Confined space monitors

SOMETHING IN THE AIR?

By David Dehaas

Choosing a gas monitor for your confined spaces is not as hard as it would appear, but getting it right is a matter of life and death

So, you’re looking for a confined space gas monitor. There’s just one problem, if you want to be entirely accurate about the concept: There’s really no such thing.

Yes, you can look in the catalogue from your favourite safety supply house, and, yes, you will find a section or a few pages full of gas monitors called "confined space monitors", but, says Tim Morrison, president of Safetyscope, it’s very important to understand that these are gas detectors, no more, no less. "There really isn’t any such thing as a confined space gas monitor," he says. "Traditionally, when you use the term, you’re talking about a device that looks for four things, usually oxygen concentration, flammable gases and two toxics -- carbon monoxide and hydrogen sulfide."

That means a typical "confined space gas detector" is really a combination of four separate detectors that measure the four things that most commonly lead to problems in confined spaces. But you can also buy detectors that look for any one, two or three of those things. Or that look for other gases and vapours that you have reason to believe may be in your confined space. But there is no gas detector that will screen for all toxic gases; you essentially need a separate detector or module for each suspected toxic gas.

At least 70 per cent of the gas monitors sold for use in confined spaces do check for oxygen, explosive gases, carbon monoxide (CO) and hydrogen sulfide (H2S), says David Wagner, product manager for portable instruments at Industrial Scientific Corporation, "because they’re by far the most common hazards. But the one thing the manufacturer cannot do is guarantee the suitability of the instrument for the application. If you test for CO and H2S, but what you’ve got in the space is sulfur dioxide, well, the gas detector is not going to tell you that."

"When you select a gas monitor to test confined spaces before entry, you have to understand what toxics there may be in the space," says Morrison, whose company specializes in confined space safety. That means you have to do an audit, first, have a good understanding of what might be in the space, and then use the appropriate monitor to test for those things. For that, he says, "you need industry knowledge, you need to know what gases may be present, and you really need to know how to read an MSDS (material safety data sheet)."

That’s why you need an audit and needs assessment before you choose a gas detector to test your confined spaces. "It depends on what’s there," says Greg Boyko, product manager for portable gas detection at Draegar Canada Ltd. "If you’re in the oil fields, you’ll check for H2S for sure. In the steel industry, a big problem is CO. But if you work in a brewery, the problem is probably not going to be flammables, it’s oxygen deficiency and carbon dioxide (CO2) that will be the most likely concern."

You also need some legal knowledge. Every jurisdiction in Canada has regulations covering confined space entry, and they all include very specific requirements for assessing the hazards in the space, testing the atmosphere and, in many cases, keeping records of the testing.

How it works

A gas detector is a small device -- they range from single-gas detectors about the size of a small TV remote control to more complex ones carried from a shoulder strap -- that can sample the air and detect a variety of contaminant gases and vapours at incredibly small concentrations. When activated, they use a small air pump to draw a sample of air and analyse it for the target gas or vapour.

Most detectors have a digital readout that tells you how many parts per million of the contaminant gas are present in the sample. This number can be compared to the threshold limit value for the contaminant to decide whether the level is safe, whether ventilation of the space is required and/or whether workers need to wear breathing protection. Most detectors also have an "alarm level" that is either preset at the factory or by the user; when a predetermined danger level is reached (too much of a contaminant, or too little oxygen, for example) an audible alarm goes off warning workers to leave the space immediately.

Many gas monitors have a data logging feature that stores information as readings are taken. This data can be downloaded, with the proper software, to a desktop computer and can be printed out. This gives a permanent record that is very useful if you are required to do continuous monitoring, says Wagner. "It gives you a record of exposure," he says, "that can be crucial if there is ever an incident or an investigation. It tells you what was there, and what the exposure was."

Gas detectors use several different kinds of technology to do the job, explains Boyko. "Most of the sensors out there are electrochemical," he says, "which is somewhat analogous to the way a battery works. You have two electrodes, one on either side of an electrolyte, which is a gel with one component missing." The gel acts like the chemicals in the battery, but it does not allow any current to flow because of the missing component.

The "missing component" is the gas or vapour the module is designed to test for. "If it’s CO, for example, the gel absorbs the substance and now the gel will allow a current to flow and that’s what registers on the detector. The more of the substance you have, the more of it that is absorbed into the gel, the more current you get," says Boyko. And, of course, the level of current from the module is processed by the electronics in the detector and translated into a reading for the amount of contaminant.

