The blower door test is a method for checking how airtight a building is. For this purpose a fan is installed airtight in the front door, and a pressure differential is created and increased step by step up to 50 pascal (equivalent to a pressure of 5 mm water column).
The values measured are listed and entered into a coordinate system (flowrate/pressure differential). The intersection point at 50 pascal is read off for negative and positive pressure. Usually these values are close to each other, unless there is a check-valve effect from a leak or wind effects are too high. The mean is the measured ACP50 value, i.e. the rate of air change at a pressure differential of 50 pascal.