Sure-Aire™ Flow Monitoring System
15
®
Greenheck’s Sure-Aire™ differential pressure electronics provides either a 2-10 VDC or 4-20 mA analog output
signal. The output signal can be configured linearly proportional to either the pressure range or the flow within the
setup. The ranges for Greenheck’s Sure-Aire™ electronics are listed by model on cover.
Calculation Using Manual Pressure Gauge or Third Party Transducer
Calculating Flow from Differential Pressure
The volumetric flow through the fan (cfm) can be calculated from the equation:
where K is the K-Factor for the specific fan model and size,
Δ
P is the measured differential pressure across the inlet
cone (in. wg), and
ρ
is the density of air (lb/ft³
°
F). K-Factors for Greenheck models are found on the back cover.
CFM = K
ΔP
ρ
Calculating Flow from Voltage Signal
If using the analog signal linear to flow, the max flow is automatically calculated based on the pressure range and
K-factor input into the electronics. The max value is displayed in the setup menu.
If output signal is linear to pressure, the corresponding equations are used to calculate the flow.
Calculating flow for 2-10 VDC output signal:
Calculating flow for 4-20 mA output signal:
CFM = K
(V - 2) P
max
8
ρ
CFM = K
(mA - 4) P
max
16
ρ
Analog Output Signal - Optional
K
Constant dependent on fan model and size (see page 9)
Pmax
Maximum pressure of electronics (in. wg)
ρ
Air density (lb/ft³ °F) [0.075 at 70°F and 0 ft elevation]
V
Output voltage of 2-10 VDC signal
mA
Output current of 4-20 mA signal
Density Corrections
Air density,
ρ
, is affected by elevation and temperature. The Sure-Aire differential pressure electronics allows the user
to input the elevation for the application. This elevation input automatically updates the density used for the flow
calculation.
The Remote Temperature sensor will adjust the air density value in the electronics based on the sensor measurement
when Temperature Compensation is set to “Yes”. This density compensation will affect the flow rate displayed on
the transmitter. If Temperature Compensation is set to “No”, the air density value will be a function of standard air
(70
°
F/
21
°
C
).
The density being used by the Sure-Aire differential pressure measurement can be viewed on the main menu by
scrolling up or down through the settings.
