As a trainer on gas detector use, one question that I hear repeatedly is, Why do I have to “bump test” my gas monitor daily? Another is, when should I calibrate it?
To ensure that a gas monitor will perform as intend- ed in the presence of gas, most manufacturers of portable gas monitors and regulatory agencies alike agree that a bump, or functional test, should be car- ried out on the instrument prior to each day’s use. Manufacturers offer confined space entry kits that provide all the equipment necessary for this opera- tion, and docking stations can bump test as well as record the events.
However, many workers remain puzzled about how often they should bump test or when to subject the instrument to a full calibration. Manufacturer’s rec- ommendations can vary, with language that cites the need for “periodic” or “frequent” testing, and inter- vals between calibrations up to 180 days. Questions abound. Is a monitor with a 180 day calibration interval better than one recommending 30 days? What does OSHA or other governing agencies require? These are simple questions that involve safety, reliability, ease of use, and maintenance costs for a company.
The fact is, there is no one size fits all answer to the question about testing an instrument. Sensor tech- nology, applications and functions of the monitor must be considered.
Sensor technologies used in today’s gas monitors have improved significantly over the last decade. Advances such as electrochemical over solid state, enhanced poison resistance, and less cross-interfer- ence have resulted in more reliable performance. This evolution is one reason why calibration inter- vals have lengthened. Still, gas monitors must oper- ate in harsh conditions and are not impervious to damage. Along with physical shock to the instru- ment, sensors can be damaged by gas concentrations that exceed the detectable limit. Filters and sensor ports can become obstructed by liquid, dirt and dust producing no change in readings even though the atmospheric conditions may have fluctuated. Proper bump testing and verification of accuracy between calibrations can prevent a false sense of security.
The frequency of this testing may depend on the monitor itself. Most if not all confined space moni- tors used today are direct reading and detect for oxygen, LEL (lower explosive level), and H2S and CO at a minimum. Catalytic bead combustible sen- sors are particularly prone contamination that
The power plant and two sister facilities, Bobbitt says, “now use an air powered VAC-U-MAX model with a pulse cleaning system on the filters, that with the push of a button releases the dust from the filter and they can resume cleaning.”
Compliance when Regulations aren’t Clear Fugitive dust “is a moving target that changes depending on the nature of the process and how well plants manage keeping the dust contained,” says Stevenson. Most NFPA guidelines for com- bustible dust state that a layer of dust the thickness of a paperclip is enough dust to cause a significant secondary explosion. The problem he says, “is that
it doesn’t account for the different Kst values between different dusts. Some are more reactive than others. Some are more easily suspended into a cloud. Some tests found that depending on the type of dust, even half of the thickness of a paper clip would be too much.”
Kst values classify dusts according to their explosiv- ity—the rate of pressure rise of a dust in the test vessel upon being ignited.
In situations where many different dusts are han- dled, testing all of them can be prohibitively expen- sive. For instance, in a high performance rubber plant where several different products are manufac- tured within the same plant, the dust in each area of the plant may have different Kst values in each area.
To Calibrate (or Not to Calibrate) your Portable Gas Detector
19 For this circumstance it is recommended to work with an expert in the field to select samples for test- ing that represent the worst case.
This is why, Bobbitt says, that when you are dealing with explosive dust, you may need a Class II Div 2 vacuum in a non Class II Div 2 area. “You might have explosive dust in small quantities, and it might take a very hot and prolonged source of ignition, but with the new combustible dust initiative, facilities need to be very careful that they comply because there is a lot of question as to what compliance means.”
“Although the regulations for combustible dust aren’t real clear,” Bobbitt says, “I find that a lot of companies are simply just trying to get better at general housecleaning.”
Cv Technology’s Stevenson agrees. “The one thing you can do very simply and easily is to keep every- thing clean—it is as simple as that. If you clean the place up and protect your dust collectors, you’ve gone a long way toward minimizing the chance for an explosion even if you do nothing else and those are pretty straight forward easy things that everyone can do,” he says.
For more info about combustible dust industrial vac- uum cleaners or to learn about VAC-U-MAX pneu- matic conveying systems, call 1-800-VAC-U-MAX (800) 822-8629 or (973) 759-4600; e-mail info@vac-u-max.com; or visit www.vac-u-max.com.
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inhibits response to the target gas. Oxygen sensors are also prone to failure where it does not respond to atmospheric changes. Monitors that employ these sensors should be tested prior to each day’s use.
In 2004 the Occupational Safety & Health Administration (OSHA) posted a statement on “Verification Of Calibration for Direct-Reading Portable Gas Monitors” their website as a “Safety and Health Information Bulletin” and can be viewed at www.osha.gov/dts/shib/shib050404.html. OSHA stipulates that this is not a regulation and creates no legal obligations but is advisory in nature and is to assist employers in providing a safe and healthful workplace.
Portable gas monitors are designed to protect work- ers against potentially life threatening occupational environments. Verifying the proper performance is an essential part of any gas monitoring safety pro- gram. Following this protocol will ensure confi- dence in the workers as well as everyone responsi- ble for keeping them safe.
Andrew Saunders is a senior technical trainer for BW Technologies by Honeywell and Honeywell Analytics. Circle 122 on Card
