gas giants

In the next 20 years, concerns about global climate change and energy security will support hydrogen as a cost-effective replacement for fossil fuels. Concerns over safety will create a need to monitor and sense this odorless, colorless gas in the vicinity of hydrogen storage facilities and the general user environment. Researchers at Intelligent Optical Systems Inc. (Torrance, CA) have developed photonic hydrogen sensors that are examples of an entire class of optical sensors, for measurands as diverse as oxygen, temperature, and toxic chemical vapors.

The hydrogen sensing system described here takes advantage of the use of advanced optical techniques to detect gases with better reliability and efficiency than ever before. Most optical gas sensors are passive, relying on the inherent spectroscopic properties of the gases themselves. While this technique can be very reliable, it is limited to the detection of gases that absorb light at wavelengths for which high-energy light sources are available. Over the past two decades, researchers have solved this limitation with active optical gas sensors, which enhance their optical response to gases by using a chemically sensitive indicator that changes its optical properties in the presence of the gas being measured. Advantages of these compact, low-mass sensors include intrinsic safety in hydrogen-containing environments, specificity to hydrogen, reversibility, robustness, wide detection range, sensitivity to low concentrations, lifetimes on the order of years, and response times of 3 s or less.

In a recent NASA-funded project, we developed a hydrogen and oxygen detection system for launch-vehicle safety.¹ The sensor systems are based on optical fiber networks tipped with optrodes, miniaturized sensors whose optical properties depend sensitively and specifically on the hydrogen concentration. We fabricated hydrogen optrodes by incorporating a proprietary colorimetric chemical indicator into porous glass rods. As hydrogen gas diffuses through the porous structure, it interacts with the indicator, changing the indicator's optical absorbance spectrum.

Beer's Law tells us that the fraction of light, I/I₀, transmitted by the optrode at a particular wavelength depends on the size of the optrode, the concentration of the indicator, C_i, and the concentration of hydrogen, C_H, as

I/I₀ = exp(-aC_iC_HL) [1]

where a, the optical absorbance, is a wavelength-dependent property of the chemical indicator and L is the optical path length (length of the optrode). Thus, the color of each optrode is directly related to the concentration of the gas. Furthermore, optrode response can be "tuned"—for example, Equation 1 shows that increasing indicator concentration or optrode length increases optrode sensitivity (defined as the change in absorbance induced by a change in hydrogen concentration). In the case of our recently developed hydrogen optrodes, this color change is fully reversible and takes place without any damage to the optrode glass material, so the sensor system can accurately and reproducibly monitor variations in hydrogen concentration.

Figure 1. Hydrogen optrode laboratory tests show consistent results for hydrogen levels between 0 and 4%.

Laboratory tests of the optical transmission of several different optrodes exposed to different levels of hydrogen showed response times on the order of a few seconds and good repeatability (see figure 1). Signal levels are identical when hydrogen concentrations are the same. These characteristics—fast response and good reproducibility—are paramount when optrodes are being considered for use in practical sensing systems.

Figure 2. Optrode systems can be set up in transmission (top) and reflection (bottom) configurations.

For the tests above, our system sandwiched each optrode directly between a blue LED and a photodetector with an optical filter to shield the detector from excitation light. This design can also be used to create the hydrogen-sensing equivalent of a smoke detector—a self-contained unit that can be mounted anywhere to detect and warn of hazardous-gas concentrations. Unlike electrically based sensors, however, optrodes are also suited for applications in which the presence of even small electrical currents could potentially lead to the generation of explosion-producing sparks. In these situations, the optrode can be linked to a light source and a detector through optical fibers, either in a direct equivalent of the fiber-free transmission geometry, or in a "single-ended" reflectometry-based approach (see figure 2). Both of these approaches have been reported by a number of groups working on optrodes for a wide variety of chemical substances.

Optrodes may be packaged together with temperature, oxygen, and other optrodes, or used separately, depending on the desired application. One potential product, suitable for multipoint hydrogen leak detection, consists of optrodes, fiber-optic cables, and a compact optoelectronic unit that sends light to the optrodes and converts the resulting optical outputs to gas concentrations. Another is a compact hydrogen "sniffer" that can be connected to a vest-pocket computer to provide convenient measurement and data logging in a hand-held format. Optrode-based gas sensing technology can also be reduced to the size of a quarter—including sensor points, light channeling, optoelectronics, and processing—by manufacturing it as a porous-glass integrated-optic chip, using technology pioneered for environmental monitoring.²

Optrodes mounted on the ends of optical fibers are ideally suited for the detection of explosive gases such as hydrogen; because no electrical connection is needed at the test area location, there is no risk of igniting the gas. This intrinsic safety becomes an important advantage when optical sensors are compared with the electrochemical and pyroelectric hydrogen sensors that are currently in use.

In early 2001, we installed a 14-point demonstration sensor system for oxygen and hydrogen on Rocket Motor Test Stand No. 2B at the NASA Stennis Space Flight Center (Slidel, MS).³ During one test firing, the system detected hydrogen vented from one of the rocket motor's roll-control nozzles less than 10 s after initiation of the firing sequence. This is well below the 2-minute response times now inherent in the current launch-pad hydrogen-monitoring system, which uses long tubes to suck air samples into a remotely located mass spectrometer. Ultimately, the inherently safe optrode approach to hydrogen sensing could be used to protect any testing or launch facility that uses cryogenic propellants.

Optrode-based chemical sensors have already found niche applications in fields ranging from environmental monitoring to medicine. In the coming years, these sensors will find wide acceptability as users benefit from the advantages of the optrode approach to hydrogen sensors. oe

References

1. SPIE Proceedings series on "Chemical, Biochemical, and Environmental Fiber Sensors."

2. E. Mendoza, D. Robinson, et al., Proc. SPIE Vol. 2836, p. 76 (1996).

3. NASA Contract NAS13-01038.