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Chapter 1. Ex Chek
Description
1.1 Ex Chek capabilities
The Ex Chek is a single sensor gas detector
designed to detect combustible gases and
vapors. The Ex Chek includes numerous
features designed to meet specific user
requirements. This chapter will broadly
discuss the use and capabilities of the Ex
Chek.
1.2 Methods of sampling
The Ex Chek may be used in either diffusion
or sample-draw mode. In either mode, the
gas sample must enter the sensor
compartment for the instrument to register a
gas reading.
In diffusion mode, the atmosphere reaches
the sensor by diffusing through the sensor
port on the front of the instrument. Normal
air movements are enough to carry the
sample to the sensor. The sensor reacts
quickly to changes in the concentration of
the gas being measured.
It is also possible to use the Ex Chek to
sample remote locations with the hand-
aspirated sample-draw kit that is available
separately. During remote sampling, the
gas sample is drawn into the sensor
compartment through the probe assembly
and a length of tubing.
Use of the sample draw kit is covered in
section 2.4.
1.3 Combustible gas sensors
As an environment becomes contaminated
with combustible gases or vapors,
concentrations can climb until they
eventually reach ignitable or explosive
levels. The minimum amount of a
combustible gas or vapor in air that will
explosively burn if a source of ignition is
present is known as the Lower Explosive
Limit (LEL) concentration. Ex Chek
combustible gas readings are given in
percent LEL, with a range of zero to one-
hundred percent explosive. The Ex Chek
combustible gas sensor is non-specific and
responds to all combustible gases and
vapors.
Combustible sensors contain two coils of
fine wire coated with a ceramic material to
form beads. These two beads are strung
onto the opposite arms of a balanced
Wheatstone Bridge circuit. The "active"
bead is additionally coated with a palladium
based material that allows catalyzed
combustion to occur on the surface of the
bead. The palladium catalyst is not
consumed in the combustion reaction, it
simply enables it to occur. It is not
necessary for the combustible vapor to be
present in LEL concentrations in order for
this reaction to occur. Even trace amounts
of combustible gas present in the air
surrounding the sensor will be catalytically
burned on the surface of the bead.
The "reference" bead lacks the palladium
outer coating but in other respects exactly
resembles the active bead. A voltage is
applied across the active and reference
elements, causing them to heat. If
combustible vapors are present, the active
bead will be heated by the reaction to a
higher temperature. The temperature of the
untreated reference bead is unaffected by
the presence of gas. The difference
between the temperatures of the two beads
will be proportional to the amount of
combustible gas present.
Since the two beads are strung on the
opposite arms of a Wheatstone Bridge
electrical circuit, the difference in
temperature between the beads is perceived
by the instrument as a change in electrical
resistance.
It is important to note that catalytic "hot
bead" type combustible sensors require the
presence of oxygen (at least 10 percent by
volume) in order to detect accurately. A
