There is a lot of confusion surrounding this issue. In the pdf manual for the 417 (pg 26, section 5.4), to obtain the cfm measurement, we are told to enter the 'free space' of the grill area (see image below). I'm assuming they mean the AK, which is what grill manufacturers provide. The Testo rep I talked to says they are the same thing. Grill manufacturers disagree, and state that AK is defined by 'effective area' and can only be defined in their labs. That the number will be different from the 'free area'.

I would assume that the grill manufacturers know what they're talking about. However, it really becomes moot when the data they provide is generic across all of a given line of grills. For instance, Hart & Cooley's C Series grills all use the same engineering data file with all the same charts. So, the curved blade C Series engineering data states the grills with no damper, multi-shutter and OBD damper uses the exact same same AK, which, of course, isn't possible.

So, even if one could identify which grill was which in the field, even if one could carry and consult all of the tables for all of the different grills made by all manufacturers (quite the task in itself...), true accuracy could not be obtained because the data for the grills isn't accurate. Perhaps the tables for one segment of a given line is accurate, but then all of the other grills in that line could not be accurate simply because the same data is used.

Can generic data be used to obtain an acceptable degree of accuracy (see 2nd image below)? Is close enough good enough?

Or, can a DIY hood, if properly made, produce accurate results, as in this thread:

http://hvac-talk.com/vbb/showthread....63#post1417563

Just how does one obtain the best results from this instrument?

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