Several types of hydrogen monitors and detectors have been described, which utilise a variety of principles related to the physicochemical properties of hydrogen. Some of these devices are only useful at low hydrogen concentrations, a typical upper limit being of the order of 3-4%. Others are able to operate over a wider, and more useful, range of concentrations, but generally either have low sensitivity and thus cannot detect small changes, or exhibit a slow response time and thus cannot respond to transient changes. Further, some of these devices require a supply of pure hydrogen as a standard reference, and some of them only operate at elevated temperatures.
It has also been proposed to use solid sensors in gas detection devices.
Ichinose et al., in CA 1,078,019, describe a gas detecting element including a gas detecting body made of a zinc oxide based semiconductor with a catalytic material coated onto its surface. This device measures the variation of the surface resistance of the semiconductor material when in contact with the gas. This device appears to be limited to detecting a combustible gas such as iso-butane, but also refers to hydrogen and carbon monoxide.
In U.S. Pat. No. 4,636,294 there is described a device for the detection and measurement of hydrogen sulphide. This device requires an anode, a cathode, a solid electrolyte, and a reference electrode in contact with the electrolyte. The reference electrode should be shielded from the gas being tested. It is also suggested that "sacrificial reference electrodes such as silver" can be used in such a device.
Alberti et al. in U.S. Pat. No. 5,453,172 describe a solid state gas sensor which comprises a solid protonic conductor sandwiched between a catalysing electrode on one side, and a solid state reference electrode on the other side. An end of the sandwich comprising these adjacent materials is exposed to the gas to be measured, and the emf generated across the outside layers of the sandwich is measured. This device is said to be suitable for hydrogen. The protonic conductor is preferably zirconium hydrogen phosphate, the catalysing electrode is preferably platinum or palladium, and the reference electrode is preferably titanium hydride or zirconium hydride. It is also indicated that there are drawbacks with using a silver electrode in contact with zirconium hydrogen phosphate which can lead to a total reduction of Ag.sup.+ in such a system.
Currie et al. in WO 94/28403 describe an integrated monolithic gas sensor, which comprises a substrate carrying several deposited thin films. The thin films include an electrically conductive heating element, a conductive reference electrode, and a second conductive electrode. These three are electrically isolated from each other. A thin film ionic conductor, and a thin film reactive gas sensitive layer are placed between the reference electrode and the second conductive electrode to form an electrolytic cell in which an electrolyte reaction including as reagent the gas to be detected produces an emf between the two electrodes indicative of the concentration of the gas. The sensor also includes a micro thermometer formed of a deposited thin film wire having a temperature dependant resistance. When the gas to be detected is carbon dioxide, the reactive gas layer may be sodium carbonate, and the ionic conductor may comprise a complex zirconium phosphate of the Nasicon type, of general formula Na.sub.3 Zr.sub.2 Si.sub.2 PO.sub.12.
A process for the preparation of polycrystalline ceramic materials which conduct electricity by the mobility of hydronium ions or hydrogen ions is described by Kuriakose et al in U.S. Pat. No. 4,724,191. This process includes methods for preparing both the so-called Nazirpos family of compounds, which are complex sodium zirconium silicophosphates corresponding to the general formula Na.sub.(1+x) Zr.sub.2 Si.sub.x P.sub.(3-x) O.sub.12, and other complex materials including polyantimonic acids, alumina containing ceramics, and complex sodium silicates of the general formula Na.sub.5 ReSi.sub.4 O.sub.12, in which Re represents yttrium or gadolinium. Whilst it is stated that the polycrystalline materials obtained by the described process "are capable of use as a membrane in devices such as hydrogen fuel cells, hydrogen detectors, and steam electrolysers" no preference is expressed amongst the many ceramics that can be made by the described process, and further there is no disclosure of how a device capable of both detecting gaseous hydrogen, and measuring the amount of hydrogen present, can be constructed.
A hydrogen concentration cell in which an electrolyte from the Nazirpos family is used has been described (J. Can. Ceramic Soc., 55, 34-37 (1986), and Solid State Ionics, 45,299-310 (1991)). This cell was constructed by applying platinum paint to both sides of a disc of the ceramic, curing it in hydrogen at about 100.degree. C., and sealing the disc onto the end of a glass tube with a silicone rubber resin. Electrical contact with the two platinised surfaces is stated to be obtained by means of a spring loaded platinum wire. This device functions as a concentration cell, in which one side (the side exposed within the glass tube) is exposed to hydrogen gas at a known partial pressure as a standard, and the other side of the cell is exposed to hydrogen gas either at a lower partial pressure, or in combination with another gas, for example nitrogen. Under these circumstances an emf is developed between the two electrodes indicative of the difference of the partial pressures of hydrogen at the two sides of the cell. This device has several disadvantages. It is difficult to ensure reliable electrical contact to the platinised ceramic surfaces with the spring loaded platinum contacts. The two sides of the ceramic disc have to be physically isolated from each other as different gas systems are in contact with each side of the cell. A reference gas of pure hydrogen at a known partial pressure also has to be provided.