Instrument inaccuracy due to improper or irregular maintenance and calibration can lead to exposure to hazardous levels of toxic gases or to an oxygen-deficient atmosphere. This exposure can cause workers to suffer serious injuries or illness, and even death. Flammable gas explosions are often catastrophic, resulting in worker injuries and death, or destruction of property. The best way to verify that a portable gas detector detects gas accurately and reliably is to test it with a known concentration of gas. This procedure will verify whether the sensors in the instrument respond accurately and whether the alarms function properly.
A reading becomes unreliable when an instrument's reference point shifts. The general rule of thumb is that “calibration drift" happens to all sensors over time. An instrument that experiences calibration drift can still measure the quantity of gas present, but it won’t convert this information into an accurate numerical reading. Calibration checks or full calibration with a traceable gas concentration will verify or update the instrument's reference point. Operators should conduct these procedures daily, or more frequently if needed, to ensure that the instrument will continue to produce accurate readings.
This is a qualitative function check in which a challenge gas is passed over the sensor at a concentration and exposure time sufficient to activate all alarm settings. The purpose of this check is to confirm that gas can get to the sensor and that all the instrument's alarms are functional. The bump test or function check does not provide a measure of the instrument's accuracy. When performing a bump test, the challenge gas concentration should trigger the portable gas detector’s alarm.
BSEE has recently sent out an alert regarding gas release hazards on various offshore production equipment. Refer to BSEE alert 328 for more information. Below is a quick summary of what the alert covers.
- Proper documentation of portable gas detectors, including bump tests prior to each use.
- Check calibration date to ensure portable gas detector is within manufacturers specifications.
- Insure employee is properly trained and competent to use the portable gas detector available.
In the event a portable gas detector shows any sign of malfunction, place the unit out of service for repairs or replacement. Always remember you have the right to use STOP WORK AUTHORITY.