Flammable gas detectors work on an entirely different principle, explains Boyko. For one thing, the flammables detector will detect any flammable gas or vapour, and it won’t tell you what you’ve got -- just that there is a flammable atmosphere. "A flammable gas detector uses a catalytic heat sensor," says Boyko. "You have two poles connected by a platinum wire that is heated to between 500 and 700 degrees Celsius. When a flammable gas is exposed to the wire, it "burns" catalytically and produces heat. This changes the conductivity of the wire, and that’s what the detector picks up."

The audit of gases and vapours that could be in the confined space is important before you choose a detector. "First and foremost, you have to make sure that the detector is suitable for the hazards you’re trying to protect your people from," says Wagner. "Depending on the gases that may be present, you have to ask if there is any cross-sensitivity, or any gases that will interfere with one of the other sensors. You don’t want to get any false readings."

Also, the flammable gas detector will not operate in an inert environment -- such as if a space has been purged with nitrogen to remove a flammable gas or vapour. "The flammable gas detector works by catalytically burning the gas," says Wagner, "and that won’t work in an oxygen-depleted atmosphere." In other words, you would get a false-negative reading for explosive gases.

Oh, and one more problem: Let’s say you’re using a flammable gas detector to warn of explosive levels of methane (which you would do if there was any conceivable source of the gas). Says Morrison, "The problem with a flammable gas is that the indicator will measure 50 or 60,000 parts per million [before the alarm sounds], but the toxic threshold is much lower. The IDLH for methane [the level at which it is immediately dangerous to life and health] is 1,200 parts per million, so you have a toxic gas problem way before you have a flammable gas problem."

Calibration

If you have a gas detector, you will have to calibrate it from time to time to ensure that it is giving accurate readings. "It’s actually a very easy process," says Morrison. "The problem is that it isn’t done often enough."

To calibrate a given detector, you will need a calibration gas: a small cylinder containing a known concentration of the gas the monitor is designed to detect. Let gas out through a valve and pass it over the detector; it should read out the known concentration (or alarm, depending on the test). If the detector reads out a slightly different level, it’s a simple matter to adjust the device until the reading matches the known concentration.

Some detectors make it even simpler. Once set in calibration mode, they adjust themselves when exposed to the calibration gas. Most gas detector manufacturers also provide calibration kits containing test gases, instructions and attachments for testing.

It should be noted, however, that there may be other very nasty things in the air of a confined space that you can’t detect with a gas monitor because they’re not gases. Such as mists, or liquids that have been turned into tiny droplets that float in the air. They’re still in the liquid state, but the droplets are so small that they act almost like a gas, and can be inhaled, can settle on the skin and be absorbed, or, if they’re flammable, they can be readily ignited. Then there’s dust, tiny particles of any solid substance. It, too, floats in the air and, at the wrong concentration, can be ignited (especially if it’s, say, wood or coal dust or flour or sugar or even powdered aluminum). And, finally, there are the fumes -- tiny particles of solids, often produced by processes such as welding, that can be inhaled.

Which brings us back to the fact that there’s no one, all-purpose device that will check out the atmosphere in all confined spaces. The point is that you will have to use a gas detector for those gases and vapours that may be present in your confined space. That will include oxygen levels, may include flammables and could possibly involve any of hundreds of possible toxic contaminants that could be lurking in your space.

Tim Morrison sums it up well: "Gas detection is not really supposed to warn us of hazards we didn’t know were there, it’s supposed to verify how good a job we did on the hazard assessment, and to measure how well we’re doing controlling the hazards."

David Dehaas is the editor of ohs canada.

What to look for

What gases and vapours do you need to check a confined space for? There are three classes of things to check out with a gas detector in a confined space. 

Oxygen: You should always check for oxygen concentration, to make sure that it is in the acceptable range close to the 20.9 per cent found in normal air. Many hygienists consider a range of 19.5 to 23 per cent acceptable; British Columbia’s regulation calls for "about 20.9 per cent" and the industrial regulation in Ontario mentions 18 to 23 per cent. Below the acceptable level, workers risk asphyxiation from too little oxygen, and can be quickly overcome. Low-oxygen atmospheres are generally caused by one of several processes: burning (oxidation) of a material in the space; biological action of organisms; rusting or oxidation of a substance; or displacement of the normal atmosphere in the space by another another gas produced in, or introduced into the space (especially "inerting" gas such as nitrogen, used to purge spaces or prevent spoilage of foodstuffs in tankers).

