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©Retrotec Inc. 2012
Appendix A: The gauge calculates Flow based on Pressure
readings from the fan
The air flow being produced by the calibrated fan is a value that can be calculated based on the
pressure developed by the air moving across the inlet side of the fan towards the exhaust. Because the
fan is calibrated, there are known values describing the mathematical relationship between the fan
pressure, which is measured across the fan inlet, and the resulting air flow through the fan.
The Fan Pressure needed to calculate the fan flow is the difference between the pressure at the pickup
and the pressure in the vicinity of the inlet side of the fan. The fan pressure pickup is located inside the
fan near the inlet side of the fan. Self-referencing fans have the reference port built in, and it is placed
to measure the ambient pressure in the vicinity of the inlet side of the fan. Fan Pressure,
FP,
is
determined by setting the gauge Mode to “PrB” and reading the resulting pressure on the gauge when
the room is at the test pressure.
Each fan and range combination has a different flow equation. For each fan and range combination
available, the variables needed to calculate flow are listed in the table below.
The form of the flow equation depends on where the pickup for the Fan Pressure connected to the
yellow “Ref B” port on the gauge is located with respect to the green “Input B” port of the gauge.
Case 1: Flow Away from Operator
If the flow of the fan is away from the operator holding the gauge, then the gauge and the operator are
on the inlet side of the fan. Readings on Channel A in this case will always be negative, measuring the
induced pressure in the room.
Since both the fan pressure pickup and the reference for the fan pressure (yellow port on the fan and
green port on the gauge) are on the inlet side of the fan, “PrB”, will show the correctly referenced fan
pressure and can be used directly as the fan pressure,
FP
, value in the following flow equation:
𝐹𝑙𝑜𝑤 𝐴𝑤𝑎𝑦 𝑓𝑟𝑜𝑚 𝑂𝑝𝑒𝑟𝑎𝑡𝑜𝑟 𝐶𝐹𝑀 = (𝐹𝑃 − |𝑅𝑃| × 𝐾1)
𝑁
× (𝐾 + 𝐹𝑃 × 𝐾3) × 𝐾4
Where:
FP
is the fan pressure from Channel B displayed as “PrB”
RP
is the room pressure from Channel A displayed as “PrA”
Case 2: Flow Toward Operator
If the flow of the fan is toward the operator holding the gauge, then the gauge is on the exhaust side of
the fan while the fan pressure pickup is on the opposite (inlet) side. The exhaust side is the side on
which the room pressure is being induced by the fan. Readings on Channel A in this case will always be
positive, measuring the induced pressure in the room.
Because the gauge which has the green “Input B” port is in the room with the induced pressure, the
pressure difference measured on Channel B will include the induced room pressure as well as the
actual Fan Pressure across the inlet. “PrB” will thus show a pressure difference that is larger than the
actual fan pressure, larger by an amount equal to the induced room pressure being measured on
Channel A.
