Originally Posted by
Eric Arnsdorff
Andrew - David explained it. I only reported the flow rates.
Nothing was changed for the comparison.
I used the anemometer at the end of an open hose (a practice I've seen done on youtube, etc.) with all other flow paths closed off. The inner diameter at the measurement point was used to determine the flow area and calculate the flow rate.
The pitot tube measurement requires proper straight upstream and downstream distances to straighten the flow along with fully developed turbulent flow. I measured this in a 6" duct section that came close to proper straight upstream and downstream lengths. I traversed the duct and took multiple readings to calculate a velocity measurement. The inner duct diameter was used to determine the flow area and calculate the flow rate. One could even argue that the pitot measurement had extra flow from the leakage of the closed dust gates and such.
Bill I fundamentally agree with what you said and I can tell a significant difference functionally in my old and new setups. The improved function is really what matters. The thing is that my personal bias comes into play without an accurate measurement. Doing all the work would make me have an emotional response to think it was better. Having an accurate measurement gave a definite quantitative assessment rather than my biased qualitative judgment. It was extra work and I still haven't spent the time to assemble the information which may be of use to others interested in doing something similar. It also helps me understand some of the choices in this space and may help others as they put together their systems.
Additionally, I'm not sure how one can ever trust an inaccurate measurement. If you compare 2 measurements which have the same bias (such as measuring with the same tape measure) then the comparison is valid and this fact is often used appropriately. But in this case the anemometer doesn't create a consistent bias even if it is positioned identically. There is some level of error cancellation in the comparison but still it just puts question on an already highly questionable measurement. It not only leaves doubt but has certainty that it is incorrect.
Additionally, it may be of use to others and having a reasonably accurate measurement may help others understand how to do something similar.
All the above has already been done and the information published on the Australian Woodworking forum, search for "Ubeaut Forums". The person who did it is a physics professor who specialised in clean rooms development and the surrounding air flow needs of clean rooms from what I understand. Ducting systems for WW's have been studied and discussed there for many years with all the information gained being available on the above forum. It is most probably the single best repository of information in the world but I am not allowed to put any links here.
Chris
Everything I like is either illegal, immoral or fattening