Sensor, defined vapor

The sensitivity of the sample or the sensor to vapor transfer must also be considered. Here, the quantity of material represented by the sample or by the sensor is important. Vapor pressure is established by the presence of a particular number of molecules in a defined volume of space. The transfer of water molecules into the vapor phase may cause a measurable change in the gravimetric water contents of the sample and sensor. It is necessary that the sample water content (or the initial sample weight) be known. It may also be necessary to... 

Multivariate A multivariate measurement is defined as one in which multiple measurements are made on a sample of interest. That is, more than one variable or response is measured for each sample. Using a sensor array to obtain multiple responses on a vapor sample is a multivariate measurement. 

The sensitivity of these sensors was defined as a signal change upon exposure to the known concentrations of vapors. Sensitivity of the 2.8-nm CdSe nanocrystals was 0.8 PL counts/Torr of methanol with almost no detectable sensitivity to toluene. The sensitivity of the 5.6-nm CdSe nanocrystals was 2.9 PL counts/Torr of methanol and 8.8PL counts/Torr of toluene. Although this environmental sensitivity was compatible with earlier reported sensors based on polished or etched bulk CdSe semiconductor crystals3940 and polymer-nanocrystals composites,16 the sensor reported here had a more selective response to polar and nonpolar vapors due to the multiwavelength PL from different-size nanocrystals incorporated into the polymer film. The response and recovery kinetics of PL from the 2.8-nm nanocrystals in PMM A upon exposure to methanol were very fast (<0.5 min). However, 5.6-nm nanocrystals in the same sensor film exhibited a much longer response and recovery times upon interactions with methanol, 4 and 20min, respectively. The 5.6-nm nanocrystals had 4-min response and 0.5-min recovery times upon interactions with toluene. 

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]. 

Chen and Luo demonstrated a multi-CPNW sensor approach which includes multiple precisely defined and individually addressable heterogeneous CPNWs. A collection of PPY, PEDOT, and CPs based on their variations were used to prepare multimodality chemical-sensor systems, which showed different responses to different gases or vapors with high sensitivities (Figure 10.49) [69]. Such a combinatorial approach can potentially provide finger-printing for different species for accurate and reliable detection of various chemical analytes. 

The downwind instrumentation grid of multiple levels of sensors was defined by three arrays, at 50, 150, and 250 m from the downwind end of the vapor barrier fence. On these downwind array towers Type K thermocouples were positioned at four heights on the 50-and 150-m arrays and three heights on the 250-m array, to measure cloud temperature as it moved downwind. The exeeption to this was the tower located on the centerline of the 150-m array, which had four levels of RTD sensors rather than thermocouples. Between tests 3 and 4, two towers and their associated instrumentation were moved from the 50-m array and added to the 150-m array to increase the width of the 150-m array. This was done based on the wider than anticipated clouds created during the first three experiments. 

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) ... 

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