Det-Tronics 505 Combustible Gas Detector Transmitter with Combustible Gas Sensor CGS - Manual (Page 5)

2 95-8472 3.1
SENSOR RESPONSE
Figure 2 shows the typical response of a catalytic gas
sensor to various levels of methane. Note that a reading
of 40% LFL will be given at 2.0% by volume methane and
also at 80.0% by volume methane, which is well above
the upper flammable limit. Although gas concentrations
above the upper flammable limit will not propagate a
flame, it is likely that somewhere in the protected area
there may be a flammable concentration.
All catalytic sensors require oxygen to accurately measure
combustible gas concentrations. Sensor response and
accuracy will decrease when the oxygen level is less
than 10%. Figure 3 shows the effect of oxygen enriched
and oxygen deficient atmospheres on the response of
a typical catalytic gas sensor. Do not use catalytic gas
sensors in areas where the oxygen level is less than 10%
by volume.
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 H2S,
(hydrogen sulfide), Cl2 (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)
0 2 4 6 8 10 12 14 16 18 20
100%
80%
60%
40%
20%
0%
MILLIAMPERES DC A1945
CALIBRATE MODE
LFL
PERCENTAGE
3.4
FAULT
Figure 1—Transmitter DC Current Output