Fire/Arson Investigations : Fire/Arson Investigations
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Re: Significant error rate
Posted by: dcarpenter (IP Logged)
Date: February 13, 2007 10:44AM
I would like to expand on John's posting.
A fundmental understanding of fire dynamics is necessary for proper analysis of fires. Based on my reviews of analyses that use fire dynamics (and computer fire models), there is an oversimplification and a lack of understanding of the phenomenon that leads to misuse (or "a disguise"). I think there is a belief that the fire science included in NFPA 921 is all that needs to be known. There is also a lot of areas that we do not know as much as we think we know.
For example, there is limited data on unventilated fires. In my review of arson cases, I see a trend towards wrongful convictions in situations where a fire occurs in the early morning hours in a house that is initially closed up. Occupant discovers "fire", traverses the house and goes outside. At this point, a new source of ventilation is created can change the behavior of the fire in the short term. The investigator collects data on a rapidly growing fire and "knows" conditions are not tenable for humans. Boom, Arson!
There are two extremes related to unventilated fires that become ventilated. We have data and know somethings about backdrafts. On the other extreme are fires that slowly grow after being ventilated. Some data is available for this scenario also. While we do have observations from real fire experience and we can apply fundamental knowledge to develop hypotheses reated to the behavior of fire for unventilated fires that become ventilated between these extremes, we do not have any experimental data to help advance the knowledge. I hope to change this state by experimentation that has not yet been done.
The problem is that most fire investigators assume we have what we need to analyze all fires and are ignorant of what we do not know about fire behavior. This leads to oversimplification and misinterpretation of fire origin and cause. This is an educational problem. There is also the problem of implicit or explicit pressure to achieve a "known" result. This is not a problem isolated to the public or private sector. This is a political and/or economic problem.
There is also a public perception that fire investigators (public and private) have all the necessary knowledge and training to reach technically valid conclusions. I think many in the community, including some of the leadership, have also convinced themselves of this also.
If we are to advance the profession and increase our abilities to achieve technically valid conclusions, there are some of the major obstacles that need to be cleared. The problem is that a critical mass within the community must recognize that there are significant problems and have the motivation and leadership to address the problems to ensure long-term advancement of the profession. If there was a "DNA" for fires, we would know our "error rate" and I venture a guess that we would all be surprised by the results.
With respect to the use of computer fire models, due to the availability and ease of use of models such as CFAST and FDS, they are getting more attention and use in fire investigations. The problem is that even a simple model is hard to use correctly. I have always tended to look for the subtle errors in the use of models since I assume that if you run the model, you would know how to use it. What I find are not-so-subtle errors in the application and use of such models. Thus, there does not seem to be a filter for use. There is great value in the use of models in the investigation of fires. Not all fires, but in many cases. The application of models requires significant effort that many are not willing to endure. What I am concerned about is that modeling results that are performed by competent users and are scientifically valid in thier use and results, get washed away in the tide of misapplication and ignorance. This again can be attributed to the general problems previously discussed.
Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
(410) 884-3267 (fax)
www.csefire.com
A fundmental understanding of fire dynamics is necessary for proper analysis of fires. Based on my reviews of analyses that use fire dynamics (and computer fire models), there is an oversimplification and a lack of understanding of the phenomenon that leads to misuse (or "a disguise"). I think there is a belief that the fire science included in NFPA 921 is all that needs to be known. There is also a lot of areas that we do not know as much as we think we know.
For example, there is limited data on unventilated fires. In my review of arson cases, I see a trend towards wrongful convictions in situations where a fire occurs in the early morning hours in a house that is initially closed up. Occupant discovers "fire", traverses the house and goes outside. At this point, a new source of ventilation is created can change the behavior of the fire in the short term. The investigator collects data on a rapidly growing fire and "knows" conditions are not tenable for humans. Boom, Arson!
There are two extremes related to unventilated fires that become ventilated. We have data and know somethings about backdrafts. On the other extreme are fires that slowly grow after being ventilated. Some data is available for this scenario also. While we do have observations from real fire experience and we can apply fundamental knowledge to develop hypotheses reated to the behavior of fire for unventilated fires that become ventilated between these extremes, we do not have any experimental data to help advance the knowledge. I hope to change this state by experimentation that has not yet been done.
The problem is that most fire investigators assume we have what we need to analyze all fires and are ignorant of what we do not know about fire behavior. This leads to oversimplification and misinterpretation of fire origin and cause. This is an educational problem. There is also the problem of implicit or explicit pressure to achieve a "known" result. This is not a problem isolated to the public or private sector. This is a political and/or economic problem.
There is also a public perception that fire investigators (public and private) have all the necessary knowledge and training to reach technically valid conclusions. I think many in the community, including some of the leadership, have also convinced themselves of this also.
If we are to advance the profession and increase our abilities to achieve technically valid conclusions, there are some of the major obstacles that need to be cleared. The problem is that a critical mass within the community must recognize that there are significant problems and have the motivation and leadership to address the problems to ensure long-term advancement of the profession. If there was a "DNA" for fires, we would know our "error rate" and I venture a guess that we would all be surprised by the results.
With respect to the use of computer fire models, due to the availability and ease of use of models such as CFAST and FDS, they are getting more attention and use in fire investigations. The problem is that even a simple model is hard to use correctly. I have always tended to look for the subtle errors in the use of models since I assume that if you run the model, you would know how to use it. What I find are not-so-subtle errors in the application and use of such models. Thus, there does not seem to be a filter for use. There is great value in the use of models in the investigation of fires. Not all fires, but in many cases. The application of models requires significant effort that many are not willing to endure. What I am concerned about is that modeling results that are performed by competent users and are scientifically valid in thier use and results, get washed away in the tide of misapplication and ignorance. This again can be attributed to the general problems previously discussed.
Douglas J. Carpenter, MScFPE, CFEI, PE, FSFPE
Vice President & Principal Engineer
Combustion Science & Engineering, Inc.
8940 Old Annapolis Road, Suite L
Columbia, MD 21045
(410) 884-3266
(410) 884-3267 (fax)
www.csefire.com
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