tain point, the detector will fail. Also,
since the electrochemical H2S detector
requires a small amount of O2 to function, it will not work correctly for applications where O2 is absent for extended
periods (for example, for H2S detection
within a natural gas pipeline).
regarding cleaning, routine maintenance
and calibration.
Combined H2S/CO Sensor Designs
A commonly used variant of the electrochemical H2S sensor includes a second
working electrode for the detection of
CO. The sensor includes a single counter
electrode, a single reference electrode,
and one electrode each for H2S and CO,
thereby providing a separate signal for
each detected gas. Typically, once the
gas diffuses into the sensor, it first
reaches the H2S working electrode where
most of this compound reacts, and the
H2S working electrode is designed not to
respond to CO. The sampled air continues to diffuse into the sensor, where it
reaches the CO sensing electrode and the
CO concentration is then measured.
Four-electrode CO/H2S sensors usually
include an internal filter that limits the
amount of VOC interferents and H2S that
reach the CO electrode. However, a certain
amount of H2S is still expected to remain
in the gas that reaches the CO electrode.
While optimized for the detection of CO,
the CO sensing electrode responds to both
CO and H2S. Instruments deduct the effects of H2S on the CO electrode based on
the expected breakthrough ratio. Such a
system requires calibration with both
gases before use.
Cross-sensitivity
Some cross-sensitivity may occur with
electrochemical sensors. Depending on
the interfering chemical, either a positive
or negative detector response bias is possible. However, in the case of an electrochemical H2S sensor, because the relative
signal strength is much greater for H2S,
a very high concentration of oxidizing
gas (for example, Cl2) would be required
to materially affect sensor readings.
A greater concern is the positive interfering effect on electrochemical H2S
sensor readings from certain volatile organic compound (VOC) vapors, such as
limonene, methanol or other alcohols,
including those present in some hand
sanitizers and insect repellents. Always
remember to allow these liquids to dry
completely before handling gas detectors, and never use methanol to clean or
decontaminate gas-detecting instruments. Electrochemical sensors of all
types are designed with specific use
conditions and requirements in mind, so
always follow factory recommendations
Figure 3. Response of a typical electrochemical H2S sensor to a stable concentration
of 25 ppm H2S.
[Continued: 32]
H2S Monitoring Concerns
The lowering of the ACGIH TLV for
hydrogen sulfide in 2010 caused
concern among industrial hygienists
about their detection instruments.
Below, manufacturers share their responses to the following question
from The Synergist: “What is the
number one complaint you hear
from industrial hygienists about H2S
monitoring?”
Bryan Bates, President and CEO,
Gas Clip Technologies, Inc.:
It would have to be a tie between
two. The first complaint concerns the
problems associated with having to
switch out existing “disposable”-type
instruments in order to comply with
the changing TLV for H2S, and the
second is the hassle and cost asso-
ciated with enforcing facility SOPs
for required bump testing of H2S
monitors/detectors for contractors.
To solve these issues, we recom-
mend our customers use disposable
H2S detectors with alarm set points
that can be changed and an elec-
tronic “bump test” frequency alarm
LED-flashing indicator. Both func-
tions require a supervisory action
and cannot be changed accidentally
by the instrument user.
Bob Henderson, President,
GfG Instrumentation:
The most common question is about
the new 2010 TLV for H2S, and
whether or not an instrument can be
successfully used with take-action
alarm settings of 1.0 ppm. The an-
swer is a qualified “yes.” It all de-
pends on the specific instrument.
Some gas detectors already have the
necessary range and resolution to
permit users to set the instantaneous
or TWA alarm as low as 1.0 ppm (or
lower). In other cases it may be nec-
essary to update the operating soft-
ware. In some cases, however, it may
not be possible to upgrade older in-
struments to be capable of resetting
the alarms to the new limits. The only
way to determine whether this is the
case for a particular design is to ask
the manufacturer.
