Using Electrochemical Sensors
FOR TOXIC GAS MEASUREMENT
GAS Detection
Author Details
Robert E. Henderson
Vice President, Business Development
BW Technologies, 2840 - 2 Avenue S. E.
Calgary, AB, Canada T2A 7X9
Tel: (403) 248-9226 Fax: (403) 273-3708
Website: www.gasmonitors.com E-mail: bhenderson@bwtnet.com
Electrochemical sensors are one of the most common types of sensors used in portable gas detectors. Multi-sensor confined space
monitors generally contain an oxygen sensor, a flammable/combustible sensor and one to three additional electrochemical sensors for
specific toxic gases. Single-sensor instruments equipped with electrochemical toxic sensors are also extremely popular for use in
situations where a single toxic hazard is present. In spite of the very large number of electrochemical toxic sensors in use, there is still
a lot of misinformation and misunderstanding when it comes to the performance characteristics and limitations of this very important
type of sensor.
How Electrochemical Sensors Detect Gas
Substance-specific electrochemical sensors are available for many of the most common toxic
gases including hydrogen sulphide, carbon monoxide, sulphur dioxide, chlorine, chlorine dioxide,
ammonia, phosphine, ethylene oxide, nitrogen dioxide, ozone and others. "EC" sensors are
compact, require very little power, exhibit excellent linearity and repeatability, and generally have
a long life span. The detection technique is very straightforward in concept. Gas that enters the
sensor undergoes an electrochemical reaction that causes a change in the electrical output of the
sensor. The difference in the electrical output is proportional to the amount of gas present. EC
sensors are usually designed to minimise the effects of interfering contaminants, making the
readings as specific as possible for the gas being measured.
Figure 1 illustrates the major components included in a typical electrochemical sensor. The
gas enters the sensor through an external diffusion barrier that is porous to gas but nonporous to
liquid. Many sensor designs include a capillary diffusion barrier that limits and controls the amount
of gas that enters the sensor. The sensing electrode is designed to catalyse a specific detection
reaction. Depending on the sensor, the substance being measured is either oxidised or reduced at
Figure 1: Major Components of a Typical Electrochemical Sensor
the surface of the sensing electrode. This reaction causes the potential of the sensing electrode
to rise or fall relative to that of the counter electrode. Current collector wires or filaments connect
the electrodes with the external pins of the sensor. The instrument supplies power to the sensor,
and interprets the output of the sensor by readings obtained through the external pins.
Electrochemical sensors are stable, long lasting, require very little power and are capable of
resolution (depending on the sensor and contaminant) to ± 0.1 PPM or even lower.
Electrochemical sensors are normally usable over a wide range of temperatures, in some cases
from - 40 to 50 °C (- 40 to 120 °F). However, the uncorrected sensor output may be strongly
influenced by changes in temperature. For this reason instruments generally include temperature
compensating software and/or hardware for the EC sensors installed.
The simplest sensor designs use a two-electrode system. In two-electrode designs, the potential
of the sensing electrode is compared directly to that of the counter electrode. In three electrode
designs, what actually is measured is the difference between the sensing electrode and reference
electrode. Since the reference electrode is shielded from any reaction, it maintains a constant
potential. This provides a true point of comparison. The change in potential of the sensing
electrode is due solely to the concentration of gas. The current generated by the sensor is
proportional to the amount of gas present. The amount of current generated per ppm (parts-permillion) of gas is constant over a wide concentration range. This consistency in output over a wide
range explains the exceptional linearity of three-electrode electrochemical sensors.
Why H2S Sensors Don’t Wear Out Even When Exposed to High
Concentrations of Gas
Chemical equations can be a little daunting, but working through a typical detection reaction is
well worth the effort. The oxidation of H2S in an electrochemical sensor provides a good example
of the detection mechanism used in a non-consuming electrochemical sensor design:
H2S is oxidised at the sensing electrode:
H2S + 4H2O H2SO4 + 8 H+ + 8 eThe counter electrode balances out the reaction at the sensing electrode by reducing oxygen
from the air to water:
2O2 + 8 H+ + 8 e- 4H2O
Each molecule of H2S that is oxidised at the sensing electrode produces a current
flow of eight electrons. The amount of current produced is a function of the
number of H2S molecules that are oxidised at the sensing electrode. For
every 1.0 ppm of H2S in the atmosphere being
monitored, the sensor shows a raw electrical
output of 0.7 mµA (micro amps). This
relationship is linear over a very wide range
such that 10 ppm produces 7.0 mµA, 100
ppm produces 70.0 mµA and so on. The
working efficiency of the sensing
electrode is very high. This means the
sensor is usually easily able to oxidise
Figure 2: Electrochemical sensor
equipped personal gas detectors
can last up to two years without
requiring battery replacement or
calibration adjustment
AET Buyers’ Guide 2005
the incoming H2S as fast as it reaches the sensing electrode. If the concentration of incoming gas
exceeds the ability of the sensing electrode to oxidise the gas, the sensor becomes saturated, in
which case the output reaches a maximum value and can't rise any higher. However, as soon as
the concentration of gas in the atmosphere drops below this critical concentration, the sensor
rapidly recovers with no damage done to the sensor.
The sulphuric acid produced in the reaction simply accumulates in the sulphuric acid
electrolyte. Water from the electrolyte is used, but is regenerated during the course of the
reaction. The only materials consumed during the detection reaction are the molecules of
hydrogen sulphide, power from the battery of the instrument and oxygen. As long as the sensor
is located in an atmosphere containing even trace amounts of oxygen, the sensor will be able to
replenish itself directly from the atmosphere. This is the reason that non-consuming
electrochemical sensors have such long life spans. The lifespan of the sensor is not affected by
exposure to the contaminant that it measures. No part of the sensor is consumed during the
detection reaction. You can expose the sensor to H2S calibration gas every single day without
shortening or affecting the lifespan of the sensor.
Similar non-consuming reactions are used for the detection of a variety of other toxic gases
including carbon monoxide, sulphur dioxide, chlorine, chlorine dioxide, nitrogen dioxide, ozone,
phosphine and most other gases detected by means of electrochemical sensors, (more on the
exceptions later). Because the electrolyte contains a certain amount of dissolved oxygen, for short
periods, non-consuming sensors can detect the contaminant they are designed to measure even
in the absence of oxygen. This is fortuitous since many reactive gases (such as chlorine) have
very short shelf lives when packaged in calibration mixtures that include oxygen. Gas mixtures
used to calibrate sensors for highly reactive gases, such as chlorine, frequently contain no
oxygen. For example, a typical calibration gas mixture used to calibrate a chlorine sensor might
contain 5 ppm of chlorine in nitrogen. The chlorine sensor has no trouble operating in an oxygen
free atmosphere for the duration of the calibration procedure.
Certain environmental conditions may limit use of this type of sensor. For instance, a nonconsuming electrochemical sensor would not be usable for long-term monitoring for H2S in an
environment containing zero percent oxygen. Once all of the oxygen available in the electrolyte
is consumed, the sensor will lose the ability to respond to hydrogen sulphide. When re-exposed
to an oxygen-containing atmosphere, however, the sensor will regain its ability to detect H2S.
Another problem is prolonged exposure to extremely dry conditions. Water in the sensor
electrolyte is not consumed, but is necessary for the detection reaction to proceed. If sufficient
moisture is lost from the electrolyte through evaporation, the sensor may not be able to detect
gas. Because the sensor electrolyte contains moisture to begin with, short-term exposure to very
dry conditions does not generally cause damage or interfere with the proper operation of the
sensor. The sensor is normally able to replenish any water lost through evaporation as long as the
atmosphere contains even trace concentrations of water. However, when operated for prolonged
periods in excessively dry conditions (such as in a stream of chemically dried air) sensors can
eventually dry out to the point that the damage may not be recoverable.
Figure 3: Compact electrochemical sensors are available for a wide variety of common toxic gases
Figure 4: Compact dual-channel “COSH” type sensors
help make it possible for a multi-sensor instrument for
O2, combustible gas, CO and H2S to be small enough
to wear on a shirt pocket
Effects of Interfering Gases
One of the chief limitations of electrochemical sensors is the effect of interfering gases - the ones
that you are not trying to measure with the sensor - on the sensor readings. Substance-specific
sensors are ideally supposed to respond only to the gases they are supposed to measure. The
higher the specificity of the sensor, the less likely the sensor will be affected by other gases. The
composition of the electrodes and type of electrolyte, as well as the use of selective filters for the
removal of interfering gases are all ways to increase the specificity of the sensor.
