Retrotec Residential Pressure & Air Leakage - Manual (Page 64)

Page 64 of 75
©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.
Instead, 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 enclosure n is different, a small error, as shown in
Table 21 will result if the
[@ Pressure] function is used at the Achieved Pressure to estimate a result at 50 Pa.
Table 21: Errors in Estimated Flow at 50 Pa if gauge "n"= 0.65 differs from actual n
Actual Flow Exponent (n) of Enclosure
0.50 0.55 0.60 0.65 0.70 0.75
Achieved
Pressure (Pa)
10 -21.4% -14.9% -7.7% 0.0% 8.4% 17.5%
15 -16.5% -11.3% -5.8% 0.0% 6.2% 12.8%
20 -12.8% -8.8% -4.5% 0.0% 4.7% 9.6%
25 -9.9% -6.7% -3.4% 0.0% 3.5% 7.2%
30 -7.4% -5.0% -2.5% 0.0% 2.6% 5.2%
35 -5.2% -3.5% -1.8% 0.0% 1.8% 3.6%
40 -3.3% -2.2% -1.1% 0.0% 1.1% 2.3%
45 -1.6% -1.0% -0.5% 0.0% 0.5% 1.1%
50 0.0% 0.0% 0.0% 0.0% 0.0% 0.0%
55 1.4% 1.0% 0.5% 0.0% -0.5% -0.9%
60 2.8% 1.8% 0.9% 0.0% -0.9% -1.8%
65 4.0% 2.7% 1.3% 0.0% -1.3% -2.6%
70 5.2% 3.4% 1.7% 0.0% -1.7% -3.3%