Fire safety in space vehicles is of utmost importance, especially for manned flight. On the International Space Station (ISS), events that may lead to fires, especially smoldering, must be detected quickly and their location found. The analyzer used on the ISS must be automated, portable, and sensitive to the gases that are most likely to indicate the presence of a fire or pre-ignition event. In addition, after any fire event, the monitor must be useful to indicate that toxic gas levels have subsided for safe reentry of the crew to the affected area. Gases of interest may originate from the smoldering of Teflon wires, polyurethane foams, Delrin, and other plastics and furnishings in the ISS.
The Compound Specific Analyzer-Combustion Products (CSA-CP), currently used on the ISS as a warning monitor of smoldering or combustion events, is being phased out of service. As a possible replacement, Southwest Sciences has been developing a laser-based Combustion Products Monitor (CPM) analyzer using wavelength modulation spectroscopic absorption. This device is capable of real-time measurements of the four most important gases of interest (carbon monoxide, carbon dioxide, hydrogen cyanide, and hydrogen fluoride) at concentration levels relevant to pre-combustion events, with a one-second response time. This battery-operated device can be wall-mounted or handheld, uses very little electrical power, and has a multi-year lifetime without the need for consumables, recalibration, or maintenance.
The analyzer combines two lasers, a custom electronics board, and a patented multiple-pass optical absorption cell to create a low-power, compact, automated system. The specific lasers selected are capable of measuring two gases each. Optical absorbance measurements of gases use a ratiometric method so that even if the laser output intensity or mirror reflectivity changes over time, the observed absorbance remains constant. Optical detection is far more robust and reliable than with solid-state sensors. While one could use four different lasers to detect the gases of interest, the complexity for a small package is unwieldy. The best compromise is to utilize a pair of lasers that can each detect two gases at sufficient sensitivity, using a single optical absorption cell.
The overall approach is to use laser absorption spectroscopy to measure the local concentrations of the four gases simultaneously. A pair of near-infrared laser diodes is used as the light sources, and wavelength modulation spectroscopy (WMS) methods allow for high-sensitivity detection. The optical path incorporates a sealed multiple-pass cell into the analyzer package, and uses a small Teflon-lined diaphragm pump to flow gas through the cell using an external sampling probe. A sealed cell is required because hydrogen fluoride is very reactive and can adsorb or react in the sampling tube and on analyzer walls that are not the proper materials.
All components are mounted on a thin aluminum plate. Two digital signal processor (DSP) boards, one for each laser, control the lasers, acquire and process data, and store the results on a compact flash drive. An easily readable display on the top of the analyzer shows all four gas concentrations and other relevant information in real time. A rechargeable lithium battery pack provides the required four hours of portable power. The overall housing at the time of this reporting is 6.5 × 4.5 × 11.5 in. (≈17 × 11 × 29 cm).