Detcon FP-424C - Manual (Page 5)

2.1 Characteristics
The detector elements maintain good sensitivity to combustible hydrocarbons 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
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 catalyst with a catastrophic reduction in sensitivity, the detector is said to be poi-
soned. Examples of poisons are; silicone oils and greases, anti-knock petrol additives and phosphate
esters. Activated carbon filters will provide adequate protection from poisoning in the majority of cases.
Note : If it is suspected that the sensor has been poisoned its sensitivity should be verified by recalibrating.
3.0 A PPLICATION
Model FP-424C 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.
3.1 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 gener-
ally located 2 - 4 feet above grade.
3.2 Response to Different Gases
One of the characteristics of the catalytic type combustible gas detectors is their almost universal response
to virtually any and all combustible gases. The partial list of detectable gases that appears in our literature
includes a list of response K-Factors. These factors are used to determine cross calibration references when
detectors are calibrated with other than the target gas. They may also be used to calculate expected read-
ings when a gas other than the target and calibration substance occurs in the area of the sensor. The follow-
ing formulas apply when converting to either of these occurrences.
Detcon Model FP-424C Combustible Gas Sensor PG.5
R E S P O N S E C U R V E
% 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
2.7V
Zero
Set
Pot
Output
Detector/Active
Compensator/Reference
R1
R2