http://media.msanet.com/NA/USA/APR/PoweredAirPurifyingRespiratorsPAPRs/OptimAir6APAPRwithOptiFilterCartridges/490883PDF.html Above link is MSA instruction manual for respirator that you are talking about and on the front page it says " The warranties made by MSA with respect to the product are voided if
the product is not used and serviced according to the instructions in this
manual. For any additional information relative to use or repair, write or
call 1-800-MSA-2222 during regular working hours."
and on page 15
PARTICULATE FILTER AND CHEMICAL CARTRIDGE
AND BATTERY PACK SERVICE TIMES
The PAPR high efficiency filter cartridges and the battery
pack are designed to supply a minimum of six cubic feet
per minute (cfm) of respirable air to the facepiece (or hood)
for a shift. Actual service time may vary. However, air flow
must not drop below 4 cfm for a facepiece or 6 cfm for
hoods. The type and concentration of the contaminant, as
well as the relative humidity of the air, may affect how long
filters may be used. See the Accessories section for available
flow check devices.
It doesn't actually tell you to check the flow, but does tell you that they will not warrant the assigned protection factors if flow falls below 4cfm for masks. (how would you know if you don't check)
NIOSH is the orginization that sets protection factors for respiratory protection and the reg for that states that the equipment must be used in accordance with manufacturers instructions.....
I hope that was helpful.
Mike