Retrotec Blower Door - Manual (Page 10)

Page 10 of 87
©Retrotec Inc. 2015
channel of the gauge is measuring the pressure difference between the area in which the gauge is
located (on the blue port of the gauge) and the other side of the doorway, since the red tube is run
through a small hole in the cloth to the other side of the doorway and acts as a reference for the
measurement.
In its role as a fan speed controller, the gauge is connected to the fan through an electrical connector,
(yellow Speed Control Cable), and changes the speed of the fan until the pressure difference across the
doorway reaches the desired test condition. The Speed Control Cable can extend approximately 1,200
meters (4,000 feet) between the gauge and the fan.
Knowing the pressure difference tells the operator when the enclosure has reached the desired
condition (50 Pa for instance). Knowing the Fan Pressure and the fan calibration allows the operator, or
the gauge itself, to calculate the air flow (CFM for instance).
1.1 How the Calibrated Fan creates flow and Fan
Pressure
When the fan is turned on, air starts moving through the fan housing. In order for the air to be
pulled through the holes on the inlet side, there must be suction. The rotating fan blade creates a
suction pressure (which is also called Fan Pressure) between the inlet opening and the fan blade.
Range Rings and Plates are typically installed on the inlet side of the fan housing to artificially
restrict flow, control the amount of air going through the fan, and thus control the Fan Pressure.
Figure 2: Pictorial display of how a calibrated Door Fan works.
By measuring the Fan Pressure developed across the fan housing, and knowing the size of the
hole(s) in the installed Range, we can calculate the volume of air moving through the fan.
When flow is away from the operator, the Fan Pressure signal will always be correct and will not
need adjusting. The gauge can determine if flow is away from the operator because pressure
When the flow is towards the operator, the Fan Pressure reading is affected by the pressure in the
area into which the air is flowing. The area which is being pressurized will exert pressure at the
Fan Pressure
Airflow