Retrotec DucTester 341 - Manual (Page 71)

Page 71 of 83
©Retrotec Inc. 2014
The @ Pressure extrapolation feature is very useful for ensuring that results taken for flow reflect the
flow at the desired test pressure, even when the pressure was not adjusted perfectly and was within 5
to 10 Pa of the desired test pressure.
Extrapolation Error for Flow if gauge “n” doesn’t match actual n
To continue the above example: The flow at 20 Pa is 100 CFM. Assume the actual n is 0.7, but this is
unknown. If the user chooses the 340 device, n-value of 0.6 will be used.
The gauge would calculate:
Flow@25 Pa =
25 n × CFM
20 n
Flow@25 Pa =
25 0.6 × 100
20 0.6
Flow@25 Pa = 114 CFM
However, if actual n was 0.7, the flow at 25 should have been:
Flow@25 Pa =
25 0.7 × 100
20 0.7
Flow@25 Pa = 117 CFM
This value is less than 3% off from what it should be. If the test pressure was within 1 or 2 Pa of the
reference pressure of 25 Pa, the @ Pressure reading would be exact.
If a gauge has the extrapolation or flow exponent “n” set to an assumed value but the actual exponent
(the true exponent value that describes the enclosure) differs, the flow values estimated by the gauge
when extrapolating will be off by a small error amount. The tables below show the percent error that
can result from entering an incorrect exponent value in the gauge and using the
[@ Pressure] function.
If the “n” value on the gauge is set to 0.65 and the actual duct “n” is different, a small error, as shown in
Error! Reference source not found. will result if the
[@ Pressure] function is used at the Achieved
ressure to estimate a result at 50 Pa.
If the “n” value on the gauge is set to 0.60 and the enclosure “n” is different, a small error, as shown in
Table 10 will result if the
[@ Pressure] function is used at the Achieved Pressure to estimate a result at
50 Pa.
Table 10: Errors in Estimated Flow at 50 Pa if gauge "n" = 0.60 differs from actual n
Assumed n = 0.6
Achieved
Pressure
(Pa)
Actual Flow Exponent (n) of Enclosure
0.50 0.55 0.60 0.65 0.70 0.75
10 -14.9% -7.7% 0.0% 8.4% 17.5% 27.3%
15 -11.3% -5.8% 0.0% 6.2% 12.8% 19.8%
20 -8.8% -4.5% 0.0% 4.7% 9.6% 14.7%
25 -6.7% -3.4% 0.0% 3.5% 7.2% 11.0%
30 -5.0% -2.5% 0.0% 2.6% 5.2% 8.0%
35 -3.5% -1.8% 0.0% 1.8% 3.6% 5.5%
40 -2.2% -1.1% 0.0% 1.1% 2.3% 3.4%
45 -1.0% -0.5% 0.0% 0.5% 1.1% 1.6%
50 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%
55 1.0% 0.5% 0.0% -0.5% -0.9% -1.4%