Detcon TP-424C - Manual (Page 13)

setting of 235 mW and is acceptable. If the value was too low you would increase the voltage by turning
the potentiometer labeled “TEMP” clockwise, and then recalculate with the new voltage and current read-
ings. If the value was too high, you would decrease the voltage and repeat the process as necessary to reach
the desired power setting.
3.11 F
AULT
S
UPERVISION
Model TP-424C sensors incorporate fault circuitry which monitors both the heater loop and sensing film
loop of the H2S sensor element. Should one or both of these circuits become electrically “open” a fault con-
dition will occur which will result in the illumination of the corresponding fault LED located on the trans-
mitter face plate (Heater Fault and Signal Fault) as well as the 4-20 mA signal dropping 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.
3.12 T ROUBLE SHOOTING
Caution: Trouble shooting procedures require that the sensor junction box cover be removed and, there-
fore, negate the electrical classification. De-classify the area before proceeding.
Tools Required
Digital volt/ohm meter, Jeweler’s type screwdriver
Note: As described in section 3.11, fault supervision circuitry monitors the heater and signal loop of the
sensor element. If the mA output has dropped to 0 mA and the Heater Fault and/or Signal Fault LED is
illuminated, the sensor element has become electrically open and should be returned for repair/replace-
ment.
Open Heater
Verify resistance between black and white wires using ohm-meter with the sensor unplugged and at room
temperature.
Heater Film Check: normal reading range should be 65-95 ohms
Open Sensor
Verify resistance between blue and yellow wires using ohm-meter with the sensor unplugged and at room tem-
perature. Sensor Film’s normal reading range should be 5-100 k-ohms. For sensors with an “X” in the serial
number, the normal reading range should be 75 k-ohms to 100 meg-ohms. Failure would be open circuit.
If you are unsure if the sensor f ilm has an open circuit due to the limits of your measurement equipment,
then apply 5.25 VDC to the heater f ilm and apply 25 ppm H2S/Air test gas while measuring the sensor f ilm
resistance with an ohm-meter. The sensor f ilm is not “open circuit” if the resistance quickly falls to <100 k-
ohms while on the test gas.
Low or Slow response to Cal Gas
1. Check Heater Voltage Setting (should be 5.25 V).
2. Use humidifier with DI Water.
3. Check validity of span gas with a pull tube.
4. Check for obstructions through s/s sinter element (including being wet).
5. Check sensor lot # and report to Detcon.
Poor calibration Repeatability
1. Check Heater Voltage Setting (should be 5.25 V).
2. Use humidifier with Distilled Water.
3. Check validity of span gas with a pull tube.
4. Check for obstructions through s/s sinter element (including being wet).
5. Check sensor lot # and report to Detcon.
6. Check area for presence of reducing gases such as Cl2 and ozone.
Detcon Model TP-424C Hydrogen Sulfide Sensor PG.13