CAPÍTULO III. MARCO JURÍDICO
3.2.1 Los fundamentos legales de la participación ciudadana
3.2.1.5. Ley General de Asentamientos Humanos, Ordenamiento Territorial y
The performance of a gas sensor is commonly governed According to the follow-ing parameters, Sensitivity, Selectivity and Stability, reversibility, the response time, recovery time and the reproducibility [171- 173]. These parameters are largely de-pendent on the nature of the sensitive material as well as its physicochemical and mi-crostructural properties. A typical response curve, that is, variation of resistance of
sensor with time on exposure and withdrawal of analyzing gas, is schematically de-picted in figure 2.3. The above parameters are detailed below:
Figure 2.3: Schematic response-curve of a chemical gas sensor.
2.4.1 Sensitivity
Sensitivity represents the ability of the component to perceive a variation in a physical and/or chemical property of the sensitive material under gaseous exposure.
Sensitivity is therefore a parameter that expresses the variation of the sensor response as a function of the gas concentration. Since the response of the gas sensors is general-ly nonlinear, the sensitivity is not constant. The sensitivity is generalgeneral-ly defined as the ratio of the resistance of the sensing element in the target gas to that in air, in the case reducing gases, and the inverted in the case the oxidizing gases. Usually, in order to be able to compare the sensitivities of sensors we use the relative response.
In the case the relative difference measurement, the relative response (S) is calcu-lated using the formulas [174, 175].
for the reducing gas:
% 100
2.1a g a
R R
S R
for the oxidizing gas:
% 100
2.2
g a g
R R R S
In the case the relative measurement, the relative response (S) is calculated using the formulas [175- 177].
for the reducing gas:
% 100
2.3a g
R S R
for the oxidizing gas:
% 100
2.4g a
R S R
Where (Ra) is the sensor resistance under the air at a given temperature (T) and (Rg) is the sensor resistance under the target gas at the same temperature (T).
A sensor is all the more sensitive if as a small variation in the concentration of the gas will cause a significant variation in the measured resistance. Sensitivity (or relative response) is one of the strengths of MOS sensors whose variation in resistance can be measured for concentrations of the order of ppm (parts per million) or even hundred ppb (part per billion) [178].
In MOS sensors the sensitivity is highly dependent on films porosity, its thick-ness, its morphology, its microstructure, operating temperature, presence of additives and crystallite size (Table 2.4). In order to improve it, it will be of great interest to work with the most appropriate sensing material in every case and reach its optimum detecting temperature (Figure 2.4). working with nanostructure materials will give a higher surface area in front of gas. Taking into account that sensing reactions take place mainly on sensor layer surface, the control of semiconductor grain size will be one of the first requirements for enhancing the sensitivity of the sensor.
Figure 2.4: Sensitivity variation as a function of temperature at a specific concentra-tion of gas, for the determinaconcentra-tion of operating Temperature and S values.
2.4.2 Operating temperature
It is the temperature that corresponds to maximum sensitivity (Figure 2.5), obvi-ously it is The operating temperature should be specified for any sensor. It is a func-tion of the semiconductor oxide composifunc-tion and of the gas which is to be detected (Figure 2.6). It is important to define this parameter because with respect to single gas, the sensor reaches the maximum of its sensitivity at a precisely fixed temperature.
Figure 2.5: Gas sensor responses of NiO sensor for different operating temperatures under 3000 ppm of H2 and CH4 and 75 ppm of NH3 [180, 181].
Figure 2.6: Sensors response for different gases, at the corresponding operating tem-peratures [182].
2.4.3 Selectivity
This characteristic is related to the discrimination capacity of a sensor towards a mixture of gases. Selectivity plays a major role in gas identification. Generally, a ‘fin-gerprinting’ method relies on a unique signature of the target gas signal. However, gases often produce very similar sensor responses (except when comparing reducing to oxidizing gases). For example, gases such as ethanol, carbon monoxide, and me-thane have appreciable cross-sensitivity that hinders the development of a domestic gas sensor that can distinguish these species [171]. Common techniques of improving the selectivity of gas sensors include controlling the sensor operating temperature, se-lective gas filters used in series with gas sampling, and using additives. Different oper-ating temperatures allow the control of the sensitivity toward a particular gas when there is a unique TC (critical temperature/optimum temperature with maximum sen-sor response) for each gas (Figure 2.7). thus allowing the sensen-sor to produce distin-guishable signals for two gases at a selected temperature. The film morphology and sensor architecture can also play a key role in selectivity [171]. Distinguishing poorly reactive gases from reactive gases can be facilitated by placing electrodes further inside the bulk of the sensing material to allow reactive materials to be filtered by the sensing material near the surface of the sensor. Another technique uses catalysts, which gener-ally reduce the operating temperature of a gas sensor for a particular gas species and thus allow the target gas to be distinguished from other gases due to the differences in sensitivity (Figure 2.8). In other words, addition of catalysts can maximize the sensi-tivity of target gases to produce a distinct signal.
