relative humidity or wet bulb temperature.
I'm going to jump in here and declare my belief that RH is not a set number.
First of all, ideal humidity for one egg is not the same for the next egg. Each egg is porous but not necessarily as porous as the next. Not all eggshells are created equal. One factor that can affect porosity is the elevation where the hen that laid the egg lived. The higher the elevation where laid, the less porous the shell. This change is necessary for hatchability because molecules of air move faster at higher elevation. This is not always an issue, I'm just trying to note that there are many reasons, including genetics why not all eggs are created equal.
Let's consider how humidity is addressed in nature. The history of artificial incubation occupies a blink of the eye on that timeline.
What happens if a hen is sitting on eggs in a dry climate, which could be experiencing humidity well below that recommended by many for successful hatching, or for that matter if there are days of thunderstorms with 90% humidity. A hen doesn't use a hygrometer. If dry, they will sit tight longer holding in humidity as moisture transpires from the contents.
Since humidity can't be determined to necessarily matter, what does? What matters is weight loss. The amount necessary is virtually the same, regardless of what species of bird. For optimal hatching of healthy chicks, eggs must lose 15%-16% of the freshly laid weight. Some species perhaps a bit more or a bit less. For instance, ratites but not much different. While not positive, I imagine birds as disparate as eagles and hummingbirds need similar weight losses for healthy hatchlings.
So with that in mind, the best tool would be a gram scale rather than a hygrometer. In fact, I didn't even use a hygrometer for years till I discovered a reliable one. But I chose weighing eggs upon collection, onset of incubation and weekly thereafter. If they aren't losing enough, let it dry out. If they are losing too much, add water. then weigh again after some time.
I'm going to jump in here and declare my belief that RH is not a set number.
First of all, ideal humidity for one egg is not the same for the next egg. Each egg is porous but not necessarily as porous as the next. Not all eggshells are created equal. One factor that can affect porosity is the elevation where the hen that laid the egg lived. The higher the elevation where laid, the less porous the shell. This change is necessary for hatchability because molecules of air move faster at higher elevation. This is not always an issue, I'm just trying to note that there are many reasons, including genetics why not all eggs are created equal.
Let's consider how humidity is addressed in nature. The history of artificial incubation occupies a blink of the eye on that timeline.
What happens if a hen is sitting on eggs in a dry climate, which could be experiencing humidity well below that recommended by many for successful hatching, or for that matter if there are days of thunderstorms with 90% humidity. A hen doesn't use a hygrometer. If dry, they will sit tight longer holding in humidity as moisture transpires from the contents.
Since humidity can't be determined to necessarily matter, what does? What matters is weight loss. The amount necessary is virtually the same, regardless of what species of bird. For optimal hatching of healthy chicks, eggs must lose 15%-16% of the freshly laid weight. Some species perhaps a bit more or a bit less. For instance, ratites but not much different. While not positive, I imagine birds as disparate as eagles and hummingbirds need similar weight losses for healthy hatchlings.
So with that in mind, the best tool would be a gram scale rather than a hygrometer. In fact, I didn't even use a hygrometer for years till I discovered a reliable one. But I chose weighing eggs upon collection, onset of incubation and weekly thereafter. If they aren't losing enough, let it dry out. If they are losing too much, add water. then weigh again after some time.
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