Sensor, defined humidity

Therefore such sensors are called Nernstian sensors. As a reference air with defined humidity is used. In reducing gases that are in chemical equilibrium (e.g., H2, H2Oj CO, C02 water gas) the oxygen partial pressure is determined by the mass law constant Kv and this in turn depends on the temperature. In the case of H2,H20-mixtures the cell voltage is obtained by insertion of a temperature function of log Kp into the Nernst equation... 

Even if the receptor by itself has high accuracy, the sensor may be unable to execute the measurement in a defined place. Quality and total accuracy depend on the combination of receptor, the converter for measured values, and mechanical protection. Mechanical protection can take the form of pockets in water and fluid and also assembly boxes which protect against pollution, humidity, and temperature in the surroundings or against electromagnetic transmissions and noise from power-supplied pipes and cables. 

The sensor response at a defined relative humidity of 40% (23 °C humidifier temperature) is displayed in Fig. 5.13 for various microhotplate temperatures. AS in-... 

Temperature and/or humidity mapping of the chamber at the required set point is an integral part of OQ tests. Mapping includes the placement of temperature and humidity sensors at various locations within the chamber to demonstrate that the temperature and/or humidity will be consistent throughout different areas of the chamber under ideal conditions. Temperature and humidity readings should be obtained over a defined period, typically, a minimum of 24 h. 

Chemosensory applications will normally take place in an environment of complex composition. Humidity and other varying ambient conditions are in sharp contrast to the well-defined environment most typically found in related applications of imprinted polymers. Moreover, the trend in sensor technology towards miniaturisation, with the aim of future nano-scale dimensions, is a primary reason for rising perturbation sensitivity, such as new interfering forces that can be neglected in the macro range. Chemical sensors can be influenced by numerous factors, such as electrostatic effects (ChemFETs) or non-specific adsorption (SAW, surface plasmon resonance). 

Figure 20-24 shows the resistance of Zr02-MgO as a function of water vapor content (ppmw). The resistance decreases rapidly with an increase in water vapor from 10 to 10 ppmw. Compared with the ionic-type humidity sensor, the response of the semiconductor-type is rather slow because of the slow rate of chemisorption or the subsequent electron transfer process on the oxide surface. The microstructure of the elements as defined by surface area and average particle size, has a less pronounced effect on sensing characteristics than is the case in the ionic-type humidity sensors [31]. 

Sensor performance for different applications is defined by various features of the ceramic. For example, the electrical output of most pressure sensors is dependent on the bulk piezoelectric properties of a PZT ceramic. Oxygen gas sensor performance is defined by the conductivity behavior of Zr02 ceramics, which is in turn dependent on the oxygen vacancy content of the material. The performance of still other sensors, for example, ceramic thermistors, is dependent on the grain boundary characteristics of doped BaTi03 ceramics. For humidity sensors based on NiO/ZnO, the p—n junction characteristics of the interface define sensor performance. 

Sensitivity. As mentioned above, the sensitivity of a sensor is defined by the signal it generates, expressed in the concentration units of the substance measured. This corresponds to the slope of the corresponding calibration curve when the substance is the analyte, or to the so-called response curve for interferents. With some sensors the sensitivity rises to a maximum during the device s lifetime. A check of the sensitivity is therefore a valuable quality-assessment step. Intelligent sensors are expected to can y out such checks automatically in the course of routine performance tests. Since in most cases the sensitivity depends on such other parameters as the sample matrix, temperature, pressure, and humidity, certain precautionary measurements are necessary to ensure... 

In particular, Connolly et al. (2005) designed NH capacitive sensor with 500-nm-thick porous SiC film. The response in humidity was very low for RH<50 %, which was attributed to the porous dimensions. The exact sensing mechanism is still not clear, but NH levels as low as-0.5 ppm were detected. Porous alumina (AI2O3) has also been examined as a sensing material for capacitive gas sensors and in particular for humidity measurements (Nahar and Khanna 1982 Timar-Horvath et al. 2008). The Al Og-based humidity sensor was a volume-effect device based on physical adsorption. At low humidity, the walls of the pores are lined with one-molecular-thickness liquid layer. As the humidity increases, after saturating the walls, due to a capillary condensation effect, the water starts condensing in the pores (Boucher 1976 Neimark and Ravikovitch 2001). It was established that the water molecules, even at a partial pressure higher than the saturated vapor pressure tend to condense in capillary pores with a radius below the Kelvin radius r, which is defined as function (1) (Boucher 1976) ... 

The methods to measure the water vapour transmission through nonwovens and fabrics by using humidity detection sensors are defined in the following standards ... 

For example, the standard water cup methods defined in ASTM E96 are suitable for measuring fabrics of the WVTR less than 2627 g/m day in contrast, the methods using humidity sensors can measure the fabric having WVTR in a wide range this is 0.05—500 g/m day for method in ASTM E398, 0.005—1000 g/m day for ASTM F1249, and 500-100,000 g/m day for ASTM D6701. 

A traditional approach for CO2 determinations during ECMO is a sensor based on electrolytic conductivity. This principle was first described by Kempen and Kreuzer [32] in 1975. Gaseous CO2 penetrates through a semipermeable polymer membrane and dissolves in a defined water volume. The change of electrolytic conductivity depends on the CO2 concentrations. Compared to other sensor principles where humidity is a limiting factor, in this case a high humidity level is required in order to ensure the formation of a stable water film to prevent long-term drifts. 

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