Retrotec DM-2 - Manual (Page 22)

Page 22 of 72
©Retrotec Inc. 2012
3. Press [Enter]. The version of hardware and firmware as well as the date of last calibration is
displayed.
4. Press [Exit] to return to the Setup Menu.
3.1.6 n setting for estimating flow @ pressure during house and duct leakage
test
The “n” value is displayed on the top row of the gauge. Set it to 0.65 for houses, and 0.60 for
ductwork. Set to 0.5 for tests on the simulator, air handler flow and any large hole that is not
composed of long thin cracks.
A wide open hole has an n of 0.5, meaning that when the pressure is quadrupled, the flow doubles.
That is due to completely turbulent flow going through that hole (flow = square root of pressure, a
constant for that particular hole).
𝑓𝑙𝑜𝑤 = 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑛
𝑓𝑙𝑜𝑤 = 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 0.5
An n value of 1.0 represents tiny little holes, so small that the air would not be turbulent but rather
would go through the holes as laminar flow. This means that when pressure is quadrupled, the flow
will also be quadrupled.
𝑓𝑙𝑜𝑤 = 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 1
𝑓𝑙𝑜𝑤 = 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒
4 ∗ 𝑓𝑙𝑜𝑤 = 4 ∗ 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒
Houses and ducts have many holes that will have both turbulent and laminar flow going through them.
Duct holes tend to be slightly larger, whereas houses have more prevalent long tiny cracks, and
therefore tend to have lower n values.
These n values can be measured simply by doing a multi-point Blower Door or Duct Test. The result
will be an n and a C (coefficient) so flow at any pressure can then be calculated by using the equation:
𝑓𝑙𝑜𝑤 = 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑛 × 𝐶
The gauge uses the “n” value to extrapolate for flows at other pressures. Because the extrapolation
function is a ratio of two flows from the same fan, the C value cancels out.
For example: If we guess at the n value of a duct as being 0.6 and measure 100 CFM at 20 Pa (by
accident or by design), then the DM-2 will complete the following calculation to estimate the flow at 25
Pa:
𝑓𝑙𝑜𝑤@25 𝑃𝑎 = 25 0.6 ×
𝑓𝑙𝑜𝑤@20 𝑃𝑎
20 0.6