Other Oxygen Sensors

In the case of camphorquinone, the signal was unstable due to photochemical reactions which were occurring in the triplet state. Hence, oxygen measurements based on the intensities were not reliable. However, the 

Ftgure19.12. Normslizedexcitstionuideiiilssionspectraofcaiiiphor. qulnow in poly( vinyl chloride), illnsinting the effect of oxygen ooncen-tndon on phosphorescence intensity. Revised snd reprinted from Ref 39, Copyright e 1994, with permission fiom Elsevier Science. 

Fipire 19.17. Reprejeigaiive ctdoride probes. These probes are cofli- 

F%ue 19.1R. Sten-IMaer plots foe cUoride qiieodufig of SFQ. SPA. Iiirigrnin. MACA, and MAMC See Table 19.1. Revised and teprinted. nMi pcntiinion. from 49. Copgrii 01994. Academic 

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Electrochemical Microsensors. The most successful chemical microsensor in use as of the mid-1990s is the oxygen sensor found in the exhaust system of almost all modem automobiles (see Exhaust control, automotive). It is an electrochemical sensor that uses a soHd electrolyte, often doped Zr02, as an oxygen ion conductor. The sensor exemplifies many of the properties considered desirable for all chemical microsensors. It works in a process-control situation and has very fast (- 100 ms) response time for feedback control. It is relatively inexpensive because it is designed specifically for one task and is mass-produced. It is relatively immune to other chemical species found in exhaust that could act as interferants. It performs in a very hostile environment and is reHable over a long period of time (36). 

In an actual exhaust system controlled by the signal of the oxygen sensor, stoichiometry is never maintained, rather, it cycles periodically rich and lean one to three times per second, ie, one-half of the time there is too much oxygen and one-half of the time there is too Httle. Incorporation of cerium oxide or other oxygen storage components solves this problem. The ceria adsorbs O2 that would otherwise escape during the lean half cycle, and during the rich half cycle the CO reacts with the adsorbed O2 (32,44,59—63). The TWC catalyst effectiveness is dependent on the use of Rh to reduce NO and... 

One system for measuring catalyst failure is based on two oxygen sensors, one located in the normal control location, the other downstream of the catalyst (102,103). The second O2 sensor indicates relative catalyst performance by measuring the abiUty to respond to a change in air/fuel mixture. Other techniques using temperatures sensors have also been described (104—107). Whereas the dual O2 sensor method is likely to be used initially, a criticism of the two O2 sensors system has been reported (44) showing that properly functioning catalysts would be detected as a failure by the method. 

Electrochemistry plays an important role in the large domain of. sensors, especially for gas analysis, that turn the chemical concentration of a gas component into an electrical signal. The longest-established sensors of this kind depend on superionic conductors, notably stabilised zirconia. The most important is probably the oxygen sensor used for analysing automobile exhaust gases (Figure 11.10). The space on one side of a solid-oxide electrolyte is filled with the gas to be analysed, the other side... 

Other useful sensors rely on the coupling of microorganisms and electrochemical transducers. Changes in the respiration activity of the microorganism, induced by the target analyte, result in decreased surface concentration of electroactive metabolites (e.g., oxygen), which can be detected by the transducer. 

Other specific issues to be considered when developing oxygen sensor systems for packaging applications include ... 

A number of optical chemical sensor systems have been developed for food and packaging applications and proven their utility. Some sensors, such as phase-fluorimetric oxygen sensor, have already reached high degree of maturity and demonstrated their potential for use on a mass scale. While the others still require extensive research, development and search for new solutions, so as to match basic practical requirements for such sensors. The experience and lessons learned with current optical sensor systems must be... 

Household appliances can also benefit from improvements in other areas. For example, oxygen sensors that measure the 02-concentations in exhaust gas have been developed that combine a Nernst type lambda gauge (which can measure only the ( -concentration at one lambda-point) with an amperometric 02-pumping cell. 

While this paper will concentrate on oxygen sensors as used in automotive applications, there is increasing interest in their use in the measurement and control of industrial and other furnaces in order to reduce fuel costs by maximizing the combustion efficiency. They have also been used for many years to measure the oxygen content of molten glass, of molten steel and other metals and for numerous other applications where a measurement of the oxygen partial pressure is desired. 

Because of this type of behavior, a sharp transition at stoichiometry but low sensitivity and temperature effects either rich or lean of this point, the oxygen sensor is most useful in controlling at the stoichiometric point. It is of limited usefulness at other exhaust compositions. However, as shown in Figure 5, this is exactly the point at which a three-way or dual bed catalytic converter is most efficient. Only when the exhaust composition is near the stoichiometric point will both the oxidation of the HC and CO and the reduction of the NO occur satisfactorily. 

One other type of oxygen sensor has received considerable attention as an alternative to the galvanic type of sensor. This is the resistive type of sensor which uses a metal oxide whose resistance is dependent on the oxygen partial pressure (6). While a number of different oxides have been used, titanium oxide appears to have the best combination of properties for automotive applications (y. 

The principle of their operation is the same, but the method of implementation of the sensor is largely dependent on the conditions of the application. Thus, a zirconia sensor for measurement of 02 in molten steel (1,600°C) has to be designed in such a way that the thermal expansion coefficients of the different layers in this device are matched. On the other hand, a room-temperature potentiometric oxygen sensor can be constructed (Yamazoe et al., 1987) by using another set of materials ... 

The production of gas sensors. The production records of various types of gas sensors for past five years in Japan are listed in Table I except for the oxygen and humidity sensors. The sensors produced in the largest quantity are of the semiconductive type, followed by the catalytic combustion and thermistor types. These sensors have been mostly applied to domestic uses such as gas leakage alarms or gas control systems for LP gas and town gas which are extensively used for cooking and heating in Japanese houses. This is why these sensors are manufactured on a large scale. Other electrochemical sensors have been developed mainly to monitor other gases. 

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