TOWARDS A TEST METHOD FOR HYDROGEN SENSOR PERFORMANCE NATHAN D. MARSH AND THOMAS G. CLEARY Fire Research Division, National Institute of Standards and Technology, 100 Bureau Drive, Stop 8664, Gaithersburg, MD 20899, USA

Introduction Reliable detection of an accidental hydrogen gas release and mitigation of the hazard through designed safety systems is a key component of hydrogen powered systems in commercial, residential, and transportation uses. Inexpensive hydrogen gas sensors based on a range of sensing technologies are becoming increasingly available, but there is a need to characterize sensors in conditions relevant to their end-use application.

Where Might Sensors be Located? Benning Road, SE, Washington, DC

Challenges to Hydrogen Sensor Performance In ISO/CD sensor performance is tested for:  Sensitivity  Response time  Recovery time  Environmental changes (temperature, humidity, pressure)  Nuisance sources, i.e. substances which may trigger a false alarm.

Fire Emulator / Detector Evaluator Previously used for the evaluation of smoke and fire detectors, this apparatus was modified for the evaluation of hydrogen sensors. Sensors are mounted in the test section and are powered and monitored by a computerized data acquisition system.

Sensors and Exposures  Temperature rise from 25 °C to 50 °C followed by a return to 25 °C  100 % relative humidity with condensing water vapor  Carbon monoxide (120 µL/L to 2000 µL/L  ) and carbon dioxide (2000 µL/L to 0.02 L/L)  Propene (3000 µL/L to 0.01 L/L)  Nitrogen dioxide (250 µL/L  )  Hydrogen (250 µL/L)  Hydrogen (250 µL/L) with temperature rise from 25 °C to 50 °C followed by a return to 25 °C  Hydrogen (250 µL/L) with 100 % relative humidity and condensing water vapor  Hydrogen (250 µL/L) with carbon monoxide (50 µL/L) and/or carbon dioxide (600 µL/L)  Hydrogen (250 µL/L) with propene (120 µL/L) 8 hydrogen sensors were purchased from 4 manufacturers and exposed to a variety of environmental conditions and gas compositions. TCD: Thermal Conductivity Detector; MOS: Metal Oxide Semiconductor; CAT: Catalytic Bead Pellistor; EC: Electrochemical Cell; Multi: Multiple integrated technologies (film resistor and MOS capacitor, Pd/Ni film); Sensors F and G incorporate a molecular sieve. SensorTechRange (vol fraction) ATCD0.0 % to 100 % BMOS0.0 % to 2.0 % CMOS0.0 % to 2.0 % DCAT0.0 % to 2.5 % EMulti0.4 % to 5.0 % FMOS0.0 % to 0.20 % GCAT0.1 % to 4.0 % HEC0.0 % to 4.0 %

Examples of Hydrogen Sensors

Typical Result of an Exposure Test Circles: CO; no symbol: Sensor A (TCD); light triangles: Sensor B (MOS); medium Xs: Sensor C (MOS); dark squares: Sensor D (CAT)

Sensitivities of Hydrogen Sensors Error bars represent standard deviation of three exposures on three days Sensors were exposed to a mixture of air and “forming gas” i.e. 5 % H 2 in N 2

Sensor Response to Nuisances Error bars represent the resolution of each sensor

Extreme Challenges Error bars represent the resolution of each sensor

Synergistic Effects? Error bars represent the resolution of each sensor

Performance of Hydrogen Sensors Sensor A (TCD) was not sensitive to 250 µL/L H 2 However it was sensitive to condensing water vapor, reading the equivalent of 3000 µL/L H 2 at 25 °C and 100 % relative humidity. Sensors B, D, and H (MOS, CAT, and EC respectively) experienced the most cross-sensitivity, responding to temperature, humidity, CO (at high concentration), propene, and NO 2. Sensor B and C (both MOS) read inconsistently high in the presence of H 2. Sensors C (MOS) experienced some cross-sensitivity. Sensor D is also inversely temperature sensitive: increasing the temperature by 25 °C reduces the baseline by a voltage equivalent to 200 µL/L H 2. Sensors E (Multi) and G (CAT) were not sensitive to any challenge gases or conditions. However they were also not sensitive enough to detect 250 µL/L of H 2 in the FE/DE. Sensor F was not sensitive to challenge gases or conditions but was sensitive to hydrogen.

Response Times of Sensors Squares: Sensor F (MOS); Circles: Sensor E (Multi); Filled symbols: response to hydrogen flow initiation; open symbols: response to hydrogen flow cessation. Arrows indicate order of tests. Times are not corrected for the response time of the calibration cell.

Conclusions Tested sensor cross-sensitivities to heat, moisture, and various gases in low concentrations were measured. Sensors were not susceptible to poisoning by CO or NO 2, even at gas concentrations that would be hazardous for humans. The extent to which the observed cross-sensitivities would lead to nuisance alarms or missed alarms is still unknown. Further testing at the desired hydrogen alarm concentrations needs to be performed. Sensor response times were on the order of one to three minutes, with relaxation times observed to be faster. Ultimately, performance evaluations need to consider dynamic environmental and concentration changes to assess temporal sensor performance.