Testing CFM; S&P TD100 and TD100X

I came up with a cheap and simple way to test the throughput of an inline fan, so I could see how my carbon filter and ductwork affected it. It's definitely not too precise, but way better than nothing. And the results were surprising.

The basic idea: Take a large garbage bag, tape up the mouth around a small cylinder of cardboard (so you can fit it over the end of the fan), and time how long it takes to fill.

For example, I have a 55 gallon trash bag. That's 7.35 cubic feet (found that online). I took an S&P TD100 (4 inch) inline fan, which is rated at 100 CFM, and timed how long it took to fill the bag. (This is a very imprecise measurement -- when is the bag "full"?) I did this 4 times and averaged the results: 4.85 seconds. The math is then 7.35 / 4.85 (to get cubic feet per second) x 60 (to convert to cubic feet per minute). This is:

7.35 x 60 / 4.85 = 91 CFM

So I measured the fan at 91 CFM. It's rated at 100 CFM, so I am at least in the right ballpark.

I'll skip the calculations for the rest of my measurements and just give the results.

Exhausting through about 8 feet of 4 inch insulated duct, with a 90 degree bend: 65 CFM.

I have a homemade carbon filter. The carbon is packed into the wide end of a 6" to 4" reducer. My first attempt had 2" of carbon, which is about 2 lbs. (So a cylinder of carbon with a diameter of 6 inches, 2 inches deep.) It has a prefilter made of a thin layer of polyester quilt batting. With the filter (but not the ductwork): 21 CFM.

Wow. So I cut the carbon in half -- same as before, but with 1 inch of carbon (about 1 lb): 26 CFM.

I never did check with both the ductwork and the filter -- it seemed clear it wasn't going to be enough (I have a 56 cubic foot tent and am looking for an air change every 3 minutes, which is about 19 CFM).

So I returned the TD100 and bought a TD100X, which is rated at 135 CFM. I put it into use right away, so never did get a measurement of the fan on its own. But here are the measurements I have:

With the 1 inch carbon filter described above and an S&P backdraft damper: 24 CFM

...as above with about 90 inches of 6 inch insulated ductwork on the exhaust side, with a 90 degree bend: 22 CFM

...as above with about 80 inches of 4 inch insulated ductwork between the carbin filter and the fan, with a 90 degree bend: 22 CFM

...and lastly, as above but without the backdraft damper: 30 CFM.

I think the S&P fans are known to be "weak" -- dropping off fast with increases of static pressure, and a true centrifugal fan would probably behave very differently.

Anyway, figured I'd throw this out there in case it helps anyone else.

This may work for very small fans and blowers, but not for those of us with equipment rated near or above 1000cfm.

LOL my blower would pop that bag like bubble wrap

Very cool idea in action though

Yeah -- good point. I guess it's pretty much useless for anything bigger than a 4 inch fan -- most of them are rated at 190 CFM, which in theory would fill the bag in 2.3 seconds. That's best case, though -- once you start adding filters and ductwork to it, it slows it down some, so it starts to get more reasonably measurable.

I don't know if they make bigger trash bags, but I suppose you could try putting two or three of them together with duct tape.

(They make "flow meters" -- the professional answer -- but they're pricey.)