During the incubation process, we need to ventilate the machines to exchange the air. This is needed to supply the eggs with oxygen, to remove carbon dioxide and excessive moisture, and in case of limited cooling capacity to remove the heat.
In modern machines, often the carbon dioxide level in the machine is used as an indicator to adjust the ventilation rate. This seems logical, as the embryo is producing carbon dioxide, which means with an increase in carbon dioxide production the ventilation will increase as well.
By itself this system works well, but we have to realise that there is a fundamental difference between the production of carbon dioxide as a result of metabolic process and the production of water as a result of physical processes. The difference is that only life embryos will produce carbon dioxide, but all eggs will release water.
When we base the ventilation rate of our machines on carbon dioxide, the actual ventilation rate will be dependend on the size of the embryo, so the day of incubation, but also on the level of fertility. If the fertility is low, there will be less embryos producing carbon dioxide, and the ventilation will be more limited. This is no problem for the removal of carbon dioxide and the supply of oxygen, but it can be for the removal of moisture. As all eggs are losing water, a limited ventilation because of a limited number of embyros producing carbon dioxide means that less moisture is removed with ventilation and the relative humdiity will go up.
If the range of relative humidity in the machine is set very tight, than at a given moment the relative humdity will go up too much and the ventilation will not be based anymore on carbon dioxide but will be driven by relative humdity. However, if the priority of the setting is to control the carbon dioxide and the range of relative humidity is set relatively wide, it can be that the ventilation of the machine becomes too limited and the relative humdity inside the machine will continiously be on the high side of the range, resulting in a too limited moisture loss.
The correct approach is to realise that steering the ventilation of the machine on carbon dioxide levels is actually dependend on the level of fertility in that machine. A lower level of fertility means less production of carbon dioxide, which means that the required set point for carbon dioxide must be lowered to match the air exchange for moisture loss.
In older machines this is often not a problem as the air exchange is relatively high to increase the heat exchange of the machines. In modern machines with more cooling capacity the ventilation is often not directed by the heat load in the machines, which can create problems if the settings of the machine for carbon dioxide and relative humdity are not chosen carefully.