Study of Technology for Detecting Pre-Ignition Conditions of ... - NIST

.
3.4 Laser Attenuation
CPSC-IAG95-1145
In. these experiments, laser attenuation is the decrease in laser-light intensity due to
interference or obscuration by particles in the path of the beam. Laser attenuation was calculated
by subtracting the voltage measured during the test from the pre-test background-zero wdue and
dividing the result by the same background-zero value. Figure 27 shows laser attenuation, with
units of volts-per-volt, as a function of time for the two He-Ne lasers for a test of oil heated on
an electric range with the range hood off. A sharp increase in attenuation begins at 450 seconds.
The increase, while sharp, is fairly linear until ignition.
Figure 28 shows a comparison of the laser-attenuation measurements 17.8 cm above the
burner for all of the food and range-hood combinations heated on the electric range. The sugar
curves are different than those of oil and bacon in that they rise very sharply after two minutes
of heating, but the rate of rise decreases within a minute to slopes similar to the other foods. The
oil and bacon curves are similar except for the period of time a&er heating begins until the curves
rise. Bacon requires about two minutes less to generate significant smoke levels.
Figure 29 shows a comparison of the attenuation measurements 17.8 cm above the burner
for all of the food and range combinations heated on the high-output gas range. The sugar
ignited much earlier than the oil and bacon. Local heating, melting and charring was observed
in the pan of sugar where the gas flame impinged on the pan. Less time was required for the gas
range to cause a local area to ignite than the electric range required to heat all of the sugar. The
bacon and oil curves are similar in shape to those for the electric range except that they are
stretched in time.
All of the foods produced large quantities of smoke that created a smoke layer in the
upper region of the room. The smoke layers were clearly visible, up to 60 cm in thickness, and
noxious in odor. When this occurred, the room air became unbreathable and even caused
discomfort in its diluted form outside of the room. Characteristics associated with these large
quantities of smoke are important features of the pre-ignition environment. Light attenuation and
the related phenomenon of scatter show considerable potential as pre-ignition signatures.
Threshold levels of attenuation or scattering would be most appropriate since the slopes of the
attenuation curves are similar for different finds on a given range.
3.5 FTIR Gases
Relative changes in concentration of general hydrocarbons, carbon dioxide, and water were
obtained from the FTIR data analysis. Absolute concentrations were not calculated due to the
difilculty associated with calibration of the FTIR with known quantities of the required gases over
the range of plume temperatures encountered during testing. The qualitative trends of the species
as ignition approached were determined. The area referenced in the following figures is the area
under the curve of absorbance plotted versus frequency. The area is calculated by integrating
between the wavenumbers (frequency units of cm-l) that bound the spectral band, or peak, of a
specific gas species. The area, with units resulting from the product of absorbance and
frequency, has little quantitative meaning without calibration, but relative changes in an area
reflect relative changes in the gas concentration.
45