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© Retrotec 2022
“MN DB”
“MN 3/4”
“Exhaust” (Fan Flow Meter)
Images of devices compatible with the gauge
2.6.4. Estimate Results with [@ Pressure]
The [@] pressure key enables the gauge to calculate and display the estimated [Channel B] Result that
would be achieved, as if the target pressure was reached (also known as “extrapolation”). The [@
Pressure] key only appears on the Home screen if [Set Speed] or [Set Pressure] functions are used and the
gauge is measuring pressures on Channel A and B.
• This function is useful in situations when the fan has reached 100% speed and the enclosure
pressure still has not reached the target pressure.
• Additionally, this function is useful since the measured [Channel A] pressure will usually be slightly
over or under the desired/set pressure, making it difficult to report results that exactly at the
specified pressure.
Tapping [@ Pressure] will toggle the extrapolation function on or off. The label under [Channel B] display
will change to indicate whether [@ Pressure] “is On” (e.g., CFM@25.0Pa) or “is Off” (e.g., CFM).
Details of how the gauge calculates the flow at exactly the target pressure, when [Channel A] has not
reached exactly the set target pressure, can be found in Appendix D
.
• If the [Set Speed] function is used, [@ Pressure] extrapolates the [Channel B] Result to the [Default
@ Pressure] set in the “Settings” screen, using the “n value” chosen in [Settings].
• If the [Set Pressure] function is used, [@ Pressure] extrapolates the [Channel B] Result to the
pressure entered for [Set Pressure], using the “n value” chosen in [Settings].
The @ Pressure function provides unique advantages over non-extrapolated results:
• There is no need to achieve an exact [Channel A] pressure (e.g. to a precision of 0.1 Pa)
• [Channel B] Results can be obtained even if the test pressure cannot be reached**
**Although the gauge can extrapolate results to any pressure, it is more accurate when the actual
enclosure pressure read on [Channel A] is close to the target pressure. As a rule of thumb, extrapolation
can be used if the actual enclosure pressure reaches at least 80% of the target pressure. If 80% cannot be
reached, more fans are needed to complete the test (See Retrotec’s Manual for Large Building Multi Family
Testing).
Example #1 - Inaccuracy
A 50 Pa building pressure is desired, but only 20 Pa can be reached (due to severe leakiness). In this
instance, the extrapolated results for flow “@ 50 Pa” do not represent the reality of the testing conditions
and might be highly inaccurate.
In the same situation, a 45 Pa building pressure is achieved. The extrapolated “@ 50 Pa” pressure is now
much more accurate, and provides an acceptable result.
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