Amperometric gas sensors (AGS; sometimes called electrochemical gas sensors) have properties that make them attractive for long-term monitoring applications and real-time measurement, process control, and safety applications. They are simple in structure, reliable and rugged in use, and modest in cost. Power consumption is low -- in fact, an AGS is actually an electric generator, oxidizing or reducing traces of electroactive gases to produce a measurable electric current. Furthermore, they have been proven in use: AGS have been used in gas measurement systems for oxygen, carbon monoxide, hydrogen sulfide, and other toxic gases for two decades. The sensitivity limit of an unadorned AGS is in the range of low parts per million by volume (ppmv) for most gases. This has been sufficient to make these sensors irreplaceable in toxic gas alarms.
Figure 1. Schematic and conventional structures of an amperometric gas sensor.
The structure of an AGS is shown in Figure 1. Topologically, it is identical to the fuel cells used in spacecraft and vital power backup systems. Three electrodes are immersed in a suitable electrolyte. The working electrode is made of platinum or gold deposited on the back of a porous membrane. Gas can diffuse through the membrane, contacting the electrode and electrolyte at the same time. An oxidation or reduction occurs at this point, which results in electrons being left on, or removed from, the electrode. The remaining electrodes (counter electrode and reference electrode) are used to maintain charge balance in the sensor and to control its operation, respectively. The charge differences between the working and counter electrodes can be used to generate an electric current which is the output signal of the sensor. Typically, an external electric circuit is used to translate the feeble current into a measurable voltage, and to maintain the working electrode at the correct voltage for optimum operation.
Selectivity (the ability to respond to only one gas) has been a continuing concern with the AGS. They are dependent on a catalytic electrode for operation, and to date only two electrode materials have proven satisfactory for commercial sensors: platinum and gold. Gold-based sensors respond to ozone and to nitrogen and sulfur compounds (e.g., NO2, NO, HONO, HNO3, O3, H2S, SO2, HCN); platinum-based sensors respond to all of those gases plus carbon-oxygen compounds (CO, formaldehyde, alcohols, etc.).
The selectivity of the two basic sensor types can be modified by changing the potentials applied to the electrodes, but only up to a limit. Other tricks have been developed to enhance selectivity, such as the use of chemical reactors to remove interferences or to convert the gas into another that is more easily measured.
New developments in construction of the basic electrode types have achieved unusually good detection limits for some gases, not by increasing the signal, but by suppressing the electrode noise. Reliable detection limits for environmental NO2 of 0.010 ppmv have been achieved, for example, and interference from NO, ozone, and sulfur dioxide can be eliminated by the use of chemical reactors.