The main disadvantage to positive duct test pressure is that it blows off register seals or makes them leak. An LBL study showed a wide dispersion of results when the same duct test was repeated many times.  Depressurizing will allow the use of lower sticking masking materials that are less likely to pull off the paint. In some cases plastic food wrap will work.

The main advantage to positive duct test pressure is that the flex duct doesn't contract as much. In cases where the flex runs to the ceiling this contraction effect might cause problems.

Testing using depressurization will usually result in less leakage because gaps and seams will be pulled closed”

What is an extrapolated result? (e.g. Q4)

Why is an extrapolated result different in FanTestic than on a DM32?

Q4 is a Flow result that is extrapolated to 4 Pa. For extrapolation of what the result would be at 4 Pa, a best-fit line on a graph (with a known slope) is needed.

The slope value is different for different enclosures, since the slope is dependent on enclosure leakage. To generate a line on a graph, you will either need to:

1. Measure multiple points of data to compute a slope ('n' value), or you
2. Guess a slope ('n' value)

FanTestic performs #1, whereas your DM32 performs #2. 

In other words, FanTestic software records multiple measurements to create a graph

One question we get a lot is about leaking flex duct that connects the ducTester to the return. We have tested the most ripped up flex available in our warehouse; it had way bigger pinholes and it measured 0.68 CFM at 25 Pa. When you measure a typical leakage of 100 CFM at 25 Pa or more, this is not even 1%.  So we believe it is not worth spending money on replacing.

I am doing duct testing and use the DW/143 standard (UK).  FanTestic keeps telling me the test is not compliant, what am I doing wrong?

FanTestic checks that you have held the ducts at the test pressure for at least 900 seconds.  If you do not, the test will show as not compliant.  If you did a manual test and are entering the information into FanTestic, be sure to enter the time over which the test pressure was held.  If this is greater than 900 and your other test conditions are correct, the test will show compliant. 

Compliance means the test conditions comply with the stated requirements for a test  in DW/143.  Pass or fail means that the measured results are within the required level.