by Martine Bissonnette, M.Sc. Chemist
Detection of chemicals when entering a hazardous area is of utmost importance, especially in confined spaces. The hazards encountered in confined spaces were discussed in detail in the Spring 1996 TDG Newsletter. This article describes the operating principles and use of multigas detectors. The hazards to be detected can be classified into three categories: toxic, asphyxiating and combustible. There is quite a variety of multigas detectors available on the market but their sensors operate using similar principles. The instruments differ primarily in the available options and cosmetic aspect. Although use of the instruments is very simple, thanks to electronic technology, knowledge about possible interferences is required for correct interpretation of the results.
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.
Operation of the Instruments
Instruments containing electrochemical cells should not be left out in temperatures below freezing since the electrolyte can freeze and cause permanent damage to the cell and to the electronic controls. Normal operating temperature range of the multigas monitors is from 0 to +40°C. Sensors should be allowed enough time to respond, the average being less than 20 seconds. However, if a sampling probe is attached more time should be allowed since the air sample has to travel the length of the probe before reaching the sensors. The multigas monitor should be allowed to run in a clean atmosphere for a few minutes when sampling is completed to ensure that the cells and the pumping system are flushed. It is very important that the instrument be calibrated at least every three months and after each use to ensure proper operation of the sensors. The multigas monitors are easy to use and do not require extensive training of the operators. Result interpretation is usually straightforward except for situations when many chemicals may be present or when the universal MOS sensor is used; one has to know which chemical was used to calibrate in order to be able to interpret the results. Use of electronic equipment such as UHF and VHF radios, especially if they are in the transmission mode, can cause interferences. Instrinsic safety is also of utmost importance because of the possibility of entering a potentially explosive atmosphere.
Features of Multigas Monitors
Multigas monitors 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 instruments cost anywhere between $800 and $6,000; the price depending on the number of sensors, special features and accessories. Replacement of the sensors costs approximately $250 for an oxygen sensor and up to $400 for a specific toxic gas sensor.
In a Nutshell
The multigas detectors provide an effective way of determining the safety of an area where dangerous goods are present. The instruments require low maintenance and are very reliable when properly calibrated. However, the operator has to be aware of the possible interferences when interpreting the results.
Publication: TDG Dangerous Goods Newsletter, Vol. 16, No. 3, Winter 1996.
* Martine Bissonnette is a former Emergency Response Advisor with CANUTEC.