Hydrogen is a highly flammable gas. Hydrogen gas is colorless, odorless, and not detectable by human senses. It is lighter than air and hence difficult to detect where accumulations cannot occur. Nor is it detectable by infrared gas sensing technology. Coupled with the challenge of detection are the safety risks posed by the gas itself. There are several hazards associated with hydrogen, ranging from respiratory ailment, component failure, ignition, and burning. Because its minimum ignition energy in air at atmospheric pressure is about 0.2 mJ, hydrogen is easily ignited. Hydrogen concentrations of 4% to 75% by volume in air are potentially explosive.

Oil refineries are some of the largest producers and consumers of hydrogen gas. Not surprisingly, refineries use large volumes of hydrogen, which may be produced on site or purchased from hydrogen production facilities. Lead-acid storage batteries also emit hydrogen gas while discharging and recharging. In addition to the fire hazards, hydrogen can produce mechanical failures of containment vessels, piping, and other components due to hydrogen embrittlement.

Types of Hydrogen Gas Sensors

There are various types of hydrogen microsensors, which use different mechanisms to detect the gas. Palladium is used in many of these, because it selectively absorbs hydrogen gas and forms the compound palladium hydride. Palladium-based sensors have a strong temperature dependence which makes their response time too large at very low temperatures. Palladium sensors have to be protected against carbon monoxide, sulfur dioxide and hydrogen sulfide.

  • Optical fibre hydrogen sensors
    • Fiber Bragg grating coated with a palladium layer – Detects the hydrogen by metal hindrance.
    • Micromirror – With a palladium thin layer at the cleaved end, detecting changes in the backreflected light.
    • Tapered fibre coated with palladium – Hydrogen changes the refractive index of the palladium, and consequently the amount of losses in the evanescent wave.
  • MEMS hydrogen sensor – The combination of nanotechnology and microelectromechanical systems (MEMS) technology allows the production of a hydrogen microsensor that functions properly at room temperature. The hydrogen sensor is coated with a film consisting of nanostructured indium oxide (In2O3) and tin oxide (SnO2).
  • Thin film sensor – A palladium thin film sensor is based on an opposing property that depends on the nanoscale structures within the thin film. In the thin film, nanosized palladium particles swell when the hydride is formed, and in the process of expanding, some of them form new electrical connections with their neighbors. The resistance decreases because of the increased number of conducting pathways.
  • Thick film sensor – Thick film hydrogen sensors rely on the fact that palladium hydride’s electrical resistance is greater than the metal’s resistance. The absorption of hydrogen causes a measurable increase in electrical resistance.
  • Chemochromic hydrogen sensor – Reversible and irreversible chemochromic hydrogen sensors, a smart pigment paint that visually identifies hydrogen leaks by a change in color. The sensor is also available as tape.
  • Diode based Schottky sensor – A Schottky diode-based hydrogen gas sensor employs a palladium-alloy gate. Hydrogen can be selectively absorbed in the gate, lowering the Schottky energy barrier. A Pd/InGaP metal-semiconductor (MS) Schottky diode can detect a concentration of 15 parts per million (ppm) H2 in air. Silicon carbide semiconductor or silicon substrates are used.
  • Metallic La-Mg2-Ni which is electrical conductive, absorbs hydrogen near ambient conditions, forming the nonmetallic hydride LaMg2NiH7 an insulator.

Parameters to consider in Selection of Hydrogen Gas Detector

  • Reliability: Functionality should be easily verifiable.
  • Performance: Detection 0.5% hydrogen in air or better
  • Response time < 1 second.
  • Lifetime: At least the time between scheduled maintenance.
  • Cost: As low price as possible
  • Measurement range coverage of 0.1%–10.0% concentration
  • Operation in temperatures of -30°C to 80°C
  • Accuracy within 5% of full scale
  • Function in an ambient air gas environment within a 10%–98% relative humidity range
  • Resistance to hydrocarbon and other interference.
  • Lifetime greater than 10 years