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© Retrotec 2022
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. [Channel B] 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].
A
ny time the gauge sees a positive pressure on [Channel A], it automatically subtracts the value on
[Channel A] from the value on [Channel B] before calculating flow and other results. The value displayed
for “B” is left as the raw measured value and does not have the [Channel A] value already subtracted. If
using a raw measured value read from [Channel B] on a Retrotec gauge, use the following equation to
adjust the Fan Pressure and calculate the flow:
=
(
)
(
× 1)
× ( + ( ) × 3) × 4
Where:
FP is the fan pressure from Channel B displayed on gauge
RP is the room pressure from Channel A displayed on gauge
Case 3: Self Referencing Fan
Self-referencing fans can be identified because they have ports for two pressure measuring tubes on the
fan itself. If a self-referencing fan is the chosen Device then the fan includes a green port as well as the
yellow port and the Fan Pressure signal sent to the gauge is always the correct value and never needs
adjusting. Setting the Device on the gauge to 3000SR or DU200 tells the gauge not to subtract the room
pressure from the pressure value from Channel B before calculating flow.
=
(
× 1
)
× ( + × 3) × 4
W
here:
FP is the fan pressure from Channel B displayed on gauge
RP is the room pressure from Channel A displayed on gauge
In all cases:
The absolute value of FP must be greater than the minimum fan pressure, “MF”, shown in Table 8
,
and
greater than the calculated factor using the K2 value from the table as follows:
|| |
| × 2
To determine the flow for a particular fan pressure FP:
Check that the measured value for FP meets the two conditions based on MF and K2. If so, find the N, K,
K1, and K3 values in Table xxx (based on the type of fan and the Range Configuration being used), enter
those values along with the values measured for FP, and RP into the Flow equation stated above and
calculate the result.
Table 4: N and K coefficients for all gauge supported fans.
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