Det-Tronics PIR9400 PointWatch Infrared Hydrocarbon Gas Detector - Manual (Page 7)

5 95-8440 9.3
DESCRIPTION
DETECTION METhOD
The PointWatch Model PIR
9400 operates on the infrared
absorption principle. A beam of modulated light is
projected from an internal infrared source to a reflector,
which sends it back to a pair of infrared sensors. One of
the sensors is designated reference and the other active,
with different optical filters in front of the two sensors to
make them selective to different infrared wavelengths.
The reference wavelength is unaffected by combustible
gases, while the active wavelength is absorbed by
combustible gases. The ratio of the active to the reference
wavelength is computed within the detector to determine
the concentration of gas present. This value is then
converted into a
4–20 mA current output for connection
to external display and control systems.
CURRENT LOOP OUTPUT
During normal operation, the Model PIR
9400 Detector
has a current output from 4–20 mA that is proportional to
gas concentrations from 0 to 100% LFL. A current output
other than 4–20 mA indicates either negative gas level, a
fault or over-range condition, or that the detector is in the
calibrate mode as indicated in Table
1 .
OPERaTING MODES
Warmup
When power is applied to the detector, it enters a
Warmup mode (for approximately one minute) in which
it performs diagnostic checks and allows the sensors
to stabilize before beginning normal operation. The
current output during this period is
0 mA. At the end of
the warmup period with no faults present, the detector
automatically enters the Normal operating mode. If a
fault is present after the warmup, the detector current
output will indicate a fault.
Normal
In the normal operating mode, the
4–20 mA signal
level corresponds to the detected gas concentration.
The detector continuously checks for system faults or
initiation of calibration, and automatically changes to the
appropriate mode.
Fault
Faults detected during warmup, normal operation, or
calibration are indicated by the current loop output as
shown in Table
1 .
Calibration
All PointWatch Detectors are calibrated at the factory with
50% LFL methane at 2.5% by volume, and are shipped
with the internal gas selection switch set for methane
gas detection. For additional information on calibration
for other gases, refer to the “Linearized Output Options”
section of this manual.
Whenever calibration of the PointWatch Detector is
required, a momentary connection of the calibration
lead wire to DC negative (common) of the power supply
initiates the zero and span calibration sequence.
NOTE
It is not recommended to physically connect or
touch the calibration lead wire to DC common
in the field to begin calibration. This practice
is often less than precise, and may result in a
spark or other undesirable result. For optimum
ease of installation and calibration, always use
a PIRTB (furnished with magnetic reed switch,
indicating LEDs, and terminal strip), available from
Det-Tronics.
The factory default setting for the output current during
calibration is an inhibited state. See Table
1 for specific
information. Note that a live current output during
calibration can also be programmed, although this is not
usually recommended. Refer to the “Calibration” section
of this manual for details.
The calibration sequence for a particular Model PIR 9400
Detector installation is typically determined by the type of
termination box installed with the detector:
• For non-intrusive one-person calibration, select the
PIRTB with Tall Cover. This termination box includes
a magnetic reed calibration switch and calibration
LED (visible through a viewing window on the cover).
By activating the magnetic reed switch with a
calibration magnet and then viewing the LED through
the window, a one person, non-intrusive calibration
can be performed. See Figure
6 .
• For intrusive or two-person calibration, select
the PIRTB with Short Cover. This termination box
typically requires removal of the termination box
cover to view the calibration LED, or it requires
two people to accomplish a remotely initiated
non-intrusive calibration. The short cover termination
box includes a magnetic reed calibration switch,
calibration LED and a solid cover (no viewing
window). This termination box can also be used for
sensor separation. See Figure
7 .