School of Mechanical Engineering, The University of Western Australia ABSTRACT: This paper discusses the selection of a hydrogen concentration sensor for the use in the University of Western Australia’s Renewable Energy Vehicle (REV). Prior to selecting a sensor, it is important to consider the available sensing methods and the speciﬁc properties of the measurand, hydrogen. The selection process leading up to the purchase of two diﬀerent hydrogen sensors from Neodym Technologies, is documented and ﬁnally the method of sensor calibration is outlined.
The University of Western Australia’s Renewable Energy Vehicle (REV) project aims to show the viability of using renewable energy as a means of transport. The vehicle will resemble the cars of today, but will be solely powered by a hybrid of hydrogen fuel and solar energy. The proposed car’s completion date is late 2005, allowing it to be driven around Australia in 2006. The REV requires numerous amounts of measured physical quantities for both data logging and controlling the car’s systems. For each measured physical quantity, a sensor is required to convert this quantity into an electrical signal. Safety is always ﬁrst priority, and for this reason hydrogen leak safety sensors were given the highest priority on the list of required sensors.
HYDROGEN CONCENTRATION MEASUREMENT
Hydrogen gas is extremely ﬂammable having an EL1 of 4.1–74.8 % by volume in air. The minimum energy of hydrogen gas ignition in air at atmospheric pressure is about 0.02 mJ and it has been shown that escaped hydrogen is very easily ignited , the ignition temperature in air is 520–580 ◦ C. In high concentrations, hydrogen may exclude an adequate supply of oxygen to the lungs, causing asphyxiation. Hydrogen gas is colourless, odourless and insipid, so the victim may be unaware of its presence. It is therefore, crucial that any hydrogen leaks are detected quickly and accurately. Hydrogen gas does react with oxygen to form water, though this reaction is extraordinarily slow at ambient temperature. At high temperatures or with an appropriate catalyst, hydrogen and oxygen gas are highly reactive. The standard hydrogen concentration in air at standard pressure is 0.00005 %, but hydrogen emissions from lead acid batteries  and fossil fuel burning may result in higher levels. A hydrogen sensor needs to detect over the general level of ambient hydrogen levels (0.00005 %) and in a variety of environments. Hydrogen gas is the lightest element having a relative density of 0.07. This means the gas is extremely buoyant and will accumulate near the ceiling of an airtight room. Hence, the sensors will need to be mounted in the apex of the monitored space, as this is where the highest concentrations of hydrogen occur. If a sensor was positioned at ground level, it may detect hydrogen gas as low concentrations as it passes, but dangerous levels would accumulate at the roof, which would remain undetected. Three hydrogen gas sensors will be needed; one general leak detector for the hydrogen workshop; two safety leak detectors in the driver’s cabin and the fuel cell compartment of the car.
3 SENSING PRINCIPLES
There are a number of sensing principles used to detect hydrogen gas, the common sensing principles are Metal Oxide Semiconductor, Catalytic Bead, Thermal Conductivity, Electrochemical, and Acoustic Wave. Each have their advantages and disadvantages and these are discussed below.
Metal-oxide semiconductor (MOS) sensors are composed of a heater resistor to warm the sensor to its working temperature (between 200–500 ◦ C), and a sensitive resistor made of a metal-oxide layer deposited on the heater. The electrical resistance of the metal-oxide layer changes, depending on the temperature and the hydrogen content in the surrounding air. MOS sensors suﬀer from several...