being replaced by superior technologies such as
infra red (IR).
Immune to poisons such as H2S, for example, IR
technology is more robust and stable. The
increased accuracy of IR is also able to reduce
maintenance and calibration intervals. Now,
instead of the need for six-monthly or, in many
cases, the more frequent calibration intervals
required by catalytic devices, IR systems such as
the Draeger PIR3000 can extend calibration inter-
vals to one year. Together with an overall life
expectancy of more than 15 years, this technology
dramatically reduces lifetime ownership costs.
The use of open path IR technology, where a
multiple beam of infra red is transmitted over a
distance of up to 200m to a separate receiver, can
have other advantages. Where large areas need to
be covered in perimeter monitoring, for instance,
it can significantly reduce cabling costs.
These advances in technology now mean that
even users of old pellistor systems, regardless of
manufacturer, can upgrade to IR technology.
Designed to detect flammable gases and
vapours, the ATEX approved DraegerSensor IR,
for example, has been specifically developed to
allow existing pellistors or catalytic bead sensors
to be replaced easily, without creating technical
problems.
Even in the harshest of conditions, these new
poison resistant, fail-safe sensors are easy to
install, simply unscrew the traditional pellistor and
screw in the DraegerSensor IR.
The inclusion of a Wheatstone Bridge, which is
able to duplicate a standard pellistor output, also
means that existing cables and control devices,
regardless of manufacturer, can still be used.
Requiring no adaptation to either the control
system or the electrical wiring, this innovative
sensor features 3-wire connection which elimi-
nates the need for extra cabling. In addition,
because of the availability of metric M25 as well as
3
/4 NPT thread with the new sensor, it can even be
used with existing junction boxes.
Enhancing plant safety
Gas detection systems work by generating an out-
put signal that is proportional to the concentration
of the relevant gas. This is transmitted either as
analogue, using 4-20mA technology, or digitally
via HART or other protocols such as PROFIBUS or
Foundation Field Bus. Received by a controller, the
data will be displayed, stored and evaluated. The
controller will then initiate the alarm which might
entail a company-wide alert, the closing of valves
or a complete shutdown process.
For failsafe applications, open path gas
detection systems measure the concentration of
flammable gases across a multiple beam of IR
light, in units of LELm. Representing the integral
gas concentration along a line of sight, this does
not give the concentration of the substance but,
instead, provides the amount of gas present in the
atmosphere. The amount of gas is an extremely
useful measure as it directly indicates the hazard
of a gas cloud. The more gas there is, the more
severe the explosion is likely to be.
Transmitters can be installed in critical locations
that are prone to possible leakage such as flanges,
54 INTERNATIONAL FIRE PROTECTION
Draeger PIR 7000
GAS DETECTION
The inclusion of a Wheatstone Bridge, which is able to
duplicate a standard pellistor output, also means that
existing cables and control devices, regardless of
manufacturer, can still be used.

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