Calibration Drift and Causes
When an instrument's reference point shifts, the reading will shift accordingly and be unreliable. This is called "calibration drift," and it happens to all sensors over time. An instrument that experiences calibration drift can still measure the quantity of gas present, but it cannot convert this information into an accurate numerical reading. Calibration checks or full calibration2 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. Calibration drift occurs most often because of:
Operators should validate a DRPGM's operability when any of these conditions occurs, or is suspected, during use. When attempting to calibrate an instrument after exposure to these conditions, the sensor often will either display a failure message or will not allow the operator to fully adjust the display reading. Harsh operating and storage conditions can affect instrument performance, leading to inaccurate readings or even failure. While a DRPGM may appear undamaged during visual inspection, it actually could be damaged internally. At this point, the operator should replace the damaged sensor or have qualified personnel service the sensor. Be sure to follow the manufacturer's instructions regarding sensor replacement and servicing.
Worker Safety: The Number One Reason for Proper and Regular Calibration
The primary reason for proper, regular instrument calibration is to provide accurate gas-concentration readings that could prevent worker illness, injury, or death. Correctly calibrating an instrument helps to ensure that the DRPGM will respond accurately to the gases it is designed to detect, thereby warning users of hazardous conditions before the conditions reach dangerous levels. Some DRPGMs have two levels of alarms – warning and danger. The warning alarm alerts the operator and workers that the work environment has a detectable elevated concentration of toxic gas and is, therefore, potentially hazardous. The danger alarm indicates that the toxic-gas concentration exceeds the programmed hazard threshold, and that the toxic gas in the work area is above the warning level and approaching a hazardous level. Whether a DRPGM provides a warning or danger alarm at the proper concentration depends on its detection capabilities, its ability to translate its findings into an accurate reading, and the presence of interfering gases (see "Calibration Drift and Causes" above).*
This Safety and Health Information Bulletin is not a standard or regulation, and it creates no new legal obligations. The Bulletin is advisory in nature, informational in content, and is intended to assist employers in providing a safe and healthful workplace. Pursuant to the Occupational Safety and Health Act, employers must comply with hazard-specific safety and health standards promulgated by OSHA or by a state with an OSHA-approved state plan. In addition, pursuant to Section 5(a)(1), the General Duty Clause of the Act, employers must provide their employees with a workplace free from recognized hazards likely to cause death or serious physical harm. Employers can be cited for violating the General Duty Clause if there is a recognized hazard and they do not take reasonable steps to prevent or abate the hazard. However, failure to implement any recommendations in this Safety and Health Information Bulletin is not, in itself, a violation of the General Duty Clause. Citations can only be based on standards, regulations, and the General Duty Clause.
In order to ensure the safe and accurate operation of Fixed Gas Detectors, they need to be periodically calibrated against reference test gases that are traceable to National Physical Laboratory standards. The recommended calibration period for Status Scientific Controls gas detectors is six months although the frequency may need to be increased where continual exposure to the target gases is experienced.
Oxygen deficiencies, explosive atmospheres, and exposure to toxic gases and vapors injure hundreds of workers every year. The atmospheric conditions that lead to these accidents and fatalities are usually invisible to the workers who are involved. The only way to ensure atmospheric conditions are safe is to use an atmospheric monitor. The only way to know whether an instrument is capable of proper performance is to expose it to test gas. Exposing the instrument to known concentration test gas verifies that gas is properly able to reach and be detected by the sensors.
It verifies the proper performance of the instrument’s alarms, and (if the instrument is equipped with a real-time display), that the readings are accurate. Failure to periodically test and document the performance of your atmospheric monitors can leave you open to regulatory citations or fines, as well as increased liability exposure in the event that a worker is injured in an accident.
According to the United States Department of Labor (SHIB 09-30-2013):
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