Detcon FP-700 - Manual (Page 6)

Model FP-700
FP-700 Instruction Manual Rev. 3.0 Page 2 of 40
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
and Class I, Zone 1, Group IIB+H
2
.
Figure 4 ITM Circuit Functional Block Diagram
Senso r
C ell
Z ero
Adjust
O u tp u t
Inp u t
Vo ltag e
C o m p ensato r /
Ref erenc e Bead
Detec to r /
Ac tive Bead
I/O
Circuit
Protection
RS-485
4-20mA
Display
Micro-
Processor
Temperature
Control
Pre-Amp
Analog 4-20mA Out
Modbus™ RS-485 Output
Power In
Power Supply
Plug-In
Sensor
Element