Detcon FP-624C - Manual (Page 5)

3.1.2 Characteristics
The detector elements maintain good sensitivity to combustible gases in air in the lower explosive limit range, as shown in
the response curve illustration below. However, for gas concentrations above the LEL range, the bridge output decreases.
Ambiguous readings above LEL range conditions dictate that alarm circuitry be of the latching type wherein alarms are
held in the “on” position until reset by operations personnel.
The performance of the detector elements may be temporarily impaired by operation in the presence of sub-
stances described as inhibitors. These are usually volatile substances containing halogens and the detectors may
recover after short periods of operation in clean air. When the inhibiting substance produces a permanent effect on
the catalyst with a catastrophic reduction in sensitivity, the detector is said to be poisoned. Examples of poisons
are; silicone oils and greases, anti-knock petrol additives and phosphate esters. Activated carbon f ilters will provide
adequate protection from poisoning in the majority of cases.
3.2 A
PPLICATION
Model FP-624C MicroSafe™ sensors are designed to detect and monitor combustible gas in ambient air in the range of
0-100% LEL. Minimum sensitivity and scale resolution is 1%. Operating temperature range is -40° F. to +175° F. While
the sensor is capable of operating outside these temperatures, performance specifications are verified within the limit.
3.2.1 Sensor Placement/Mounting
Sensor location should be reviewed by facility engineering and safety personnel. Area leak sources and perimeter mounting
are typically used to determine number and location of sensors. The sensors are generally located 2 - 4 feet above grade.
Model FP-624C Combustible Gas Sensor PG.5
% Methane in Air Concentration
0
20
40
60
80
100
0 20 40 60 80
B
ri
d
g
e
O
u
tp
u
t
%
0
4
8
12
16
20
20 40 60 80 100
% LEL (lower explosive limit)
m
A
D
C
S
ig
n
a
l
O
u
tp
u
t
Response Curve Response Curve
Functional
Block
Diagram
Functional
Block
Diagram
Analog 4-20 mA Out
Power In
Relays Out
Pre-Amp Display
Temperature
Compensation
Alarm & Fault
Relays
RS-485 & 4-20mA
Micro-
processor
Transmitter
Power Supply
Sensor
Element
I/O Circuit
Protection
Serial RS-485 Out
2.2V
Zero
Set
Output
Detector/Active
Compensator/Reference