The increased awareness of potentially dangerous atmospheric conditions is a leading contributor to the amplified use of single-gas and multi-gas monitors in the workplace around the world.1 Gas monitoring instruments are designed to protect personnel from these unseen hazards that include, but are not limited to, oxygen enrichment or depletion as well as the release of toxic or combustible gases.

It is vital to worker safety that these instruments are maintained and calibrated properly. In fact, the International Safety Equipment Association (ISEA) recommends, at a minimum, verification of sensor accuracy before each day’s use.

Eliminate calibration drift
Calibration accuracy is relative to a known concentration of traceable calibration gas. Gas monitors measure atmospheric gas concentrations by comparing the response generated by a calibration gas to the measured concentration of a gas in an air sample. The reference point or measurement scale is the monitor’s response to the calibration gas.

Most portable single-gas and multi-gas monitors have two alarm levels — warning and danger. The warning alarm indicates that a detectable concentration of gas is present; the danger alarm indicates that gas concentrations have reached the hazard threshold. An instrument’s ability to alarm at the proper setpoints depends on its ability to detect and translate the hazard into accurate readings.

Calibration drift happens to all monitors, but if the reference point shifts, the reading also shifts and is unreliable. Regular calibration updates the monitor’s reference point, and daily calibration advises if the reference point is within acceptable limits. Causes of calibration drift or conditions that trigger the need for instrument bump test, or full calibration include2:

• Chronic exposure to extreme environmental conditions, such as high/low temperature and humidity, and high levels of airborne particulates.
• Exposure to high (over range) concentrations of target gases and vapors.
• Chronic or acute exposure of catalytic hot-bead LEL sensors to poisons and inhibitors, including volatile silicones, hydride gases, halogenated hydrocarbons, and sulfide gases.
• Chronic or acute exposure of electrochemical toxic gas sensors to solvent vapors and highly corrosive gases.
• Harsh storage and operating conditions, such as when a portable gas monitor is dropped onto a hard surface or submerged in liquid. Normal handling/jostling of monitors can create enough vibration over time to affect electronic components and circuitry.
• Change in custody of the monitor (either bump test or calibration check is acceptable).
• Any other conditions that potentially affect the performance of a monitor.

Test frequency
Frequency of calibrations has been a confusing point for end users. The ISEA issued a position statement on instrument calibration that states, “A bump test or full calibration of direct-reading portable gas monitors should be made before each day’s use in accordance with manufacturer’s instructions, using appropriate test gas.”

If environmental conditions such as sensor poisons are present, more frequent testing should be conducted. The ISEA allows for less frequent calibration verification under certain conditions, but the interval between tests should not exceed 30 days.

Less frequent verification may be appropriate according to ISEA if the following criteria are met:

• During a period of initial use of at least ten days in the intended atmosphere, calibration is verified daily to ensure there is nothing in the atmosphere to poison the sensor(s). The period of initial use must be of sufficient duration to ensure that the sensors are exposed to all conditions that might adversely affect them.
• If the tests demonstrated that no adjustments are necessary, the interval between checks may be lengthened, but should not exceed 30 days.3

Bump tests vs. full calibration
There are two methods of verifying instrument accuracy: a functional or bump test and a full calibration; each appropriate under certain conditions. A bump test verifies calibration by exposing the instrument to a known concentration of test gas. The instrument reading is compared to the actual quantity of gas present (as indicated on the cylinder).

If the instrument’s response is within an acceptable tolerance range of the actual concentration, then its calibration is verified. (Note: It is recommended that users check with the detection equipment manufacturer for the acceptable tolerance ranges.) Instruments should be “zeroed” before the bump test in order to give a more accurate picture of the bump test results. When performing a bump test, the test gas concentration should be high enough to trigger the instrument alarm.

If bump test results are not within the acceptable range, a full calibration must be performed. A full calibration is the adjustment of the instrument’s reading to coincide with a known concentration (generally a certified standard) of test gas.

For verification of accuracy, calibration gas should always be certified by and traceable to the National Institute of Standards and Technology (NIST). In most cases, a full calibration is only necessary when an instrument fails a bump test or after it has been serviced. The full calibration and bump test should be conducted in a clean, fresh air environment.

Ensuring functionality
Following some basic guidelines will ensure the proper functionality of portable gas monitors. It is important to:

• Train workers/operators on the proper procedures for performing a bump/functional test and full calibration.
• Provide the correct tools for performing an instrument test.
• Ensure the calibration gas/test gas is within the manufacturer’s concentration range.
• Check the expiration date of the gas. Never use an expired bottle of gas for any of the above-mentioned tests.
• Follow the manufacturer’s recommendations on the flow rate for the regulator to be used on the test gas bottle for their particular instrument.
• Follow the manufacturer’s recommendations for frequency of calibration.

Although an instrument may appear to be working from the outside, it is important that each employer establish guidelines to ensure the monitor their employees are using is working accurately. Today’s instruments offer fully automatic calibration, and most manufacturers have computer-based testing equipment for their instruments. This type of testing equipment automates and documents instrument testing and enables instrument performance checks to be quick and simple procedures.



References:
1 U.S. Department of Labor Safety and Health Information Bulletins SHIB 05-04-2005
2 Ilbid.
3 International Safety Equipment Association- Statement on Bump Testing and Verification of Calibration for Direct Reading Portable Gas Monitors