Methane sensors

In context with methane detection during offshore oil drilling, another infrared fiber optic methane sensor was reported25. The detector comprises 3 main units a microcomputer-based signal processing and control unit, a nonconducting fiber optic gas sensor, and an optical fiber cable module. The system operates at an absorption line of methane where silica fibers have very low losses. 

Methane Sensor. Methylomonas flagellata utilizes methane as its sole source of energy and oxygen is consumed by the respiration as follows ... 

Pt. Pd Octamethyl- cyclitetra- siloxane Decamethyl- Ambient 1 atm NR Deactivation of methane sensors used in coal mines 73... 

M.Kawahata and R.Lazzaro, Poisoning of Catalytic Methane Sensors , prepared for Bureau of Mines, Washington, DC, 1981. 

Basu S, Basu PK (2009) NanocrystaUine metal oxides for methane sensors role of noble metals. J Sensors 2009, 861968... 

Deng YQ, Neved TG, Ewen RJ, Honeybonme CL, Jones MG (1993) Sulfur poisoning, recovery and related phenomena over supported padadium, rhodiinn and iridium catalysts for methane oxidation. Appl Catal A 101 51-62 Ehrhardt JJ, CoUn L, Jamois D (1997) Poisoning of platinum surfaces by hexamethyldisdoxane (HMDS) application to catalytic methane sensors. Sens Actuators B 40 117-124 Firth JG, Jones A, Jones TA (1973) The principles of the detection of flammable atmospheres by catalytic devices. Combust Flame 21 303-311... 

Dutronc P, Lucat C, Menil F, Loesch M, Horillo MC, Sayago I, Gutierrez J, De Agapito JA (1993a) A potentially selective methane sensor based on the differential conductivity responses of Pd and Pt-doped tin oxide thick layers. Sens Actuators B 15-16 384-389... 

The variety of measurable substrates has been expanded by applying enzymes or bacteria as catalysts for the formation or consumption of redox-active compounds in the sensor. Glucose oxidase has been used for glucose sensors (Cronenberg et al. 1991), cultures of methane oxidizers in methane sensors (Damgaard and Revsbech 1997), and pure cultures of incomplete denitrifiers in nitrate and nitrite sensors (Larsen et al. 1997, 1996). 

Garda, M.L and Masson, M. (2004) Environmental and geologic application of solid state methane sensors. Environ. Geol., 46,... 

The retention time of the non-adsorbing methane (ti) is the measure of the column void volume or holdup. Ethylene is adsorbed by the catalyst, hence it does not reach the detector until the available surface is saturated, at which point ethylene breaks through and is detected by the sensor (t2). The adsorbed volume of ethylene is given simply by ... 

Mabbit, Allen, and Andrew Parker. Methane, and Carbon Dioxide Detection Using LED Sources. Sensor Rev. 16 (1996), pp. 38-41. 

A very recent example of the first case is presented by Vilaseca et al. [71] where an LTA coating on a micromachined sensor made the sensor much more selective to ethanol than methane. Moos et al. [72, 73] report H-ZSM5 NH3 sensor based on impedance spectroscopy using the zeolite as active sensing material. At elevated temperatures (>673 K) NH3 still adsorbs significantly in contrast to CO2, NO,... 

The thermal conductivity of methane is about twice as high as that of any other flammable compound of natural gas. Sensors for determining the methane number use this effect, and the principle is already in use for gas engines [2], as their performance depends heavily on the methane number. 

During the reaction of the hot catalyst surface with a flammable gas the temperature of the device increases. The Platinum coil itself serves at the same time as a resistance thermometer. The resistance increase of the coil then is a direct measure for the amount of combusted gas. Usually the amount of heat that develops during combustion is small and amounts to 800 kj/mol for methane, for example [8], Therefore the sensor is connected in a bridge circuit to a second resistor which shows the same setup as the pellistor but is catalytically inactive. The bridge voltage is then controlled by the temperature difference of the two sensors (see Fig. 5.34). 

The model analytes, which were used to show the sensor performance of the microsystems include carbon monoxide, CO, and methane, CH4. The sensor microsystems were designed for practical applications, such as environmental monitoring, industrial safety applications or household surveillance, which implies that oxygen and water vapors are present under normal operating conditions. In the following, a brief overview of the relevant gas sensor mechanisms focused on nano crystalline tin-oxide thick-film layers will be given. 

In the presence of humidity, the sensor signal for a given methane concentration decreases. This can be attributed to competing adsorption of water and methane molecules at the same sites, which holds particularly true for lattice oxygen sites [90]. 

Kim et al. have compared the hydrogen and methane sensitivity at 400-600°C of Pt and Pd-SiC Schottky diodes fabricated on n-type 6H-SiC. The Pd or Pt (<80 nm) was sputter deposited in 1-mm dots [8]. The sensitivity was measured as the change in current at a constant forward bias of 3V. The Pd-SiC Schottky diodes showed a higher sensitivity, as well as faster speed of response to both hydrogen and methane. The stability of the hydrogen response was tested for 30 days at 500°C and showed excellent results for both types of sensors. 

Hunter et al. demonstrated palladium-gate SiC Schottky sensors, but a drift was experienced when the sensors were operated at 350°C for a period of several weeks [4]. It was suggested that this was caused by a reaction between Pd and SiC. Hunter et al. tested two different approaches to remove this drift. First an alloy of Pd/Cr was tested, which caused the sensitivity to hydrogen to increase by two orders of magnitude. Second, SnO was tested as an interfacial layer under the Pd contact, which resulted an increased sensitivity to hydrogen, methane, and propylene. The longterm stability was substantially improved in both cases. 

---