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Atmospheric Monitors: “Calibration vs. Bump Testing”

Friday, January 21, 2011

“The fact that we rely on these instruments to detect hazards that may be colorless, odorless, and very often fatal, should be reason enough to motivate us to complete a very strict schedule of instrument calibration/maintenance and pre-use bump testing.”

Here at Roco, we’re often asked for an explanation of the difference between “calibration” and “bump testing” of portable atmospheric monitors. There seems to be some confusion, specifically regarding bump testing. Some folks believe that bump testing and calibration are the same thing. Others think that bump testing is no more than allowing the monitor to run its “auto span function” during the initial startup sequence – or by running a “manual auto span” in order to zero out the display if there is any deviation from the expected values.

To preface this explanation, it is important that the user maintain and operate the monitor in accordance with the manufacturer’s instructions for use. There are some general guidelines that apply to all portable atmospheric monitors and some of the information in this article is drawn from an OSHA Safety and Health Information Bulletin (SHIB) dated 5/4/2004 titled “Verification of Calibration for Direct Reading Portable Gas Monitors.”

Considering that atmospheric hazards account for the majority of confined space fatalities, it is absolutely imperative that the instruments used to detect and quantify the presence of atmospheric hazards be maintained in a reliable and ready state. Environmental factors such as shifts in temperature, humidity, vibration, and rough handling all contribute to inaccurate readings or outright failure of these instruments. Therefore it is critical to perform periodic calibration and pre-use bump testing to ensure the instruments are capable of providing accurate/reliable information to the operator.

Calibration of the monitor involves using a certified calibration gas in accordance with the manufacturer’s instructions. This includes exposing the instrument sensors and allowing the instrument to automatically adjust the readings to coincide with the known concentration of the calibration gas. Or, if necessary, the operator will manually adjust the readings to match the known concentration of the calibration gas.

In addition to using a certified calibration gas appropriate to the sensors being targeted, do not ever use calibration gas that has passed its expiration date. The best practice is to use calibration gas, tubing, flow rate regulators, and adapter hoods provided by the manufacturer of the instrument.

The frequency of calibration should also adhere to the manufacturer’s instructions for use; or, if more frequent, the set protocol of the user’s company or facility. Once the monitor has been calibrated, it is important to maintain a written record of the results including adjustments for calibration drift, excessive maintenance/repairs, or if an instrument is prone to inaccurate readings.

Each day prior to use, the operator should verify the instrument’s accuracy. This can be done by completing a full calibration or running a bump test, also known as a functional test. To perform a bump test, use the same calibration gas and equipment used during the full calibration and expose the instrument to the calibration gas. If the readings displayed are in an acceptable range compared to the concentrations of the calibration gas, then that is verification of instrument accuracy. If the values are not within an acceptable range, then a full calibration must be performed and repairs/replacement completed as necessary.

Modern electro-mechanical direct reading atmospheric monitors have come a long way in recent years in terms of reliability, accuracy, and ease of use. But they are still relatively fragile instruments that need to be handled and maintained with a high degree of care. The fact that we rely on these instruments to detect hazards that may be colorless, odorless, and very often fatal should be reason enough to motivate us to complete a very strict schedule of instrument calibration/maintenance and pre-use bump testing.

For more information on this subject, please refer to the November 20, 2002 ISEA position Statement “Verification of Calibration for Direct Reading Portable Gas Monitors Used In Confined Spaces”; “Are Your Gas Monitors Just expensive Paperweights?” by Joe Sprately, and James MacNeal’s article as it appears in the October 2006 issue of Occupational Safety and Health magazine.




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