Retrotec DM32 - Manual (Page 76)

Page 76 of 94
©Retrotec Inc. 2014
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, [Channel B] 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 on gauge
RP is the room pressure from Channel A displayed on gauge
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. [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].
Any 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:
𝐹𝑙𝑜𝑤 𝑇𝑜𝑤𝑎𝑟𝑑 𝑂𝑝𝑒𝑟𝑎𝑡𝑜𝑟 𝐶𝐹𝑀 = ((𝐹𝑃 − 𝑅𝑃) − (𝑅𝑃 × 𝐾1))
𝑁
× (𝐾 + (𝐹𝑃 − 𝑅𝑃) × 𝐾3) × 𝐾4
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.
𝐹𝑙𝑜𝑤 𝑆𝑒𝑙𝑓 𝑅𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒𝑑 𝐶𝐹𝑀 = (𝐹𝑃 − 𝑅𝑃 × 𝐾1) 𝑁 × (𝐾 + 𝐹𝑃 × 𝐾3) × 𝐾4
Where:
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: