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 |
Toxic Sensors
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.
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