DESCRIPTION
DETECTION METHOD
PointWatch 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 designat-
ed reference and the other active, with different optical fil-
ters 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 PointWatch detector to determine the concen-
tration of gas present. This value is then converted into a
4 to 20 milliampere current output for connection to exter-
nal display and control systems.
CURRENT LOOP OUTPUT
During normal operation, the PointWatch detector has a
current output from 4 to 20 milliamperes that is propor-
tional to gas concentrations from 0 to 100% LFL. A cur-
rent output other than 4 to 20 milliamperes indicates
either negative gas level, a fault or over-range condition,
or that the unit 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 cur-
4
Figure 5—Junction Box Dimensions in Inches (MM)
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