Diffusion-limited oxygen sensor

For completeness it should be mentioned that some of the theoretical conclusions for SECMIT are analogous to earlier treatments for the transient and steady-state response for a membrane-covered inlaid disk UME, which was investigated for the development of microscale Clark oxygen sensors [62-65]. An analytical solution for the steady-state diffusion-limited problem has also been proposed [66,67]. 

Figure 9.4. Stem-Volmer plot of a fiberoptic oxygen sensor at different temperatures. , 43°C a, 37°C , 31°C , 25°C. The sensing capabilities of the fiberoptic sensor are limited by diffusion processes as suggested by the decreasing value of the lifetimes with increasing temperature. (From Ref. 21 with permission.)...

Non-Diffusion-Limited Sensor Structures. In this section, we will discuss a number of devices which do not require for their operation the existence of a barrier to the diffusion of oxygen. 

One device of this type was discussed by Hickam and Witkowski(23). In the diffusion limited devices that have been discussed, the flow rate of the gas is not normally an important factor. In the Hickam device, however, the gas flow is of paramount importance and new possibilities or complications arise. The structure consists of pump (upstream) and sensor (downstream) cells cylindrically surrounding a flowing stream of gas containing oxygen. The sensor cell EMF is fedback to the pump so that oxygen is either added to or subtracted from the stream in the amount required to keep the sensor EMF at a constant value. For a calibrated device, the amount of pump current required measures the oxygen content of the gas at the inlet of the structure provided the flow rate is held constant. Alternately, if a gas of constant composition were employed, the structure could be used to measure flow rate. 

Potential applications of chemical sensors are diverse and numerous, and the environment where the sensor is used varies. Therefore, chemical sensor often requires to be tailor-made or semi-tailor-made to meet the needs in the special circumstance. For example, sensing of oxygen in an automobile exhaust or in water or in blood can be accomplished by using an electrochemical-based sensor. However, the selection of electrolyte and a diffusion-limited layer or protective membrane will be different in each case. Therefore, the platform chemical sensor technology is discussed in general. Special applications of a chemical sensor under a particular circumstance need to be addressed separately. 

A sensor based on this reaction scheme has been described by Hahn [37]. This is a Clark type sensor which operates using a double pulse method. The first pulse is to a potential at which the reduction of oxygen occurs at its diffusion limited rate. The integrated current is used as a measure of oxygen concentration. The second pulse is to a potential where the superoxide ion is oxidised back to oxygen. In the absence of COj the integrated current should match that of the forward pulse. When CO2 is present the reverse pulse is diminished. The difference between the two pulses is therefore a measure of the amount of carbon dioxide present. The interested reader is referred to the review by Hahn for further details of this system. 

Independent of the brand of polarographic oxygen sensor, a general calibration procedure has to be followed. The geometry and the membrane of the sensor define, within limits, the diffusion layers and consequently the temperature dependence of the oxygen probe. 

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