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Electrochemical Sensors
[Toxic gases & oxygen]
Delphian manufactures a number of systems which incorporate
electrochemical sensors including the 755 system for
hydrogen
sulfide, the 795 system for
carbon
monoxide and the 770 system for
sulfur
dioxide. These systems are available with
analog
controllers, digital controllers, as
standalone
systems or attached to our
SAGE system - our
computerized gas monitoring system.
(This article is reprinted with permission
from THE GAS MONITORING HANDBOOK
by Gerald L. Anderson & David M. Hadden, published by Avocet Press Inc in
1999)
Electrochemical sensors are used primarily to
detect oxygen and toxic gases. Each sensor is designed to be specific to the gas
it is intended to detect. Electrochemical sensors are essentially fuel
cells composed of noble metal electrodes in an electrolyte. The electrolyte is
normally an aqueous solution of strong inorganic acids. When a gas is detected
the cell generates a small current proportional to the concentration of the gas.
In its simplest form, an electrochemical
sensor consists of a diffusion barrier, a sensing-electrode (sometimes called
the working-electrode, measuring-electrode, or anode), a counter-electrode
(sometimes called the cathode) and an electrolyte. In an environment free of
chemically reactive gases, oxygen diffuses into the cell and adsorbs on both
electrodes. The result is a stable potential between the two in which little,
theoretically no, current flows. The cell’s chemical process at this point is:
O2 + 4H+ + 4e 2H2O
When a chemically reactive gas passes through
the diffusion barrier it is either oxidized (accepts oxygen and/or gives up
electrons) or reduced (gives up oxygen and/or accepts electrons), depending upon
the gas. The resulting potential difference between the two electrodes causes a
current to flow.
For instance, when carbon monoxide, a reducing
gas, diffuses to the sensing electrode, it is oxidized causing the potential of
the sensing electrode to shift in a negative (cathodic) direction. The cell’s
chemical process is:
Sensing-electrode
2CO + 2H2O 2CO2 + 4H+ + 4e-
Counter-electrode
O2 + 4H+ + 4e 2H2O
Cell reaction
2CO + O2 2CO2
The more modern form of toxic gas cell
utilizes a third electrode called a reference electrode. This electrode has a
stable potential from which no current is drawn. It is used to eliminate
interference from side reactions with the counter-electrode. In addition it
allows the sensing-electrode potential to be biased with respect to its rest
potential. Biasing is one method of controlling sensitivity to a particular gas.
In order to provide for extended storage, a shorting clip is connected across
the sensing and reference terminals. This short maintains the electrodes at the
same potential and keeps current from flowing through the cell.
The electrochemical cell typically consists of a casing containing an
electrolyte gel and three electrodes. The top of the casing has a gas permeable
membrane as well as a gas capillary. The electrodes are carefully constructed to
provide maximum sensitivity and long life, through a electrode construction
which allows more surface area. This allows a larger signal, a quicker response
and permits a smaller volume of electrolyte to provide the same life available
from large sensors. Each cell is constructed using special filters, electrodes,
and electrolytes to make the cell as specific as possible. Electronics should
provide appropriate bias current to eliminate interfering gas sensitivity.
Because electrochemical reactions, like all chemical processes, are temperature
dependent, electrochemical sensors should incorporate a sensitive temperature
sensor which the electronics use to compensate for temperature variations.
Characteristics
Interfering
Gases
Interfering gases, often called cross-sensitivities,
are gases other than the target gas which will cause the cell to react. Like
virtually any sensor, electrochemical cells are not completely specific. Despite
the use of carefully engineered electrodes and electrolytes, changing the
operating potential of the sensing electrode and chemical filters, it is
difficult to develop a catalyst which will not respond to a more active gas than
the target gas. In addition, the use of a filter may often slow the response of
the cell.
Blocking
Mechanisms
Blocking is a condition which causes the cell to
function poorly or not at all until the condition is removed. Normally the block
does not damage the sensor permanently as a poison would. Some of the most
common blocking mechanisms for electrochemical cells are:
Freezing the electrolyte
As the temperatures of the cell decreases, the
chemical reaction which the user sees as a signal decreases. At some point,
depending upon the electrolyte, the cell current stops. Usually, upon
returning to a normal temperature, the cell will reactivate. If you expect to
use an electrochemical cell in temperatures below its normal operating
temperatures, the cell should be heated. In general, the lowest temperature at
which a cell can be expected to function properly over long periods of time is
0C.
