Calorimetric gas sensors

Miura N (1991) New-type calorimetric gas sensor using temperature characteristics of piezoelectric quartz crystal fitted with noble metal catalyst film. Sens Actuators B 5 211-217 Monz6n-Hemdndez D, Luna-Moreno D, Martfnez-Escobar D (2009) Fast response fiber optic hydrogen sensor based on palladium and gold nano-layers. Sens Actuators B 136 562-566 Mueller WM, Blackledge IP, Libowitz GG (1968) Metal hydrides. Academic, New York, NY Noh J-S, Lee JM, Lee W (2011) Low-dimensional palladium nanostmctures for fast and rehable hydrogen gas detection. Sensors 11 825-851... 

As can be seen in Table 11.1, noble metals Pt, Pd, and Rh are the most usable catalysts in calorimetric gas sensors designed. It was established that of all the catalysts known they have the highest activity with respect to oxidation of combustible gases (see Fig. 11.2) and provide acceptable operation temperatures. Morooka and coworkers (Morooka and Ozaki 1966 Morooka et al. 1967) showed that activity for a model reaction, propylene oxidation, correlates with the strength of the metal-oxygen (M-0) bond. Because an LEL sensor must oxidize all ambient hydrocarbon species, the highest activity catalysts hold the most promise for the application. Therefore, the choice of palladium and platinum and sometimes rhodium for application in combustion gas sensors is natural (Miller 2001). This explains why the automobile exhaust system is treated with platinum or palladium compounds and is called a catalytic converter. 

Table 11.1 Catalyst typically used in calorimetric gas sensors...

Futjes P, Adam M, Ducso C, Zettner J, Barsony 1 (2005) Thermal effects by the sensitive coating of calorimetric gas sensors. Sens Actuators B 111 96-101... 

Riegel J, Hardtl KH (1990) Analysis of combustible gases in air with calorimetric gas sensors based on semiconducting BaUOj ceramics. Sens Actuators B 1 54-57... 

Figure 5. Calorimetric gas sensor on polyimide foil (a) fabricated thin-film resistances with contact pads on polyimide (b) thin-film resistances with SU-8 photo resist as passivation film and manganese(IV) oxide as catalyticaUy active layer (chip size 10 x 10 mm )[40].

Kirchner P, Oberlander J, Friedrich P, Berger J, Rysstad G, Keusgen M, Schoning MJ (2010) Realization of a calorimetric gas sensor on polyimide foil for applications in aseptic food industry, Procedia Engineering. 5 264 267. 

Based on the operating principle of the transducer, there are different types of gas sensors electrical, optical, mass, magnetic, and calorimetric gas sensors [2]. 

Semiochemicals Sensor, calorimetric Sensor, gas Sensor, optical Separation, cafeine Separation, CO2... 

Micro-calorimetric complementary metal oxide semiconductor gas sensor... 

Catalysts Used in Calorimetric (Combustion-Type) Gas Sensors. 287... 

As we here are mainly interested in adsorption measurement techniques for industrial purposes, i. e. at elevated pressures (and temperatures), we restrict this chapter to volumetric instruments which on principle can do this for pure sorptive gases (N = 1), Sect. 2. Thermovolumetric measurements, i. e. volumetric/manometric measurements at high temperatures (300 K - 700 K) are considered in Sect. 3. In Section 4 volumetric-chromatographic measurements for multi-component gases (N>1), are considered as mixture gas adsorption is becoming more and more important for a growing number of industrial gas separation processes. In Section 5 we discuss combined volumetric-calorimetric measurements performed in a gas sensor calorimeter (GSC). Finally pros and cons of volumetry/manometry will be discussed in Sect 6, and a hst of symbols. Sect. 7, and references will be given at the end of the chapter. 

For gas sensor classification, various approaches can be used. For example, taking into accoimt transduction mechanisms, we can distinguish six general categories of sensors (1) optical sensors, (2) electrochemical sensors, (3) electrical sensors, (4) mass-sensitive sensors, (5) calorimetric sensors, and (6) magnetic sensors (see Table 1.11). 

Casey V, Cleary J, D Arcy G, McMonagle JB (2003) Calorimetric combustible gas sensor based on a planar thermopile array fabrication, characterisation, and gas response. Sens Actuators B 96 114-123 Chadwick B, Tann J, Brungs M, Gal M (1994) A hydrogen sensor based on the optical generation of surface plasmons in a palladium alloy. Sens Actuators B 17 215-220... 

Figure 7.9 Scheme of the aneroid dynamic combustion calorimeter designed by Adams, Carson, and Laye [77], A jacket B jacket lid C motor that drives the rotation of calorimetric system D rotation system E bomb (which is also the calorimeter proper) F channels to accommodate the temperature sensor, which is a copper wire resistance wound around the bomb G crucible H electrode I gas valve. Adapted from [77]. 

The third block in Fig. 2.1 shows the various possible sensing modes. The basic operation mode of a micromachined metal-oxide sensor is the measurement of the resistance or impedance [69] of the sensitive layer at constant temperature. A well-known problem of metal-oxide-based sensors is their lack of selectivity. Additional information on the interaction of analyte and sensitive layer may lead to better gas discrimination. Micromachined sensors exhibit a low thermal time constant, which can be used to advantage by applying temperature-modulation techniques. The gas/oxide interaction characteristics and dynamics are observable in the measured sensor resistance. Various temperature modulation methods have been explored. The first method relies on a train of rectangular temperature pulses at variable temperature step heights [70-72]. This method was further developed to find optimized modulation curves [73]. Sinusoidal temperature modulation also has been applied, and the data were evaluated by Fourier transformation [75]. Another idea included the simultaneous measurement of the resistive and calorimetric microhotplate response by additionally monitoring the change in the heater resistance upon gas exposure [74-76]. 

Gas test measurements were performed with this device, and a novel sensor operation mode was developed and successfully demonstrated (Sect. 4.5) calorimetric-type signals could be recorded as changes in the source-gate voltage of the transistor while maintaining a constant microhotplate temperature. 

Recently, the influence of gas adsorption on physical properties of carbon nanotubes has attracted a considerable interest. It is caused by a possibility to create gas nanosensors [1,2]. The analysis of calorimetric data has shown that the gas adsorption in space between nanotubes gives the main effect on physical properties [3]. The measurement of the conductivity is simple and convenient method to detect a response of multiwalled nanotubes with respect to an external influence. This response can be used to construct a resistive sensor. 

In this section, the fundamental principles of fluid flow and heat transfer are introduced. Additionally, two representative approaches in flow velocity measurement using pulse modality flow sensors and calorimetric-type gas flow sensors are discussed. There are a number of notations found in the text Table 1 lists the notations and their respective symbols. Moreover, the articles found in the cross-references list are useful for further understanding of the topics discussed here. 

---