FACTORS AFFECTING SENSOR
SENSITIVITY
There are a variety of factors that can cause a decrease
in the sensitivity of catalytic type combustible gas
sensors. The following information identifies the most
common substances that can have a detrimental effect
on the catalytic gas sensor. Under no circumstances
should these lists be considered as all inclusive.
Interfering or contaminating substances include
materials that can clog the pores of the sintered steel
flame arrestor and reduce the gas diffusion rate to the
sensor. Examples include:
1. Dirt or oil.
A dust cover or splash guard should be installed to
protect the flame arrestor. The dust cover may be
cleaned using an organic solvent and an ultrasonic
bath unless the contaminant is insoluble. Replace
dust cover if there is any doubt.
2. Corrosive liquids and vapors.
This can occur when substances such as H
2
S,
(hydrogen sulfide), Cl
2
(chlorine) or HCl
(hydrochloric acid) are present. A dust cover may
provide some limited protection. Routine calibration
frequency should be increased in applications
where corrosive materials are present.
3. Flame arrestor clogged as a result of painting or
cleaning.
The routine maintenance procedure should include
first powering down the system, then covering the
sensor with a plastic bag when painting or cleaning.
The bag should be removed as soon as possible
when the procedure is complete. Recalibrate the
sensors after re-powering and stabilization.
4. Polymer formation in the flame arrestor.
This can occur where monomeric vapors such as
1-3 butadiene, styrene, isoprene, etc. are present.
This may render the sensor dead.
Some substances can cover or tie up the active sites on
the catalytic surface of the active sensing element. This
occurs in the presence of volatile metal organics,
gases, hydride vapors, and volatile compounds
containing phosphorous, boron, silicon, etc.
Examples: Tetraethyl lead
Phosphine
Diborane
Silane
Trimethyl chlorsilane
Hydrogen fluoride
Boron trifluoride
Phosphate esters
Silicone oils and greases
RTV silicone sealants
Some substances react with the catalytic element metal,
forming a volatile compound. This erodes the metal
from the surface. With sufficient exposure, most or all of
the metal catalyst can be removed from the surface of
the active element of the sensor. Halogens and
compounds that contain halogens are materials of this
nature.
Examples: Chlorine
Bromine
Iodine
Hydrogen Chloride, Bromide or Iodide
Organic halides
Trichloroethylene
Dichlorobenzene
Vinyl chloride
Freons
Halon 1301
(Bromotrifluoromethane)
A brief exposure to any of these materials may
temporarily increase the sensitivity of the sensor due to
etching of the catalytic surface. This practice is not
recommended, since it is unreliable and may give a
false sense of security.
Exposure to high concentrations of gas for extended
periods can introduce stress to the sensing element and
seriously affect its performance. After exposure to a
high concentration of combustible gas, recalibration
should be performed and, if necessary, the sensor
should be replaced.
The degree of damage to a catalytic sensor is
determined by the type of contaminant, its concentration
in the atmosphere, and the length of time the sensor is
exposed. When a sensor has been exposed to a
contaminant or to a high level of combustible gas, it
should be re-calibrated at the time, followed by an
additional calibration a few days later to determine
whether a significant shift in sensitivity has occurred.
SPECIFICATIONS
TRANSMITTER
INPUT VOLTAGE—
With signal loop impedance option A: 10 to 30 vdc
With signal loop impedance option B: 17 to 30 vdc.
Refer to “Description” section for additional information
regarding impedance option.
Linear, regulated, filtered 24 vdc power source is
recommended.
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