Detcon FP-700 - Sensor Electronics Design; Figure 2 Wheatstone Bridge; Figure 3 Response Curves

Model FP-700
FP-700 Instruction Manual Rev. 5.1 Page 2 of 46
Figure 2 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 3. 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 3 Response Curves
1.2 Sensor Electronics Design
Intelligent Sensor Module
The Intelligent Transmitter Module (ITM) is a fully encapsulated microprocessor-based package that accepts a
plug-in field replaceable combustible gas sensor. Circuit functions include extensive I/O circuit protection,
sensor pre-amplifier, bridge voltage (temperature) control, on-board power supplies, microprocessor, LED
display, magnetic programming switches, linear 4-20mA DC output, and Modbus™ RS-485 output. Magnetic
program switches located on either side of the LED Display are activated via a hand-held magnetic programming
tool, thus allowing non-intrusive operator interface with the ITM. Calibration can be accomplished without
declassifying the area. Electrical classifications are Class I, Division 1, Groups B C D, Class I, Zone 1, Group
IIB+H
2
and II 2G Ex d IIB+H
2
Gb.
Sensor
Cell
Zero
Adjust
Output
Input
Voltage
Compensator /
Reference Bead
Detector /
Active Bead