Nir-diode Laser Based In-situ Measurement of Molecular Oxygen in Full-scale Fire Suppression Tests

The accurate measurement of oxygen is essential to evaluate the effectiveness of tire suppressant agents and to model the dynamics of the suppression process. We employed tunable diode laser absorption spectroscopy (TDLAS) at 760nm to detect molecular oxygen. This technique has sensitivity to oxygen, minimal interference from other species, and capability of in-situ operation. Measurements were carried out at the NRL Chesapeake Bay Detachment in a 28 m’test compartment under three scenarios: water mist only, unsuppressed fire, and fire suppressed by water mist. This paper will discuss the diode lasers properties, techniques to confine the probe region, and the basic measurement principles of TDLAS based in-situ oxygen sensors under the condition of large, rapid obscuration changes. Finally we present the first results of laboratory measurements and field tests. INTRODUCTION The use of water to extinguish a fire is not a new idea. Water is a powerful and convenient agent to suppress fire due to its high evaporation enthalpy, zero-toxicity, availability, and safe handling. Water, in the form of fine mists (droplets < I 0 0 pm in diameter), has the potential to replace halons. The effectiveness of water to suppress fire depends very much on the form in which it is used. Studies by the Navy [ I , 21 have shown that water is most effective in suppressing fire if it is applied in the form of fine droplets. The optimum droplet size is between 1Obm and 30 pm in diameter. Limitations in its use include a lack of mist delivery systems that provide appropriately sized water mist in this size range in sufficient quantities and how to transport the mist to the fire. Current studies at the Chesapeake Bay Detachment (CBD) of the Naval Research Laboratory are investigating the effectiveness of water mist versus gaseous suppression systems with regard to cost, space, and weight. For these studies two steel compartments representing flammable liquid storage rooms onboard Navy ships have been built: one with a volume of 28 m3 (1000 ft3) and a larger one with 298 m3 (10,500 ft3) [3]. Performance tests of various fire suppression systems, including water mist, and an evaluation of how to implement these systems are being carried out. To accomplish these goals, a fairly comprehensive data set is needed including the concentrations of fuel and oxygen. The Oz measurement in the presence of both liquid and vaporized water is not trivial. Extractive techniques are not capable of determining the actual oxygen concentration because the ratio between the two phases of water in the sample is not obtained. The decomposition products of fluorohydrocarbon agents attack ZrOz in-situ probes and lead to erroneous 0 2 concentrations. Also, oxygen has no mid-IR absorption and cannot be detected by conventional IR-spectroscopy. Tunable diode laser absorption spectroscopy (TDLAS) can detect oxygen concentrations in-situ under these conditions as we show in this study. TDLAS Tunable diode laser absorption spectroscopy is the combination of in-situ absorption techniques with tunable near infrared diode lasers (NIR-DL). One main advantage of direct TDLAS is that the absorption signal can be easily evaluated in such a way that it is in principle insensitive to the 492 Halon Oplions Technical Working Conference 2-4 May 2000 degree of optical obscuration. TDLAS is well suited for the investigation of gas samples with high particle loads. If the DC-signal is evaluated, the gas can be analysed in-situ, e.g., directly in rhe probe volume. completely avoiding any gas sampling, which is a common source of errors. In contrast to many other lasers, NIR-DLs are inexpensive, compact, rugged, and relatively simple to operate at room temperature. They are particularly suitable for weight and space restricted applications. The lifetime of a diode is above IO'h. NIR-DL output wavelengths can range from 635 to 1650 nm, although some wavelengths are not readily available, including those around 760 nm. Another disadvantage is the typically low emission power of 5 to 10 mW. Only for certain wavelengths (e.g., water at 810 nm) are there available diode lasers wilh 100 mW output. However, because of their fast tunability, high spectral resolution ( density, NIF-DL offer the opportunity for nonintrusive, chemical sensors with high sensitivity, specificity. and a fast time response. TDLAS has been used in many applications to measure reliably the in-situ gas concentrations of species like CH4 [4], HzO [ 5 ] . and 0 2 [6] in challenging environments. Simultaneous multi-species detection and optical temperature measurement by two line absorption techniques has been demonstrated in power plants and waste incinerators L7, 8,91. nm), and spectral power THIS STUDY An NIR-DL based spectrometer has been assembled to measure in-situ oxygen concenlration inside a 28-m' compartment which simulates a typical flammable liquid storeroom (FLSR I) located at the NRL Chesapeake Beach Detachment (CBD) facility. Initial tests as presented here have been carried out characterizing the optical boundary conditions, including the overall transmission of the measuremen[ path, and the variation of the oxygen concentralion. This has been done for three different test scenarios important to fire suppression research: water mist only, fire only, and water mist suppressed fire. Water mist was generated using a single dualfluid, high-pressure nozzle (HI-FOG, Marioff Oy). Test results show the potential of TDLAS as a tool for the in-situ determination of 0 2 concentrations in these difficult environments. BACKGROUND ABSORPTION SPECTROSCOPY The measurement principle of absorption spectroscopy is based upon a spectrally resolved measurement of the losses of the laser beam propagating through the measurement volume. These losses are recovered by wavelength tuning of the laser and are described by Lambert Beer's law: I(h, T)=l,,(h) exp(-S(T) g(h-h,,) * N * L) = I,,(h). exp(-a,,) ( 1 ) where I,,(h) is the initial laser intensity, I(h) is the measured intensity after pa absorbing medium of thickness L characterized by S(T), the temperature dependent line strength, N the number density of absorbers, and g(h, h,) describing the shape of an absorption line, centred at wavelength ho. In a gas cell, NIR-DL-based O2 detection is possible with a sensitivity of 8 ppmV perm of absorption path [IO]. In real world environments, sensitivity is reduced because molecular absorption is usually orders of magnitude smaller than the disturbances that interfere with the Halon Oplion, Tcchnicai Worhinp Cr,nlcrence 1-4 May Z O W 493 signal. Wavelength unspecific absorption leads to time dependent variations in the optical transmission of the measurement path. The NIR oxygen transition at 760 nm is quite weak, and an absorption path of more than a meter is typically required to achieve a reasonable detection limit. The poor visibility environment of a fire test chamber produces variations in background absorbances 100 times larger than the molecular absorption signal of oxygen. Scattering, refractive index gradients, thermal lensing, mechanical vibrations, and deformation of the alignment will occur to some extent and affect the beam profile, the pointing stability, and will lead to light loss. The detector signal can be increased by background radiation from the fire. Problems can be caused by electromagnetic pickup, high voltage/current switching, ground loops, and thermal drift in the laser power supply, detector or signal processing electronics. Techniques have been developed to compensate for or avoid most of these problems. The ability to follow transmission fluctuations by high-speed wavelength tuning achieved by simple modulation of the laser current is one of the major advantages of diode lasers. Repetition rates in the kHz range allow acquisition of an absorption profile in less than a millisecond, which is sufficiently fast to “freeze” all transmission variations within an absorption scan, making correction by division with a background transmission signal possible. Background radiation can be minimized by the use of narrowband optical filters. Additional noise reduction is achieved by averaging consecutive absorption profiles and extracting the absorption signal, i.e., the area underneath the absorption line, by a fitting algorithm. This absorption cross section is proportional to the concentration of the absorber, in this case the oxygen molecules (Figure I). If the temperature change in the measurement region is large ( x a . 30 K) the temperature dependence of the observed absorption line has to be considered. It is therefore important to select a suitable rotational line and to know its properties.