Fire Tests Conducted To Measure Heat, O2 Levels
What temperatures can you expect to encounter in a dwelling fire? How low will the percentage of oxygen drop when the fire goes into the third, or smoldering, stage? To get some answers, we conducted two confined fire tests in separate rooms of a single family house with the aid of the Minnetonka, Minn., Fire Department.
Prior data suggests that these temperatures can reach 1300°F and the oxygen level can drop below 15 percent during the third stage. These figures varied based on fire load.
For the tests, the fire load consisted of wood and paper products. Plastics were not introduced. Type K thermocouples (T/C) were inserted at the top, middle and bottom of an exterior wall. Each T/C extended into the room about 4 inches and the hole was sealed with insulation. A 1/4-inch copper tube was also inserted through the center hole and was used to sample the air in the room for oxygen content.
In the second fire a pressure transducer was connected via copper tube through the same hole used to sample oxygen. All the signals were monitored by strip chart recorders.
The two fires were ignited and allowed to enter the free-burning second stage, producing upper T/C temperatures of 720 and 910° F, respectively. Flames due to fire gases had spread across the entire ceiling area. The rooms were closed off and the fires entered the third stage as the oxygen and temperature diminished. After a dip in oxygen and temperature, air seeping in from cracks resupplied the fires. They grew in intensity but were easily controlled and extinguished.
Humidity affects maximum fire temperatures and elapsed time, as indicated during the two fires. The first fire had a lower temperature and took longer to burn. Several days prior to the test, water had been applied to control a fire in this room. During actual fire conditions, high humidity in a structure will play a part in the amount of structural damage and ease fire control.
The two oxygen analyzers measure in different aspects. One measures amount of oxygen, which was 13 percent minimum. The other analyzer measures amount of oxygen left after unburned fire gases are burned off, which was 7 percent. The difference was due to the flammable gas content of the air, which was equivalent to 6 percent oxygen. These gases were generated as the fire entered the third stage when oxygen dropped below the requirements for complete combustion. These are the gases that must be considered dangerous and toxic when fighting fires in any stage of burning.
This is why ventilation from above the fire is very important. A sudden inrush of oxygen from the fire level will ignite these heated gases and an explosion follows, known to us as a backdraft.
Due to the possibility of pressure variations in the room as the fire progressed, we used a semiconductor pressure transducer, which is highly sensitive, to verify any pressure change that would develop. Even with this device, we observed no measurable pressure change in the room, although we suspected pressure fluctations from the temperature and oxygen content variations.
Fire fighters must be careful of all operations during third-stage fire conditions. But as indicated by our lower T/C temperature levels, a fire fighter can survive in a third-stage fire with self-contained breathing apparatus and protective gear. If the fire fighter opens a door or window during this time, however, a back draft can occur.
In summary, it is hard to tell when a fire has entered the third stage and dangerously low levels of oxygen are present. Structure size and fire load are main considerations along with response time. Never hesitate to ventilate a fire if it has not self-ventilated. It can save on amount of structure damage and fire fighting time. Most importantly, ventilation can reduce the chance of injury or loss of life.