Davidr said, of the trash bag and stopwatch method of measuring air flow:
>>Personally I view this method to be at the least very inaccurate.
There is "precision" and "repeatability" issues in addition to whether the measurement would agree with a flow hood. Could you speak to whether you consider this repeatable? I think it would be.
>>When placing a trash bag over any diffuser you immediately change the airflow
>>ditribution of the duct system due to an unidentifiable back pressure.
I submit that a garbage bag in the process of being filled, will have such little back pressure compared to other methods, that this is not a significant issue. The bag is absolutely limp to begin with, and should not begin to offer resistance until nearly filled. That's what I speculate anyway.
Let me add that operator error may be a big thing with any measurement method, including expensive flow hoods. I have had 2 separate flow hood measurements done on one of my systems, troubleshooting a reported problem. The results are so far different that I have to believe one or the other made a measurement error. One said 723 CFM (for a 3.5 ton system), the other said 1320. I firmly believe the skill level of my 2 sets of techs, was an issue. I trust that *you* would apply enough expertise with a flow hood, that your readings would be repeatable and reasonably accurate.
Best wishes -- P.Student