A pellistor is a

solid-state Solid state, or solid matter, is one of the four fundamental states of matter. Solid state may also refer to: Electronics * Solid-state electronics, circuits built of solid materials * Solid state ionics, study of ionic conductors and their ...

device used to detect gases which are either

combustible A combustible material is something that can burn (i.e., ''combust'') in air. A combustible material is flammable if it ignites easily at ambient temperatures. In other words, a combustible material ignites with some effort and a flammable mat ...

or which have a significant difference in

thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...

to that of air. The word "pellistor" is a combination of pellet and

resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias activ ...

Principle

The detecting element consists of small " pellets" of

catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

loaded

ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelai ...

whose

resistance Resistance may refer to: Arts, entertainment, and media Comics * Either of two similarly named but otherwise unrelated comic book series, both published by Wildstorm: ** ''Resistance'' (comics), based on the video game of the same title ** ''T ...

changes in the presence of gas. Many of them require gentle heating in use, so they are four terminal devices with two connections for a small heating element and two for the sensor itself. Gas-Sensor

To avoid any risk of explosion, the sensitive element is usually enclosed in a wire mesh housing. More robust sensors for use in high risk environments may have solid steel housing with a gas port of sintered metal granules. Both of these work in a manner similar to the Davy safety lamp; gas may percolate through the permeable mesh, but the passages are too long and narrow to support the propagation of a flame.

History

The pellistor was developed in the early 1960s for use in mining operations as the successor of the flame safety lamp and the canary. It was invented by English scientist Alan Baker.

Types

Catalytic

The catalytic pellistor as used in the catalytic bead sensor works by burning the target gas; the heat generated produces a change in the resistance of the detecting element of the sensor proportional to the gas concentration.

Thermal conductivity

The

(TC) pellistor works by measuring the change in heat loss (and hence temperature/resistance) of the detecting element in the presence of the target gas.

References