Detcon FP-424C User Manual

Model FP-424C

detcon inc.

4-20 mA Combustible Gas Sensor (0-100% LEL)

Operator’s Installation & Instruction Manual

May 18, 2010 • Document #2243T • Version 3.8

CAUTION: FOR SAFETY REASONS THIS EQUIPMENT MUST BE OPERATED
AND SERVICED BY QUALIFIED PERSONNEL ONLY. READ AND UNDERSTAND
INSTRUCTION MANUAL COMPLETELY BEFORE OPERATING OR SERVICING.

ATTENTION: POUR DES RAISONS DE SECURITE, CET EQUIPEMENT DOIT ETRE
UTILISE ENTRETENU ET REPARE UNIQUEMENT PAR UN PERSONNEL QUALI­FIE. ETUDIER LE MANUEL D'INSTRUCTIONS EN ENTIER AVANT D'UTILISER,
D'ENTRETENIR OU DE REPARER L'EQUIPEMENT.

phone [phone] • fax [phone] • www.detcon.com • [e-mail]

Table of Contents

1.0 Description

2.0 Principle of Operation

3.0 Application

4.0 Specifications

5.0 Installation

6.0 Start-up

7.0 Calibration

8.0 Fault Supervision

9.0 Trouble Shooting Guide

10.0 Spare Parts List

11.0 Warranty

12.0 Service Policy

Detcon Model FP-424C Combustible Gas Sensor PG.2

1.0 DESCRIPTION

Alumina Bead

Platinum Wire

Catalyst

Construction of

Detector Bead

Sintered Stainless Steel Can

Header

Gold Plated Pins

Beads

BRDG

FP-424C

+

% LEL

4

-

20 mA DC

T

E

X

A

S

H

O

U

S

T

O

N

VOLT

detcon inc.

ZERO

BRIDGE

SIGNAL

FLT

DET

FLT

FINE

COARSE

Detcon Model FP-424C combustible gas detection sensors are designed to monitor ambient air for com­bustible hydrocarbons in the lower explosive limit range (0-100% LEL). The sensor assembly consists of a
plug in replaceable matched pair of catalytic beads mounted in a stainless steel housing, a 12-28 VDC 4-20

A DC control transmitter, an explosion proof junction box, and a machine fitted rain shield.

m

1.1 Catalytic Detector

The catalytic detector is supplied as a matched pair of elements mounted in a plug-in replaceable housing.
One element is an active catalytic detector and the other is a non-active compensating element. Each ele­ment consists of a fine platinum wire embedded in a bead of alumina. A catalytic mixture is applied to the
detecting element while the compensating element is treated so that catalytic oxidation of gas does not
occur. The beads are mounted in a plug-in module that is enclosed by a sintered porous stainless steel
flame arrestor. The plug-in sensor module uses gold plated pins and mounts inside the stainless steel sen­sor head via mating gold plated sockets.

1.2 Control Transmitter Circuit

The control transmitter is a remote mounted electronic circuit designed and packaged as a plug-in module.
The transmitter assembly consists of several circuit functions: input power conditioning, sensor heater con­trol, sensor signal conditioning, sensor fault supervision, and fault LEDs. A metallic face plate with pull
ring identifies test points and calibration adjustments. The plug-in module design supports easy field level
maintenance and repair. The mating plug is gold plated for maximum resistance to corrosion.

1.3 Terminal Connector Board

The terminal connector board is mounted in the explosion proof enclosure and includes: the mating con­nector for the control circuit, reverse input and secondary transient suppression, input filter and terminals
that do not require lugs for all field wiring.

Detcon Model FP-424C Combustible Gas Sensor PG.3

1.4 Explosion Proof Enclosure

BLUE
YELLOWMA

24VDC

To Controller

Te r mi n a l s

BLACK

WHITE

Sensor

–
+

4-20MA

The transmitter electronics are packaged in a cast aluminum explosion proof enclosure and given a baked
epoxy paint finish. The explosion proof enclosure has 3/4” NPT female entry ports. The enclosure is fitted
with a threaded cover. Electrical classification is Class I, Div. 1; Groups B, C, D.

1.5 Rain Shield

The machine fitted rain shield is designed to help prevent obstruction of the sintered stainless steel flame
arrestor as a result of liquid or solid contact. Material of construction is Dupont Delrin.

2.0 PRINCIPLE OF OPERATION

In operation, FP-424C series sensor assemblies are installed in industrial areas where early detection of gas
leaks is most likely. The sensor is powered by 12-28 VDC. Air and gas diffuses through the porous sintered
flame arrestor and contacts the active and compensating detector elements. Electronically, the detector ele­ments form part of a bridge circuit as shown below. The balance potentiometer is used to set the output of
the bridge circuit referenced to zero gas. As the active detector element reacts to the presence of com­bustible gas and the compensating element does not react, the bridge voltage output changes. This voltage
change is conditioned by the transmitter circuit to provide for a linear 4-20 mA DC signal output. As com­bustible gas dissipates and the sensor is returned to a zero gas environment the bridge circuit returns to a
balanced condition. In application, sensor operation is a continuous reading detection/monitoring method.

Detcon Model FP-424C Combustible Gas Sensor PG.4

2.1 Characteristics

RESPONSE CURVE

% Methane in Air Concentration

0

20

40

60

80

100

020406080

Bridge Output %

0

4

8

12

16

20

20 40 60 80 100

% LEL (lower explosive limit)

mA DC Signal Output

2

.7V

Z

ero
Set
P

ot

O

utput

Detector/Active

C

ompensator/Reference

R

1

R

2

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 poi­soned. 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 APPLICATION

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 gener­ally located 2 - 4 feet above grade.

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 read­ings when a gas other than the target and calibration substance occurs in the area of the sensor. The follow-

3.2 Response to Different Gases

ing formulas apply when converting to either of these occurrences.

Detcon Model FP-424C Combustible Gas Sensor PG.5