Detcon FP-424C - Manual (Page 13)

8.0 F AULT S UPERVISION
Model FP-424C sensors incorporate fault circuitry which monitors both the detector loop and the bridge
voltage of the catalytic bead sensor element. Should one or both of the catalytic bead circuits become elec-
trically “open” a Detector Fault condition will occur which will result in the illumination of the “DET FLT”
LED located on the transmitter face plate. Should the bridge voltage drop below 1.8 V, a Bridge Fault con-
dition will occur which will result in the illumination of the “BRDG FLT” LED located on the transmitter
face plate. Either fault condition will cause the 4-20 mA signal to drop to 0 mA. When used with Detcon
Model 10 or Model 12 controls, this drop in the mA signal will result in a fault condition on the affected
control module creating a dual redundant fault function.
9.0 T ROUBLE S HOOTING G UIDE
Detector Fault
1. Open Sensor – broken wire or contact in sensor.
2. Remove replaceable sensor element and check adjacent pin pairs with ohm-meter. Normal reading is 1-
4 ohms and failed reading is an open circuit.
3. Replace sensor if verified as Open Sensor.
Bridge Fault
1. Shorted detector bead, shorted transmitter, or faulty transmitter.
2. Make sure that there is no presence of condensation or accumulated water in the sensor housing or cond-
ulet. This could cause possible shorting.
3. Swap in new transmitter and/or new sensor.
Poor Sensor Performance (Slow Response, Drifting Sensor)
1. Check that correct Heater Voltage is applied to your sensor.
Note: Detcon has two version sensors: C-Style and the J-Style. Each uses a different heater voltage setting.
The C-Style sensor measures 0.9'' across the exposed stainless steel sinter face and has a serial number for-
mat C??-###. The C-Style sensor requires 2.7 VDC
The J-Style sensor measures 0.4”across the exposed stainless steel sinter face and has a serial number for-
mat J??-###. The J- Style sensor requires 2.2 VDC.
2. If heater voltage is incorrect then adjust it accordingly for the sensor type.
Excessive Span Drift or Slow Response
1. Check Heater Voltage Setting (should be 2.7V C-Style and 2.2V J-Style) and check heater voltage at the
sensor if remote mounted.
2. Verify correct cal gas flow rate and proper use of the cal gas adapter.
3. Check validity of cal gas via the expiration date and use pull tube if necessary.
4. Check for obstructions through stainless steel sinter element (including being wet).
5. Replace plug-in sensor if necessary.
6. Check area for presence of sensor poisoning gases such as silicon grease vapors, HMDS, high H2S, chlo-
rine or chlorinated compounds if sensor failures persist.
Drifting Zero
1. May be correct reading if there are real gas leaks or the sensor was zero calibrated when actual gas was
around and subsequently cleared.
2. Check Heater voltage is set correctly for sensor type (check voltage at the sensor if remote mounted).
3. Replace sensor if sensor has experienced large span losses that exaggerate zero drift.
4. If sensor drift is gradual and continuously positive (with a stable span) then contact Detcon for sensor
replacement.
Detcon Model FP-424C Combustible Gas Sensor PG.13