Because hydrogen molecules are so small, it is very difficult to fully contain hydrogen over long periods. Pipework requires very tight metal-to-metal joins.
For safety, it is important to be able to detect even the smallest leaks. In a closed room the gas will move upwards until it reaches the ceiling. If the ceiling is an effective barrier, the gas becomes trapped and will fill downwards displacing air until a dangerous pocket of hydrogen gas is formed that can be ignited by ceiling light fittings.
Simple leaks are identified by spraying a mix of water an detergent on fittings and looking for bubbles forming. More sophisticated hydrogen detectors depend on a property of hydrogen that is significantly different to that of air. For example, the unique absorption frequency of hydrogen atoms could be chosen for identification. However, care is needed as a sensor of this type may also be sensitive to the hydrogen atoms in water vapour. A particular chemical reaction specific to hydrogen could be selected, but a permanent reaction would mean a sensor could only be used once, which is not ideal.
Most detectors rely on hydrogen being adsorbed (or absorbed) on a surface which then changes in some electrical property (such as resistance or capacitance). The process is generally slow, with a response time that is of the order of seconds with possibly minutes required for the surface to return to the original state once the hydrogen is cleared. However, this is rarely a problem as ultrafast detection is almost never needed. For a full review of methods, refer to this recent review.
A interesting method of hydrogen detection is illustrated above. The speed of sound in air at STP is 331.3 m s-1, but sound travels much faster in hydrogen (1280 m s-1 in pure hydrogen). This is potentially a way of detecting a hydrogen leak somewhere within a room by creating a mesh of paths that cut across most of the roof area. In the example shown above, there is a cloud of hydrogen gas though which the upper sound pulse passes. The pulse is speeded up with the significant deviation from reference indicating a leak. The pulses are generated by exciting a standard 40 kHz quartz transducer (or the type used in car reversing sensors) and waiting for the echo. The displays shows the echo time in milliseconds. The diagram implies the device used is some sort of transceiver, but in practice a matched receiver/transmitter pair are placed side by side. One long-standing objection to this type of sensor is that it could be triggered by other gases and the variation of sound speed with temperature is significant and affects precision. However, by using an AI learning system, these are no longer such significant issues because a hydrogen gas leak will present a very different signature to the normal temperature change in a room and the system can be trained to sound an alarm only when the proper signature is present.
Investigations