Detcon FP-524D-HRT - Manual (Page 6)

Model FP-524D-HRT
FP-524D-HRT Instruction Manual Rev. 0.1 Page 2 of 48
Main Housing Insert
Printed Circuit Board
Gold Plated Pins
Platinum Wire
Construction
of Detector
Bead
Catalyst
Alumina Bead
Catalytic Beads
Figure 2 Sensor Cell Construction
Principle of Operation
Method of detection is by diffusion/adsorption. Air and combustible gases pass through a sintered stainless
steel filter and contact the heated surface of both the active and reference detectors. The surface of the active
detector promotes oxidation of the combustible gas molecules while the reference detector has been treated not
to support this oxidation. The reference detector serves as a means to maintain zero stability over a wide range
of temperature and humidity.
When combustible gas molecules oxidize on the surface of the active detector, heat is generated, and the
resistance of the detector changes. Electronically, the detectors form part of a balanced bridge circuit. As the
active detector changes in resistance, the bridge circuit unbalances. This change in output is conditioned by
the amplifier circuitry, which is an integral part of the sensor design. The response and clearing characteristics
of the sensor are rapid and provide for the continuous and accurate monitoring of ambient air conditions.
Sensor
Cell
Zero
Adjust
Output
Input
Voltage
Compensator /
Reference Bead
Detector /
Active Bead
Figure 3 Wheatstone Bridge
Performance Characteristics
The detector elements maintain good sensitivity to combustible gas concentrations in the Lower Explosive
Limit (LEL) range, as shown in the response curves in Figure 4. However, for gas concentrations significantly
above the LEL range (100% LEL = 5% by volume Methane), the bridge output begins to decrease.
Ambiguous readings above the LEL range dictate that alarm control logic be of the latching type, wherein
alarms are held in the “ON” position until reset by operations personnel.
Figure 4 Response Curves