Retrotec Blower Door - Manual (Page 11)

Page 11 of 87
©Retrotec Inc. 2015
gauge green port which will be picked up and included as part of the measured pressure difference.
The measured Fan Pressure, shown when Pressure is the Result being displayed on the gauge, will
thus include the actual Fan Pressure as well as the additional pressure in the room where the
operator is standing. So when the flow is towards the operator, this room pressure must be
removed from the measurement to get the true Fan Pressure, before calculating flow.
A self-referencing fan avoids the need for adjusting Fan Pressure measurements by measuring the
pressure difference at the fan inlet directly with both the green and yellow tubes connected to
either side of the fan inlet on Retrotec fans. The Fan Pressure difference is thus always correct
when measured between the yellow and green port at the gauge.
If a self-referencing fan is in use (both yellow and green tubes are connected between the fan and
the gauge), it is imperative to choose 3000SR for the Device on the gauge (or the DucTester), so the
gauge does not compensate for the room pressure in any situation.
The gauge can determine when flow is toward the operator because the readings on Channel A will
always be positive (assuming the gauge is set up with the blue port measuring the pressure of the
room in which the operator is standing, and the red tube runs to the opposite side). Thus, if a
device that is not self-referencing is chosen on the gauge, (any device except the DucTester and
those denoted SR), and Channel A readings are positive, the gauge compensates: the measured
pressure from the fan, “PrB”, is reduced by the room pressure being measured on Channel A, “PrA”.
The gauge then uses the adjusted value, the actual Fan Pressure, to calculate the airflow displayed
as Mode “Flow” or any other Mode result except “PrB”. When the Mode is set to “PrB”, the gauge
always shows the actual pressure difference measured on Channel B, which includes both the fan
pressure and the additional pressure in the room where the operator is standing, when flow is
towards the operator.
Other manufacturers’ digital gauges need to have the pressure signal from the fan corrected to the
actual Fan Pressure before the Fan Pressure value is used to calculate air flow. When flow is
towards the operator, the procedure is to subtract the Room Pressure from the Fan Pressure to
determine the actual Fan Pressure.
1.2 Range configurations restrict the fan’s air flow
A Door Fan measures flow by measuring the Fan Pressure (usually on Channel B of the gauge). As
the fan blades spin, a suction pressure develops in the fan that causes air to flow. By measuring this
suction pressure (Fan Pressure), airflow can be calculated.
When there is not much air flow through the fan, the Fan Pressure becomes too small to accurately
measure. To bring the Fan Pressure up into a more accurate range, while not changing the volume
of air being moved, a restriction is placed in front of the fan. Because of the restricted area through
which to move air, the fan has to spin faster to move the same volume of air. The suction or Fan
Pressure thus increases to where it can be accurately measured again.
The calibrated fans are provided with a set of Ranges which are metal rings and plates with holes in
them. Each inlet size has a pre-established range of air flows that it will allow the calibrated fan to
accurately measure. By providing a set of flow restricting plates with ever smaller holes (Ranges),
the Retrotec Fans can accurately measure flow from 5 CFM to 8300 CFM (2 to 3917 litres per second
or 8 to 14,100 cubic meters per hour).
Range Configurations are somewhat analogous to gears in a standard transmission. The lower the
air flow, the smaller the hole required to maintain a readable Fan Pressure.