Detcon FP-524C - Manual (Page 5)

3.1 P RINCIPLE OF O PERATION
Method of detection is by a controlled rate of diffusion/adsorption. Air and gas diffuse through a sintered stainless
steel f ilter and contact both the active and reference detector beads. The surface of the active detector promotes
oxidation of the combustible gas molecule while the reference detector has been treated not to support this oxida-
tion. The reference detectors serve as a means to maintain zero stability over a wide operating temperature range.
When combustible gas molecules oxidize on the surface of the active detector, heat is generated, effectively chang-
ing the electrical conductance of the active detector. Electronically, the detectors form part of a balanced bridge cir-
cuit. As the active detector changes in electrical conductance, the bridge circuit unbalances. This change in output is
conditioned by amplifier circuits that are an integral part of the sensor transmitter. The sensor response and clearing
characteristics are quite rapid resulting in a method of continuous and accurate monitoring of ambient air conditions.
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.
The performance of the detector elements may be temporarily impaired by operation in the presence of substances
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 cat-
alyst with a catastrophic reduction in sensitivity, the detector is said to be poisoned. Examples of poisons are; sili-
cone oils and greases, anti-knock petrol additives and phosphate esters. Activated carbon f ilters will provide ade-
quate protection from poisoning in the majority of cases.
Model FP-524C 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
2.7V
Zero
Set
Output
Detector/Active
Compensator/Reference
Functional
Block
Diagram
4-20 mA Out
Power In
Pre-Amp Display
4-20mA
Micro-
processor
Transmitter
Power Supply
Sensor
Element
I/O Circuit
Protection