Ventilation of incubators is necessary to provide the embryos with oxygen and to remove carbon dioxide and moisture. Air flows through incubators as a result of the difference in pressure between inlet and outlet. Without a difference in pressure the air would not enter and leave the machine, and air exchange would not happen.
This pressure difference can be created by the ventilator in the machine, by the difference in pressure on both sides of the machine or by a combination of these two factors, but for simplicity reasons lets consider the pressure on both sides of the machine (inlet and outlet) to be the only factor involved.
We usually measure and express the pressure in Pascals (Pa), although other units are used as well. Often we consider the pressure in relation to the pressure of the outside air, so – 5 Pa means that the pressure in for instance the plenum is 5 Pascal lower then outside air.
This is a good practice as it makes the situation independent of the outside pressure, but it bears a risk of misinterpretation as well. Usually the pressure in the setter room is positive compared to outside (a ventilator or an air handling unit is pushing air into the setter room) and in the plenum or the exhaust negative (a ventilator is pulling air from the plenum to outside). This of course creates a difference in pressure and air will flow through the machine. But the question is, what is actually the pressure difference we are dealing with?
If the pressure difference in front of the machine is +25 Pa and the pressure at the exhaust is – 15 Pa compared to outside, it means that the air is pushed into the inlet and the machine with 25 Pa and also pushed out of the exhaust into the plenum with 25 Pa. If we then want to create a negative pressure in the plenum we first have to compensate for the +25 Pa, and then add another 15 Pa difference to get to a pressure of – 15 Pa. This means that the total pressure difference over the machine (so between inlet and exhaust) is 25 + 15 = 40 Pa. But as compared to outside it looks as a positive and a negative value that can compensate each other, it is sometimes considered as a pressure difference as 25 – 15 = 10 Pa. This can create problems as due to this difference we might ventilate much more then we expect.
Also the opposite can happen, if the pressure in front of the machine is + 25 Pa and at the back is + 15 Pa, the difference is not 25 + 15 = 40 Pa but 25 – 15 = 10 Pa.
The problem often occurs when in a sheet the pressure differences are just listed as numbers. If we see +25/-15 listed, our calculating brain more or less automatically interpret it as +25 – 15 = 10 Pa, where the actual pressure difference is 40 Pa. As we have to consider the pressure difference between inlet and outlet and not between inlet/outlet compared to outside, 1 + 1 is not always 2.