Detcon FP-100 - Manual (Page 6)

Model FP-100
FP-100 Instruction Manual Rev. 0.0 Page 2 of 30
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
1.2 Modular Mechanical Design
The Model FP-100 Sensor Assembly is completely modular and is made up of four parts (See Figure 5 for
Assembly Breakaway):
1) FP-100 Intelligent Transmitter Module (ITM)
2) Intelligent Plug-in Sensor (varies by detector type)
3) 100 Series Bottom Housing Assembly
4) Splash Guard.