Troubleshooting Guide: Flammable Vapor Sensor (25VS)

The 25VS is a passive, silicone-based sensor designed to detect gasoline vapors in the event of a spill. The sensor provides an output based on changes in resistance caused by exposure to gasoline vapors. If you're experiencing issues with the sensor, follow the troubleshooting steps below.

Symptoms:

1. Gas valve does not shut off after spill.

2. False positive or false negative readings from the sensor.

3. Sensor not detecting vapors or not resetting properly.

Step 1: Verify the Sensor’s Resistance

• Normal Operation: The sensor should exhibit a base resistance of 7 to 25 KΩ at 25°C.

• Check Resistance: Measure the resistance across the sensor terminals using a multimeter.

  • If the resistance is within the specified range (7 to 25 KΩ), the sensor is likely functioning properly.

  • If the resistance is higher than 50 KΩ, the sensor has detected gasoline vapor and has triggered the trip point. This would result in the gas valve shutting off. The system should reset after exposure, but if it doesn’t, proceed with further troubleshooting.

Step 2: Check for Flammable Vapor Exposure

• Sensor Response: The sensor reacts to gasoline vapor by expanding the silicone film, which increases the resistance.

  • If the sensor has been exposed to flammable vapors, resistance will be elevated (above 50 KΩ).

  • If the system is not resetting after exposure, this may indicate that the vapor concentration was too high or that the sensor has "locked out" the response.

Step 3: Inspect the Operating Environment

• Temperature Sensitivity: The 25VS sensor can be moderately sensitive to ambient temperature, which may cause resistance changes.

  • Check Temperature: Ensure the operating temperature is within the specified range of 0°C to 65°C (32°F to 150°F).

  • If the temperature is too high or too low, this could lead to false positives or incorrect readings.

  • If the sensor is exposed to temperatures outside this range, allow it to return to normal temperature and retest.

• Humidity and Contaminants: The sensor is designed to be relatively insensitive to household cleaner vapors but may still be affected by high humidity or chemical exposure.

  • Check Humidity: Ensure the relative humidity is between 10% and 90%.

  • If humidity is outside this range, it could lead to erratic sensor readings.

Step 4: Ensure Proper Wiring and Connections

• Verify Wiring: Ensure the sensor is wired properly to the control circuit.

  • Check the sensor’s resistance and verify that it is correctly integrated into the system to send the appropriate signal to the control module.

  • Loose or poor connections could result in improper sensor readings and failure to detect vapors.

Step 5: "One-Shot" Use Consideration

• Limited Exposure: The 25VS sensor may reset after exposure to flammable vapors, but Therm-O-Disc cannot validate its performance after initial exposure in the field.

  • Reset the System: If the sensor has been exposed to gasoline vapors, allow the system time to reset. However, if it does not reset or the sensor continues to show elevated resistance (above 50 KΩ), it may need to be replaced.

  • Long-Term Exposure: If the sensor fails to reset after exposure, it may no longer be functioning properly, and replacement is recommended.

Step 6: Replace the Sensor (if necessary)

• Base Resistance: If the sensor’s base resistance is outside the 7 to 25 KΩ range, or if it fails to reset after exposure to vapors, it may be defective and require replacement.

• Considerations for Replacement: Ensure the replacement sensor is compatible with your system and follows the same operating conditions, such as the 5 VDC voltage and humidity range of 10% to 90%.

Conclusion

If the sensor continues to provide improper readings or fails to reset after exposure to gasoline vapors, it may require replacement. Regular maintenance and checking for environmental factors, such as temperature and humidity, will help ensure the proper operation of the 25VS flammable vapor sensor. If troubleshooting steps do not resolve the issue, consult the manufacturer or a trained technician for further assistance.