It is vital to perform gas detection before entering hazardous areas, especially in confined Spaces. This paper introduces the working principle and application method of multi gas detectors. The hazards to be tested fall into three categories: toxic, asphyxiating and combustible. There are a variety of multi gas detectors on the market, but their sensors work using a similar principle. These multi gas detectors differ primarily in the options available and in their appearance. Although the use of the instrument is very simple, due to the development of electronic technology, proper interpretation of the results requires an understanding of possible interference.
Electrochemical Cell for Oxygen Detection
The cell consists of a leak proof container with a polymer membrane that selectively permits oxygen to diffuse inside. Reactions 1 and 2 (shown below) occur where °M° is a metal, usually lead for the anode and gold for the cathode. As shown in reaction 2, the anode being consumed controls the life of the cell. This sensor acts as both oxygen deficiency and abundance detector, 20.9% being the normal value. The quantity of current produced by the reactions is proportional to the partial pressure of oxygen in air and is linear from 0 to 25%. Life of the sensor is about 12-18 months, whether or not the cell is installed in the instrument.
(1) Reaction at cathode
O2 + 2H2O + 4e -> 4OH-
(2) Reaction at anode
M + 2OH- -> MO + H2O + 2e
Specificity of the electrochemical cells is obtained by selecting the appropriate sensing electrode, controlling the voltage of that sensing electrode or through the use of filters that selectively remove unwanted chemicals. Cells are available for a variety of toxic substances such as sulfur dioxide, hydrogen sulphide, carbon monoxide, chlorine, nitrogen dioxide, ammonia and several others. A sensor usually consists of three electrodes separated by a thin layer of electrolyte (sensing, counter and reference electrode). The toxic gas diffuses into the cell and is oxidized at the sensing electrode (example below for carbon monoxide) while the oxygen is reduced to water at the counter electrode. The current produced is compared to that of the reference electrode and the difference converted to the concentration of the toxic chemical. Readouts are usually given in ppm and the operating range is from 0 up to 2000 ppm. Most important, the chemical has to be known to be present in order to select the appropriate sensor.
(3) Reaction at counter electrode
O2 + 4H+ + 4e -> 2 H2O
(4) Reaction of reaction at sensing electrode
CO + H2O -> CO2 + 2H+ + 2e
The metal oxide semiconductor (MOS) sensor can be used to detect both toxic and combustible gases. It operates using a heated metal oxide semiconductor. The gas molecules adsorb onto the heated surface where an oxidation-reduction reaction occurs causing a change in the electrical conductivity of the metal oxide. This change is proportional to the concentration of the gas of interest. Very low concentrations of toxic gases can be detected. However, the sensor is not specific and will respond to a large number of chemicals. This non-specificity is taken as an advantage when the sensor is used as a screening tool to determine if toxic gases are present in the atmosphere. To obtain a quantitative readout with a MOS sensor, the instrument has to be calibrated properly and one has to know which compound is present in the atmosphere.
Combustible Gas Detectors
Combustible gas detectors usually provide readouts in % of LEL (Lower Explosive Limit), in the 0 to 100% range. The lowest concentration, in air, of ignitable vapours corresponds to 100% of the LEL. Therefore, a combustible gas which has a LEL of 100% at 4% volume in air would produce a readout of 50% of the LEL if only 2% of the volume of air is that gas.
The most popular sensor is the catalytic combustion sensor which consists of a Wheatsone bridge circuit containing a heated platinum filament. More recent models, called catalytic treated beads use a coiled platinum wire embedded in a porous ceramic bead. With both types of sensor, the gas, oxidized by the filament, creates a change in the electrical resistance which is proportional to the combustible gas concentration. The sensor res-ponds to any gas or vapour which burns in the presence of oxygen. However, a non-linear response is obtained with high concentrations of combustible gas or when incomplete oxidation occurs due to insufficient oxygen supply.
The sensor can also burn out in high concentrations of combustible gases and will not operate properly with oxygen concentrations below 16%. Coverage of the surface of the catalyst’s active sites by decomposition of poisoning compounds such as silicon, lead, phosphorus and halogen compounds can hinder the activity of the sensor. Operating life of the sensor is 24-36 months.
The concentration given by the instrument is true only if the gas being detected is the same as the gas used for the calibration of the instrument. Therefore, an instrument calibrated with pentane can only approximate the pre-sence or absence of other combustible gases. The alarm settings usually take this into account by being set at 10 to 20% of the LEL.
The principle of the MOS sensor or combustible gases is very similar to that of the MOS sensor for toxic gases. The sensor will not burn in combustible gas rich atmospheres, is not subject to poisoning and will still operate in a partially depleted oxygen atmosphere. However, as for toxic gases, the sensor is not specific and will produce a reading with any gas absorbed by the metal oxide. This type of sensor is not meant to identify unknown contaminants and will only give an approximation of the concentration of a chemical known to be present.
Features of multi gas detectors
multi gas detectors can continuously detect up to five hazards simultaneously. Most versions are micro-processor controlled. Every manufacturer offers a number of features such as data logging and a variety of sensors. Under normal use, the monitor operates in the diffusion mode but when determining air quality in remote or confined areas prior to entry, a built-in or external pump is preferred. The price depending on the number of sensors, special features and accessories.