Electrochemical gas sensors are gas detectors that measure the
concentration of a target gas by oxidizing or reducing the target gas at
an electrode and measuring the resulting current.
History
Beginning his research in 1962, Mr. Naoyoshi Taguchi became the first
person in the world to succeed in the development of a semiconductor
device which could detect low concentrations of combustible and reducing
gases when used with a simple electrical circuit. Devices based on this
technology are often called "TGS" (Taguchi Gas Sensors).
Construction
The sensors contain two or three electrodes, occasionally four, in
contact with an electrolyte. The electrodes are typically fabricated by
fixing a high surface area precious metal on to the porous hydrophobic
membrane. The working electrode contacts both the electrolyte and the
ambient air to be monitored usually via a porous membrane. The
electrolyte most commonly used is a mineral acid, but organic
electrolytes are also used for some sensors. The electrodes and housing
are usually in a plastic housing which contains a gas entry hole for the
gas and electrical contacts.
Theory of operation
The gas diffuses into the sensor, through the back of the porous
membrane to the working electrode where it is oxidized or reduced. This
electrochemical reaction results in an electric current that passes
through the external circuit. In addition to measuring, amplifying and
performing other signal processing functions, the external circuit
maintains the voltage across the sensor between the working and counter
electrodes for a two electrode sensor or between the working and
reference electrodes for a three electrode cell. At the counter
electrode an equal and opposite reaction occurs, such that if the
working electrode is an oxidation, then the counter electrode is a
reduction.
Diffusion controlled response
The magnitude of the current is controlled by how much of the target gas
is oxidized at the working electrode. Sensors are usually designed so
that the gas supply is limited by diffusion and thus the output from the
sensor is linearly proportional to the gas concentration. This linear
output is one of the advantages of electrochemical sensors over other
sensor technologies, (e.g. infrared), whose output must be linearized
before they can be used. A linear output allows for more precise
measurement of low concentrations and much simpler calibration (only
baseline and one point are needed).
Diffusion control offers another advantage. Changing the diffusion
barrier allows the sensor manufacturer to tailor the sensor to a
particular target gas concentration range. In addition, since the
diffusion barrier is primarily mechanical, the calibration of
electrochemical sensors tends to be more stable over time and so
electrochemical sensor based instruments require much less maintenance
than some other detection technologies. In principle, the sensitivity
can be calculated based on the diffusion properties of the gas path into
the sensor, though experimental errors in the measurement of the
diffusion properties make the calculation less accurate than calibrating
with test gas.
Cross sensitivity
For some gases such as ethylene oxide, cross sensitivity can be a
problem because ethylene oxide requires a very active working electrode
catalyst and high operating potential for its oxidation. Therefore gases
which are more easily oxidized such as alcohols and carbon monoxide
will also give a response. Cross sensitivity problems can be eliminated
though through the use of a chemical filter, for example filters that
allows the target gas to pass through unimpeded, but which reacts with
and removes common interferences.
While electrochemical sensors offer many advantages, they are not
suitable for every gas. Since the detection mechanism involves the
oxidation or reduction of the gas, electrochemical sensors are usually
only suitable for gases which are electrochemically active, though it is
possible to detect electrochemically inert gases indirectly if the gas
interacts with another species in the sensor that then produces a
response. Sensors for carbon dioxide are an example of this approach and
they have been commercially available for several years.
iSweek(http://www.isweek.com/)- Industry sourcing & Wholesale industrial products
没有评论:
发表评论