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TN-144: Handling LEL Sensor Poisons Technical Note TN-144
rev 1 wh.03-99



1
RAE Systems Inc.
3775 N. First St., San Jose, CA 95134-1708 USA
Phone: +1.888.723.8823
Email: raesales@raesystems.com
Web Site: www.raesystems.com

Handling LEL Sensor Poisons

It is well known that certain chemicals inhibit or
poison the catalytic bead in LEL (combustible)
sensors, leading to partial or complete loss of
sensitivity. Poisoning is defined as a permanent
degradation, while inhibition is usually recoverable
by exposure to clean air. For the best sensor
performance, it is crucial for both instrument users
and manufacturers to avoid operation of the devices
in an environment containing poisons and inhibitors.

How LEL Sensors Work
The RAE LEL sensor consists of a matched detector
and compensator made from coils of platinum wire
embedded within a catalytic bead. Oxidation of
combustible gases and vapors releases heat, changing
the resistance of one element with respect to the
other, which is detected as an out-of-balance voltage
in a bridge circuit. The output signal is used for the
detection of combustible gases and vapors. The RAE
Systems LEL sensor is one of the most poison-
resistant sensors on the market, but special care to
avoid exposure to poisons is required to assure long
sensor life.

LEL Sensor Poisoning
The most serious poisons are silicon compounds,
such as silanes, silicones and silicates. Just a few
parts per million (ppm) of such materials are
sufficient to degrade the sensing performance. These
compounds are used in a wide range of products,
including lubricants, mold-release agents, polishes,
adhesives, cosmetic and medicinal creams, silicone
rubbers (including caulking and sealant compounds),
and others.

Lead compounds, such as tetraethyl lead in gasoline,
is a well-known poison which reduces LEL
sensitivity, especially for combustibles with high
ignition temperatures, like methane.

High concentrations of halogenated hydrocarbons are
thermally decomposed to HCl on the catalytic bead,
which can cause corrosion of the sensor and low
signal readings. H
2
S and other reduced sulfur
compounds, such as carbon disulfide, dimethyl
disulfide, and trimethyl disulfide, as well as
phosphate esters, nitro compounds (e.g.,
nitromethane, nitroethane, and nitropropane), are
oxidized to mineral acids that may be corrosive to the
sensor. Hot organic acids (such as acetic acid), and
direct exposure to acid gases (e.g., HCl, sulfuric acid
vapors) can also corrode sensors. Chlorinated
hydrocarbons are frequently found in solvents
including degreasing and cleaning agents. Harmful
chlorinated substances are also given off by
overheating some polymers and plastics such as PVC
insulated wire during soldering.

The forementioned materials are known to have a
detrimental effect on the catalytic bead. Silicones are
usually considered as one major poison source, and
H
2
S as an inhibitor. However, most compounds fall
into both categories to various degrees. Some
compounds may chemically react near the catalytic
bead at elevated temperature. This makes the poison
mechanism more complicated.

Operation Precautions
To keep the sensor operating for a longer time,
special attention should be made to reduce exposure
of the sensor or monitor to poisonous environments.

Here are some tips:

Follow the precautions below for Assembly
and Service.
Always use a recommended filter in front of
the sensor, and change the filter weekly, or
immediately after the exposure to a poison.
Clean the pump and replace the tubing, filter,
and gasket after a serious exposure to poisons.
Reduce the exposure time of the sensor in
poison surroundings, and turn the monitor off
when not in use.
Decrease the flow rate of gases over the sensor
during operation in poison environments, or
use a diffusion-type monitor instead.

Assembly and Service Precautions
To avoid introducing poisons to the monitor:

Do not use any molded plastic parts that have
been manufactured using silicone mold release
agents. Technical Note TN-144
rev 1 wh.03-99



2
RAE Systems Inc.
3775 N. First St., San Jose, CA 95134-1708 USA
Phone: +1.888.723.8823
Email: raesales@raesystems.com
Web Site: www.raesystems.com

Do not use silicone rubber, elastomer, or
sealants for gaskets, potting or insulation.
These materials are particularly hazardous
during mixing and setting, when they give off
substantial amounts of vapor. Avoid operating
the monitor in areas where these compounds
are mixed or are curing.
Do not use polishes, cleaners or lubricants
containing silicones in the area where the
instruments are assembled, tested or stored.
Most domestic furniture polishes contain
silicones.
Assemblers and service technicians should not
use cosmetic or medicinal hand creams
containing silicones.

Silicone lubricants are common in valves and
regulators used in gas-distribution systems.
Never use these components with LEL sensors.

Epoxy and adhesives must be poison-free.
Avoid using self-stick labels on the monitor or
any internal parts. Many adhesives used on
labels contain silicones.
Only use RAE Systems parts for replacement.

With these precautions, your RAE Systems
combustible sensors will operate reliably in RAE
Systems gas monitors. Follow the instructions in the
operations manual for routine maintenance. Always
test gas monitors before use.