Above that acceptable level, the risk of fire and explosion increases as the excess oxygen makes ignition and burning much easier. Excess oxygen can be caused by chemical processes, but is most frequently a result of leaking oxygen cylinders or pipes. 

Flammable gases and vapours: Check spaces for flammable atmospheres -- gases or vapours that may ignite, burn and/or explode. When a gas or vapour ignites in a confined space, the result is a fire or explosion, a rapid depletion of breathable oxygen and the production of toxic smoke and gases -- none of which are conducive to survival. A detector for flammable atmospheres (they’re often called LEL, or lower explosive limit, detectors) samples the air and measures for a wide variety of gases and vapours that may burn. They’re designed to sound an alarm at a given concentration of the flammable material -- usually 10 per cent of the LEL. The list is a long one: residues of fuels, solvents or other flammable liquids; methane or other gases given off by biological processes; hydrogen given off by chemical reactions; and gases, such as propane or natural gas that may leak into the space. 

Toxic gases: Then there are the toxic gases, which are vastly more complicated to test for than oxygen and flammables. You have to test for the ones that may be there, and that means an audit and assessment beforehand to tell you what to check for. Carbon monoxide produced by incomplete burning and hydrogen sulfide produced by decomposition of organic matter are the "big two", but the list of possibles is almost endless. To track them down, you need to ask questions: What was in the space before, that may still be there as a residue? What substances are used nearby, that may leak into the space? What substances may be produced or released in the space by chemical, physical or biological processes?

Buying Tips

* Before you choose any gas detector, a complete audit of the workplace, or type of workplace, must be performed to determine what gases and vapours could be present. Without this information, you cannot make an informed choice.

* "Make sure the seller will provide training," says Greg Boyko. But, he warns, training in how to use that particular detector may not make you a "competent person" as required by law. "It’s like when you buy a car down at the Ford dealership," he says, "they’ll show you all the features of that car and how to operate them, but they don’t teach you to drive."

* Many detectors come with training literature and videos, says Morrison. "It’s a good idea to have the sales people go over the video with you," he says. "That way, they’re there to answer questions." You should get as much training as you can, he says, and use the videos and materials as refresher training afterward.

* "You have to bring in the users," says Boyko, "you’ve got to give the users an opportunity to use the thing for a little while." For that, you need an opportunity for a field evaluation, he says. "When you talk to the sales reps, ask them if you can borrow one for a week before you decide."

* "Make sure the instrument is protected from the environmental factors in the space," says David Wagner. "Compare the ‘ingress protection rating’ if things like dust and water are a factor in the space."

* "Sensors can fail after six months," says Morrison, "but the average is 12 to 18 months, depending on the type and how often it’s used." Boyko agrees. "Check out the warranty on the sensor," he advises, "but also ask how quickly you can get a replacement unit. You don’t want to stop work while you wait for a replacement sensor."

* Check the "Mean Time Between Failure" rating for the device, advises David Wagner. It’s a comparative figure that gives you some indication of how reliable the instrument is.

* Sensor technology is the key issue, says Boyko. "You have to get the data sheets to check long term-drift and zero drift", he advises. "Any sensor will have drift," he says, "and wander off zero through aging." But some do not read out negative figures if the calibration drifts off below zero. "So the sensor may be off by minus five," he says, "but still read zero. Then if you’re exposed to 10 parts per million, the device will read five."

* "Look for a detector that uses normal language in its readouts," says Morrison, "rather than codes that have to be looked up in the manual."

* In calculating the price of a detector, figure in the cost of sensor modules you may need in the future, maintenance cost, replacement parts and calibration kits. Some detectors require special calibration kits while others use cheaper "off the shelf" calibration gases.

* Consider the cost of options such as remote sampling pumps and tubes.

* Consider battery type and life. Some detectors use AA batteries, some use rechargeable battery packs you can buy in a hardware store and others use special battery packs that may cost hundreds of dollars.

* If you plan to purchase a device with data-logging capacity, consider the cost of the associated software and its compatibility with your computers.

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