I got a callback from Vortex and found myself speaking with…an engineer! OMG! It’s so rare to get to speak with a technically knowledgeable person when phoning manufacturers.

Here’s the text from an email he sent, showing how he checked whether Canfilters uses the same 0.1-s contact time criterion that Vortex does:

**CAN 150**

**OD** : 395mm

**ID** : 265mm

**Bed Tickness** : 65mm

**Max rated CFM** : 1260 CFM

**Volume of carbon bed** : 101 080 CM3

**Air volume in carbon bed** = 101 080 CM3 X 0.57L = 57 615 CM3

1 CFM = 28 316 CM3/M

**Max Rated CFM** : 1260 CFM X 28 316 = 35 678 160 CM3/M

35 678 160 CM3/M / 60 = 594 636 CM3/sec

**Air volume in carbon bed** (57 615 CM3) / Max rating CM3/sec (594 636 CM/sec) = 0.09689 sec

**Contact time** : 0.096 Sec

Contact time at 0.1 = 1220CFM

First, he estimated the volume of air in the carbon layer as 57% of the volume difference between the two cylinders. Second, he calculated the contact time by dividing the air volume by Canfilter’s CFM rating. It fell a bit short of 0.1 sec, so he recalculated the CFM by dividing 0.1 sec by the air volume, getting a rating of 1220 CFM.

There’s a bit of converting between British and SI units, and I don’t know yet what the “L” in “0.57L” denotes, but the math is straightforward.

I’ll ask where the 57% approximation comes from. A tiny difference there might explain why Canfilter’s CFM rating is higher.

The exciting thing for me is we now have the tool needed to calculate a CFM rating for any cylindrical carbon filter, given its dimensions, in a consistent way. I can add it to my spreadsheet easily and we’ll see how the results compare with manufacturers’ ratings.

Possibly, once the CFMs are all calculated the same way, my CFM vs. Cost graph in post #1 will have less variance.

I believe this new info gets us a step closer to being able to calculate a filter’s effect on a fan’s maximum CFM.