For instance, on the inside, a CO sensor is very similar to a sensor used to measure H2S. The
trick is to keep the H2S from reaching the CO sensing electrode. Most substance-specific CO
sensors include an internal activated carbon filter designed to remove the H2S and other acid gas
interferents before they reach the sensing electrode. Thus, the reading of the sensor is not affected
by the presence of H2S in the atmosphere being monitored.
While inclusion of a filter is frequently able to increase specificity, removal of a filter may be
used to broaden response to a wider variety of gases. For instance, carbon monoxide sensors that
do not include a filter are sometimes marketed as "dual purpose" sensors for the simultaneous
detection of both CO and H2S. This type of sensor responds to both CO and H2S, but cannot tell
them apart. The sensor produces a single signal, which is up to the instrument user to interpret.
Even though care has been taken to reduce cross-sensitivity in substance-specific designs,
interferences still exist. In some cases, the interfering effect is positive and results in readings that
are higher than actual. In other cases, the interference is negative and produces readings that are
lower than actual. It's important to understand clearly the effects of potential interferents on the
output of the sensors installed. Users should consult the owner's manual or contact the
manufacturer of the instrument they will be using to verify the correct values to use when making
decisions based on interfering contaminants.
"COSH" Type CO / H2S Sensors are
Really Two Sensors in a Single Housing
One of the most popular electrochemical sensors is the four-electrode "COSH" type design. This
type of sensor essentially packs two separate sensors for the measurement of CO and H2S into a
single housing. The sensor contains two separate sensing electrodes, one for CO and one for H2S.
Each sensing electrode provides an independent, substance-specific signal, and can be
individually calibrated. In order to increase specificity, the sensor is internally configured so that
incoming gas passes by the H2S electrode first. Hydrogen sulphide, which would otherwise have
an interfering effect on the CO sensing electrode, is removed via the electrochemical detection
reaction at the H2S electrode, and is not present in the gas that finally reaches the CO sensing
electrode. Thus, the sensor is able to differentiate between CO and H2S, with minimal interference
between the two contaminants on the sensor outputs.
Electrochemical sensors are among the most dependable, stable and reliable type of gas
detecting sensors available. But no sensor can detect gas unless it is used. The only way of being
sure that toxic contaminants are not present in dangerous concentrations is to look for them with
an atmospheric monitor designed for their detection. Understanding your instrument is important;
using your instrument is critical.
Special Types of Electrochemical Sensors
A bias voltage is sometimes applied to sensors used to detect less electrochemically active gases
such as hydrogen chloride, ethylene oxide (ETO) and nitric oxide. The bias voltage helps to drive
the detection reaction. Biased sensors may take a significant amount of time - up to 24 hours or
more in some cases - to stabilise completely when first installed in an instrument, or if the source
of power used to maintain the biasing voltage is interrupted.
Several other gases (such as ammonia and hydrogen cyanide) are detectable by less
straightforward reactions that consume parts of the sensor. In the case of a hydrogen cyanide
sensor, the sensor includes a gold sensing electrode. The gold in the electrode is consumed
during the detection reaction. Once all of the available gold is consumed, the sensor will need to
be replaced.
In the case of the ammonia sensor, it is the electrolyte that is consumed. The lifespan of
the ammonia sensor is directly related to its exposure to NH3. An ammonia sensor that
has a lifespan of one year when continuously exposed to 2 ppm of ammonia would last only 6
months when exposed to 4 ppm, or three months when exposed to 8 ppm, etc. This type of sensor
should be used only when the normal ambient background concentration of ammonia is
sufficiently low to allow a reasonable operational life. For example, this type of sensor should
not be used at a poultry farm or nitrate fertiliser plant where ambient concentrations of ammonia
may be as high as 20 to 30 ppm. In this environment the life span of the sensor could be a matter
of weeks.
About the author:
Robert Henderson is Vice President, Business Development for BW Technologies. Mr. Henderson
has been a member of the American Industrial Hygiene Association since 1992. He is Vice Chair
of the AIHA Gas and Vapour Detection Systems Technical Committee. He is also a current
member and past chair of the AIHA Confined Spaces Committee. He is also a past chair of the
Instrument Products Group of the Industrial Safety Equipment Association.
AET Buyers’ Guide 2005