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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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What does it mean when my atmospheric monitor gives negative or minus readings?

Thursday, December 02, 2010

At some point, most atmospheric monitors will display a “negative” or minus reading for a flammable gas or toxic contaminant. First of all, it is not actually possible for an atmosphere to contain a “negative amount” of a substance. These negative readings usually result from improper use of the monitor.

Most monitors will “Field Zero” or “Fresh Air Calibrate” its sensors when powered on. Because of this, it is very important to power on the unit in a clean, fresh air environment away from confined spaces, running equipment or other possible contaminants. Otherwise, the monitor may falsely calibrate based on the contaminant that is present.For example, a monitor that is powered on in an atmosphere that contains 10 ppm of a contaminant and then moved to fresh air may display a reading of minus 10 ppm. Even more troublesome, if that same monitor is then brought to a confined space that actually contains 25 ppm of the contaminant, it may display a reading of only 15 ppm. As you can see, this could easily lead to the improper selection of PPE for the entrant and result in a confined space emergency.

As always, it is very important to consult with the manufacturer of your particular atmospheric monitor in order to determine how to use it properly. Don’t take any chances with this critical part of preparing for confined space entry or rescue operations.
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How much training is needed for attendants on air monitoring equipment?

Wednesday, September 01, 2010

Reader Jeff Machen had a question concerning how much training to give attendants on air monitoring equipment; especially when they may only be working a week long shut down? Here’s our reply from CSRT Manager Bryan Rogers.

When you’re dealing with temporary labor, it is difficult to ensure that they are well trained on something as complex as atmospheric monitoring. We checked with several equipment manufacturers, and they don’t set a specific amount of training required, but leave it up to the customer’s internal company policy and/or person(s) issuing the monitor.

We also spoke to a few of our instructors who work at different plants and refineries. The majority of these companies require a company employee to perform the initial monitoring and then again after a break in work greater than 30 minutes. In addition, they review with the attendant what to look for and what to do if there are changes in the readings or an alarm sounds. One company provides a four-hour PowerPoint presentation on monitoring and attendant responsibilities.

OSHA does not indicate a time frame for this training either. However, it does require that persons be capable of safely performing the tasks assigned. Therefore, I would say your best bet would be to cover as much of the manufacturer’s instructions as possible along with reviewing the most common problems such as…

    - Calibration conversions
    - Turning on the monitor (or “field zeroing”) in the presence of contaminates
    - Negative LEL or negative toxic readings
    - Contaminated sampling hoses
    - Clogged filters

Lastly, I would stress to the attendants the importance of contacting a supervisor if they have any questions or concerns - and, if they get any unusual results from the monitor… “Do not hesitate to have everyone exit the space while the results are investigated!”
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