Pellistor sensors

Pellistor sensors can be rapidly poisoned by exposure to contaminants such as silicones, tetraethyl-lead, phosphorus containing compounds and high concentrations of H2S, and they can be damaged by exposure to high concentrations of combustible gas. They can also simply lose sensitivity slowly over time, which is why it is so important to regularly test their response to known concentration calibration gas. 

Fortunately, there are alternative detection techniques that are not affected by these constraints. It is important to note that these alternative types of sensors should not be seen as replacements for the pellistor LEL sensor. Pellistor sensors are still the best and most cost-effective solution for many applications. 

Pellistor type combustible sensors and photoionization detectors represent complementary, rather than competing, detection techniques. Pellistor sensors are excellent for the measurement of methane, propane and other common combustible gases that are not detectable by means of a PID. On the other hand, PIDs can detect large VOC and hydrocarbon molecules that are effectively undetectable by pellistor sensors, even when the catalytic sensor is operable in ppm measurement ranges. The best approach for VOC measurement in many cases is to use a multi-sensor instrument equipped with both a pellistor LEL sensor and a PID sensor. 

Sensors following the principle of heat of reaction , so-called Pellistors , see Fig. 3.17, serve as indicators for flammable gases. With the catalytic conversion of fuel gases on the surface of a heated sensor, its electrical resistance changes proportionally to the concentration of the gas. 

Currently, pellistors are often used as guarding sensors in rooms where there is a risk of flammable gases leaking and causing explosion. Pellistors react to concentrations far below the explosion limits. As these pellistors have been specifically developed for this purpose, nearly all that are currently available work at an ambient temperature of below 50 °C. 

Pellistors are used to detect flammable gases like CO, NH3, CH4 or natural gas. Some flammable gases, their upper and lower explosion limits and the corresponding self-ignition temperatures are listed in Tab. 5.1. This kind of gas sensor uses the exothermicity of gas combustion on a catalytic surface. As the combustion process is activated at higher temperatures, a pellistor is equipped with a heater coil which heats up the active catalytic surface to an operative temperature of about 500 °C. Usually a Platinum coil is used as heater, embedded in an inert support structure which itself is covered by the active catalyst (see Fig. 5.33). The most frequently used catalysts are platinum, palladium, iridium and rhodium. 

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). 

CO Resistive sensors pellistors, metal-oxide sensors Optical sensors micro-spectrometer, IR-sources, IR-detectors, IR-filters Hybrid or integrated, surface micromachining Sn02 sintered thick film (Figaro, FIS,. ..), Sn02 thin and thick film on silicon (MiCS, Microsens) IR spectroscopy (Vaisala, Honeywell,. ..)... 

Exhaust gases Smoke Resistive sensors pellistors, oxygen-sensors, metal-oxide sensors Hybrid or integrated Hybrid or integrated... 

Microhotplates, however, are not only used for metal-oxide-based gas sensor applications. In all cases, in which elevated temperatures are required, or thermal decoupling from the bulk substrate is necessary, microhotplate-like structures can be used with various materials and detector configurations [25]. Examples include polymer-based capacitive sensors [26], pellistors [27-29], GasFETs [30,31], sensors based on changes in thermal conductivity [32], or devices that rely on metal films [33,34]. Only microhotplates for chemoresistive metal-oxide materials will be further detailed here. The relevant design considerations will be addressed. 

R. Aigner, M. Dietl, R. Katterloher, and V. Klee. Si-planar-pellistor designs for temperature modulated operation . Sensors and Actuators B33 (1996), 151-155. 

Catalytic (Pellistor) Flammable gases Air Measures the heat output due to the catalytic oxidation of flammable gas molecules. A stream of the sample is passed over the sensor which is usually a ceramic bead impregnated with Pt or Pd. The temperature variations in the sensor due to reaction are monitored. Dependent on individual design. Flammable gas detector. Usually portable... 

This is the reaction taking place at the surface of the thermal sensor, the pellistor, discussed in Chapter 3. An example of a biocatalyst is the enzyme glucose oxidase (GOD) which highly selectively promotes oxidation of D-glucose to gluconic acid. 

The coefficients for Pt are A = 4 x 10 3, B = 5.8 x 10 7, and po = 1 x 10-5 Q cm. With these parameters, the sensitivity, expressed as the temperature coefficient, is 0.4%°C 1 over a wide range of temperatures. Resistivities of other metals, as well as their temperature coefficients, are tabulated in standard reference tables (e.g., the CRC Handbook of Chemistry and Physics, 2006). Because the geometry of the resistor does not change with temperature, (3.8) is often written in terms of change of resistance R. Because of their chemical inertness and high temperature coefficient, platinum resistors are most common. They are the key part of the most successful thermal sensors, pellistors, which are discussed in Section 3.6.2. 

The CTL spectrum is different to the incandescent spectrum from solids (black-body radiation), and no luminescence is observed in an atmosphere without combustible gases. This has enabled highly sensitive gas detection together with the recent development of a photodetector with high sensitivity. This very sensitive gas sensor also has a stability and linearity similar to the Pellistor, and we call it the CTL-based gas sensor . 

Pellistor or catalytic bead sensors have a fairly long lifetime, and a wide temperature range. They require 5% to 10% of oxygen to operate. 

Electrochemical sensors, oxygen sensors, toxic sensors, pellistors, infrared sensors... 

Jones, E., The pellistor catalytic gas sensor , in Moseley, P. and Tofield, B. (eds.). Solid-state Gas Sensors, Institute of Physics, Bristol, 1987,17-31. 

In biomedical sensing, some of the solid-state devices based on thermal sensing cannot be used effectively. The reason is that the sensor itself has to be heated or is heated quite hot by catalytic surface reactions. Thus pellistors (oxides with catalytic surfaces and embedded platinum wire thermometer), chemiresistors, and Figaro sensor smoke detectors have not found many biologic applications. 

Low-temperature sensors surface plasmon resonance sensors pellistors biosensors High-temperature electrochemical gas sensors sensors for tough conditions electronic nose RT electrochemical sensors Membranes filters for aU types of sensors SAW sensors cantilever-based sensors... 

One example is the pellistor-type sensor first described by Baker (2) which is shown in Figure 12.2. This sensor uses palladium supported on thoria as the catalyst. This is deposited on the surface of a refractory bead of c. 1 mm diameter encapsulating the platinum coil. Palladium is more active than platinum for hydrocarbon oxidation and readily oxidizes methane at temperatures of about 500 °C. Encapsulation of the coil within a spherical bead in this way produces a device which is insensitive to orientation and also resistant to shock. This type of sensor is widely used in all types of flammable gas detection instruments. 

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