Retrotec DucTester 341 User Manual
Page 68
Page 68 of 83
©Retrotec Inc. 2014
Appendix A: Calculate Air Flow based on Channel B Fan
Pressure readings
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 such as the DU300 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. It is
important to set the Device on the gauge to DU300 because that tells the gauge not to adjust the “PrB”
value before calculating flow. Such an adjustment is required in the case of flow toward the operator
when the fan is not self-referenced.
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. N and K values depend on
the type of fan and the Range Plate/Ring being used.
Since both the fan pressure pickup and the reference for the fan pressure (yellow and green ports on
the fan connected to the gauge) are on the inlet side of the fan, “PrB”, will always show the correctly
referenced fan pressure and can be used directly as the fan pressure, FP, value in the flow equation.
To determine the fan flow for a particular Fan Pressure, insert the values measured for FP and DP, and
the N and K values from the table into the following equation:
Flow CFM = (FP − DP × K1)
N
× (K + K3 × FP)
Where:
FP
is the fan pressure from Channel B displayed as “PrB”
DP
is the duct pressure from Channel A displayed as “PrA”
There are two conditions for
FP
that must be met before the calculated flow can be considered valid.
The absolute value of
FP
must be greater than the minimum fan pressure,
MF
, from the table and
greater than a factor calculated from the duct pressure,
DP
, and
K2
from the table:
So before calculating flow ensure that: |FP| > MF And: |FP| ≥ |DP| × K2
Table 8: N and K coefficients for all supported fans in the gauge.
Range
n
K
K1
K2
K3
MF
Model
300
Open
0.5150
27.9000
0
0.04
0
10
102
0.5100
17.0000
0.001
0.072
-0.0002
18
Range 102 is not available for Model 341
74
0.5000
7.0500
0
0.16
0.0001
40
47
0.5000
2.9400
-0.010
0.1
0.0001
25
29
0.5000
1.1920
0
0.036
0
9
18
0.5000
0.4420
-0.010
0.036
0
35
These Ranges are optional for Model 341
11
0.5300
0.1464
-0.070
0.18
0
45
7
0.4450
0.0810
0
0.10
0.00002
25
3
0.4450
0.01812
0
0.10
0.000007
25
2
0.4450
0.00417
0
0.10
0.000003
25
1
0.4450
0.00104
0
0.10
0.000001
25