Oxygen Depravation
Oxygen is an essential ingredient in the reaction with gas. If the oxygen
supplied to the counter-electrode is cut off, the current can not be
sustained. Under normal conditions, when detecting small concentrations of gas
in ambient air, an adequate supply of oxygen can easily be achieved. Oxygen
deprivation can occur through a number of mechanisms:
In the event that the cell is exposed to
large concentrations of a reactive gas over a sustained period, or, if the
cell is flooded with one or more reactive gases, the available oxygen for a
reaction can easily be used up.
When measurements need to be made in a low
oxygen environment, a separate access of air to the counter-electrode must be
assured. Because of electrochemical sensor’s need for an adequate supply of
oxygen, it is often recommended that the oxygen content of air be sampled
first before relying on these sensors for safety.
If the cell's diffusion barrier (or flame arrestor) becomes
clogged or coated the normal supply of oxygen (as well as signal gas) may be cut
off. Sensors used in front of air inlets, exhaust fans, or in dusty areas are
likely to be clogged. A dust filter should be used and it
should be cleaned regularly to prevent the cell's oxygen supply, from being
depleted.
Vapor condensation on the cells diffusion
barrier (or flame arrestor) can also effectively cut off the oxygen (as well as
the signal gas). If the sensor temperature is lower than the atmosphere
temperature, it is likely that condensation will occur. To prevent this the cell
may need to be heated or the air being circulated to the cell may need to be
dried.
Gases which cause an opposite reaction
Gases or vapors which are electro-chemically reducible at the cathode will
cause a reverse reaction in sensor chemistry, which could mask the normal
operation of a sensor designed to detect an oxidizable gas. The same problem
will occur if an oxidizable gas were to cause a reaction in a sensor designed
to detect a reducible gas.
Poisoning
Mechanisms
A poison blocks and/or degrades the sensor’s
operation on a permanent basis. Prolonged exposure to a poison usually results
in permanently destroying the sensor. Most sensors are not directly poisoned by
a gas or vapor, but they may be poisoned indirectly. The most common are:
Solvent Vapors
High concentrations of solvent vapors which attack the plastic housing or
filters. The most common solvents which can cause problems (depending upon the
construction of the sensor) are Alcohols, Ketones, Phenols, Pyridine, Amines,
or Chlorinated solvents. Sensors used in these atmospheres may have a shorter
life.
High Temperatures
Continuous
operation at high temperatures (usually above 40oC [ 104F]) will
not only cause the electrolyte to dry out, but could cause the electrolyte to
boil. In addition, at temperatures above about 30oC, many sensors
begin to lose signal output reducing their span.
Altitude and pressure changes
Electrochemical sensors are not directly affected by
pressures within the normal range of ambient pressures + 10%. However,
sudden changes in pressure can cause more gas to be forced into the sensor
producing a current transient. These transients rapidly decay to zero as the
normal diffusion conditions are re-established. However, some transients could
trigger false alarms.
Humidity
Unlike many solid state or semiconductor devices,
electrochemical sensors are not affected directly by humidity. However,
continuous operation below 15% or above 90% relative humidity can change the
water content of the electrolyte affecting the operation of the cell. This
process occurs very slowly and depends upon the temperature, the electrolyte,
and the vapor barrier. The major problem with increased humidity occurs when the
volume of electrolyte exceeds the available free space, causing the cell to
leak. In addition, however, an increase in moisture content can also cause the
electrolyte to freeze more quickly. In dry conditions, the acid content of the
electrolyte can rise, causing crystallization, or allowing the acid to attack
the seals. In general, high temperature and low humidity conditions are most
likely to result in a poisoned sensor.
Sensor Life
Electrochemical cells are active even when stored with
a shorting clip and therefore have a limited natural life even without use.
Their normal life expectancy may be up to three years from the date of
manufacture. If they are being stored, it is advisable to keep them in a
refrigerator rather than storing them at ambient temperature.
Sensor life can be shortened by a variety of
environmental factors such as low humidity, high temperatures and exposure to
poisons.
Exposure to a signal producing gas destroys a small portion of the electrolyte,
therefore continuous exposures to the target gas or any interfering gas will
shorten the cell’s useful life. The in-board filters also have a limited life.
Prolonged exposure to gases being removed by the filter will shorten its
effective life.
Advantages
very linear
good selectivity
excellent repeatability and accuracy.
Disadvantages
limited temperature range and sensitive to changes in
temperature
humidity extremes can destabilize the sensor
sensitive to EMF/RFI
limited storage life
slow start-up if depolarized
care should be used when handling a cell as the electrolyte contains a strong
acid.
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, Northvale, N.J.,
U.S.A.
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