Figure 2.7: Variation of the values and forms of response as a function of the tempera-ture variation for different gases having the same concentration.
Figure 2.8: Selectivity tests of gas sensors based on NiO films, the concentration of NH3 is 50 ppm, and the concentration of other organic gases is higher than
300 ppm [183].
2.4.4 Stability
It is a characteristic that takes into account the repeatability of device measure-ments after a long use. The success of the sensor will be limited if the sensor perfor-mance is not demonstrated as repeatable and stable over long-term testing (Figure 2.9). Problems of stability, may be attributing to three primary areas of concern. The first is that a surface conductive sensor can suffer from surface contamination. Second, changes in the sensor characteristics (such as intergranular connectivity) can occur due to thermal expansion coefficient mismatch or interfacial reactions at the metal electrode interface. Last the film morphology may change over time due to the rela-tively high operating temperatures of the sensor, which may change over time due to the relatively high operating temperatures of the sensor, which may also cause migra-tion of additives. To avoid the effects of non-repeatability after repeated use, the sensor materials are submitted to a thermal pretreatment; during these treatments samples are submitted to high calcinations temperatures to avoid instabilities during their working life.
2.4.5 Response time
The response time is the time interval over which resistance attains a fixed per-centage (usually 90%) of final value when the sensor is exposed to full scale concentra-tion of the gas. Time response is especially dependent on the sensor characteristics such as crystallite size, additives, electrode geometry, electrode position, diffusion rates, etc. A small value of response time is indicative of a good sensor.
Figure 2.9: Long-term stability of sensor signal to ozone gas with different concentrations [184].
2.4.6 Recovery time
This is the time interval over which the sensor resistance reduces to 10% of the saturation value when the sensor is exposed to full scale concentration of the gas and then placed in clean air. A good sensor have a small recovery time so that sensor can be used over and over again.
2.4.7 Reproducibility or repeatability
This parameter is probably the most important for both physical and chemical sensors. It is the ability of a sensor to give, under defined conditions, very similar re-sponses when repeatedly applying the same input signal, this includes retaining the sensitivity, selectivity, response, and recovery time. That is to say, we talk about the repeatability or reproducibility if the sensor responds to a gas in the same way regard-less of the number of measurements and the time between measurements. In other words, the repeatability of a gas sensor reflects its ability to produce the same response for a given gas atmosphere. Reproducibility can also be applied under changed condi-tions of measurement for the same measurand to check, that the results are not an arti-fact of the measurement procedures.
2.4.8 Linearity
Linearity is the relative deviation of an experimentally determined calibration graph from an ideal straight line [185].
3.5.9 Sensitivity limit or detection limit
This is the smallest value of the measurable quantity to be detected under given conditions, or minimum detectable target gas concentration, far from the range of un-certainty of experimentation, particularly at a given temperature (Figure 2.10) [185].
Figure 2.10: Percentage response as a function of (a) ethanol concentration and (b) working temperature, (c) limit of detection as a function of the sensor operating temperature (Polycrystalline NiO nanowires) and (d) selectivity plot at 200°C for 100
ppm of gas [186].
Table 2.4 shows the effect of some factors on the parameters of the chemical gas sensor. While the table 2.5 shows a comparative study among different large families of sensors.
(c) (d)
(a) (b)
Table 2.4: Some factors affecting the parameters of the chemical gas sensor [172]
.
Sensor
parameter Some influencing factors
Sensitivity
Nature of sensitive material
Structure (porosity, morphology, crystallinity ...)
Doping (copper, platinum, cobalt ...)
Operating temperature
Stability and Repeatability
Stability of the sensitive material
Operating temperature
Measurement Conditions (temperature, humidity, pressure...) Selectivity Doping
Selective filters
Operating mode
Reversibility Nature of sensitive material vis-à-vis gas to detect
Structure
Operating temperature
Table 2.5: Comparative study of the characteristics of the large families of sensors [164, 178]
.
Classification Semiconductor Catalytic Electrochemical Thermal conductivity Infrared absorption
Sensitivity ++ + + - - ++
Selectivity - - - + - - ++
Precision + + + + ++
Response time ++ + - - + -
Stability + + - + +
Durability + ++ - + ++
Maintenance ++ ++ + + -
Cost ++ ++ + + -
Portability ++ + - + - -
Integration in an
embedded system ++ + - + - -