Windshirt Question

Richard wrote in a different thread: "I have not tested a Montbell Tachyon but, I have tested the SAME APPARENT Montbell 7 denier ballistic rip stop used in their EX Light. It tested 9.72 CFM."

Well, there ya go. Although, I have my doubts that Montbell would develop a completely different 7d nylon fabric with the same lackluster DWR solely for their wind shirts. Never know I guess.

Ryan

9.72 cfm for the MontBell fabric seems rather unlikely.

The "60" in the Norrona Bitihorn Aero 60 Jacket designation refers to an air permeability of 60 Mbar/l/m²/s. I have no idea what the conversion to cfm is.

Norrona makes two Aero 100 jackets which they rate at 100 Mbar/l/m²/s air permeability. Unfortunately they lack hoods.

Richard Nisley replied, "Divide their cm3/s/cm2 value by 0.508 for the CFM conversion."

Pertex lists the permeability of the Quantum GL and Microlight fabrics at 1.0cc, so that is equivalent to 1.968 or basically 2CFM. That is what, 6% of the 35CFM of the older Houdini fabric? That's terrible!

Richard, thanks once again for your patient tutelage here. Your input has really been an education in clothing technology and much appreciated. You really should write a book.

Paul,

The specification for the Norrona Bithorn Aero 60 jackets at 60 mbar/l/m2/s equates to 7.5 CFM. The specifications for the two Aero 100 jackets at 100 Mbar/l/m²/s equates to 12.5 CFM.

Richard,

I am confused.

Although Norrona does not specify the parameters for their air permeability tests, I suppose they have been testing their jackets according to DIN EN ISO 9237. Hence the pressure difference for testing their apparel should be 100 Pa (compared to 200 Pa for technical fabrics and filter).

So my calculations for the Norrona Bitihorn Aero 60 are:
@ 100 Pa (DIN EN ISO 9237): 60 l/m²s / 5.08 = 60 l/m²s x .197 = 11.81 ft³/ft²min
@ 125 Pa (ASTM D737): 11.81 ft³/ft²min x 1.25 = 14.76 ft³/ft²min

The same calculation yields 24.6 CFM for the Bitihorn Aero 100, which fairly matches my experience with it (soon after the Aero 60 (medium) I also bought the Aero 100 (large) for running, though the Aero 60 is my go-to windshirt as it's generally more to my liking when moving at "normal" speed).

BTW, the Aero 100 fabric seems VERY similar to the pre-2013 Houdini nylon …

Cheers,
Steffen

Well, the question is: do they test according to DIN EN ISO 9237 ? Because if they test according to DIN 53887, Richard is wright.

Woubeir,

Not sure …

It seems to me as if Richard has calculated both CFM values with a conversion factor (x .625) for a DIN EN ISO 9237 test environment @ 200 Pa (the specification for technical fabrics). The factor accounting for a test protocol @ 100 Pa pressure difference (apparel) would be quite different (x 1.25). FWIW, DIN 53887 (predecessor of DIN EN ISO 9237) has been spec'd @ 200 Pa for all kinds of fabrics.

Maybe I just messed up. Hopefully Richard can chime in.

Cheers,
Steffen

For the conversion from DIN EN ISO 9237 to ASTM D 737, I found /4.

"9.72 cfm for the MontBell fabric seems rather unlikely."

Not really. That's a pretty poor number which matches most people's reports.

Ryan

I sent an email to Norrona Customer Service this morning. I will post their response when I get a reply.