How to convert CFAST output OD (1/m) to visibility (m)

[Jason]

With CFAST6 the output parameter related to smoke concentration is OD (1/m), how I can link OD to the visibility (m)?

 

CFAST6 User's Guilde (NIST Special Publication 1041) page 66:

 

""OD" is the obscuration density, which is a measure of the amount of smoke.""Optical depth per meter is a measure of the visibility in the smoke. This is covered in detail in the comment on visibility in the section on fires and species specification."

My understanding is, if there is no smoke, then OD=0 (1/m), visibility should be infinite. If there are more smoke in the air, then OD become greater than 0. My questions is, is there a formula to convert the OD value to visibility in meters? How shall I interpret OD value per meter?

Jason

 

[Richard Peacock]

Most typically, the work of Jin is used to assess visibility. He found there is an approximate relationship between obscuration and visibility distance so that

kV = C

where K is the smoke extinction coefficient (the natural log equivalent of the OD parameter in CFAST), V is the distance at which a person can identify an exit sign, and C is a constant. For a light-emitting sign, a value of 5-10 is typical; for a reflecting sign, 2-4. See the following references:

Jin, R., "Visibility Through Fire Smoke in Main Reports on Production, Movement and Control of Smoke in Buildings," Japanese Association of Fire Science and Engineering, 1974, pp. 100-153.

Jin, T., "Visibility and Human Behavior in Fire Smoke," Chapter 02-04 in SFPE Handbook of Fire Protection Engineering, Third Edition, Society of Fire Protection Engineers, Bethesda, MD, 2002.

[Ivan Aparicio]

Sorry, is the relationship between the extinction coefficient (K) described in NFPA 2-04 and the Optical density in CFAST (OD) as shown below?:

K = ln(OD)

hence, is the visibility calculated as:

V = C/ln(OD)

?

Thanks,

Ivan

[Richard Peacock]

The reference I typically use is a Master's Thesis by Geiman.  Page 10 of the paper has equations for conversion between the various forms:

http://drum.lib.umd.edu/bitstream/handle/1903/113/dissertation.pdf;jsessionid=7ACEFE518428645502A42A08CA3513C4?sequence=1