Proper gas detection equipment can help identify hazardous atmospheres and protect your workers. When choosing a gas detector, the single most important factors to consider are the sensors and their capabilities. I've done a bit of research on this topic and this week I'll share my findings on the types of combustible gas sensors available and compare the pros and cons of each.

Instrument Flexibility

Gas detection manufacturers produce instruments with a variety of sensor configurations. These days, it's not uncommon to find units with one to six gas monitoring options with interchangeable sensors. But they're not one-size-fits-all. You need to be fully aware of the different sensor types and the capabilities of each. Before using a gas monitoring instrument, there are two things you need to know:

1. The hazards that are likely to be present in the particular space you're testing; and
2. The capability of the instrument you're going to use to detect such hazards.

Understanding Sensor Technology

Sensor technology is the foundation of any instrumentation - whether portable or fixed. It's also the cornerstone of accurate compliance reporting. Remember, though, that every sensor has its limitations. If your sensor selection is inadequate or inappropriate for the application, then everything downstream of the sensor will be compromised. So you must match the capability of the sensor with your requirements; otherwise, you'll get inaccurate data and prematurely wear out your sensor.

Combustible Gas Sensor Options

In confined space work, you must monitor for combustible gases. There are three types of combustible gas sensors available for you to choose from: catalytic, metallic oxide semiconductor and infra-red. Let's take a look at their advantages and disadvantages:

1. Catalytic combustible gas sensors detect combustible gases by causing an actual combustion of gases within the sensor chamber.

Catalytic sensors consist of a flame arresting material encasing two chambers, each of which contains a coiled wire filament. One chamber, whose coil is typically coated with platinum or palladium, is designed to allow air to enter. The other chamber, whose coil is not coated, is sealed to prevent air from entering.

Both coils are heated, typically to 500°F or higher. When combustible gases are exposed to the coil, they will ignite and raise the temperature even higher. This temperature increase and the change of the coil's electrical resistance are displayed as "percent LEL."

Pros

• Offer good linearity
• Can react to most combustible gases

Cons

• Work best in concentrations between 1,000 and 50,000 PPM
• Don't measure trace amounts of gas (under 200 PPM) and therefore are of no use determining toxic levels
• Require a minimum of 16% oxygen content in the air to work accurately
• Sensor can be damaged by lead or silicone
• Readings can be affected by humidity and water vapor condensation
• Tend to lose their linearity after a year or so
• Not recommended for use in an acetylene atmosphere

Note: The flame arrestor will prevent ignition of most gases outside the sensor, except acetylene. It is extremely important to check the approvals to determine the types of hazardous locations the detector can function within.

2. Metallic oxide semiconductor (MOS or "Solid State") combustible gas sensors consist of a housing (either a stainless steel sintered cup or plastic) containing an electric conductor. This conductor is made up of a heating element, typically operating between 150°F and 350°F, and a bead that contains a mixture of metal oxides.

As the electrical current travels through the bead when exposed to clean air, a base resistance is established. When a gas comes into contact with the sensor surface, a change in sensor resistance occurs. The sensor resistance can change significantly even with small quantities of gases (less than 200 PPM).

Pros

• Long operation life (typically 3 to 5 years)
• Very rugged with capacity to recover from high concentrations of a gas that could damage other types of sensors

Cons

• Require oxygen to work accurately, although not as much as the catalytic
• Some sensors' heating elements have a high demand for power that requires large battery packs
• Readings may be affected by humidity and water vapor condensation

3. Infra-red combustible sensors have recently begun appearing in some instruments. These sensors work by reflecting light off a mirror and measuring the amount of light adsorbed during refraction.

Pros

• Work well in low oxygen levels or acetylene atmospheres

Cons

• Quite expensive
• Typically require a constant flow across the sensing assembly and may be slow to clear from alarm
• Unable to detect hydrogen

Conclusion

Depending on what you need, there is quite a selection for combustible gas sensor technology. Next week, we'll look at various toxic sensors and discuss their pros and cons.

Two Products Recalled for Fire Hazards

By Catherine Jones

1. Dell notebook computer batteries

Got Dell? Don't go unplugged.

In case you missed last week's buzz, the US Consumer Product Safety Commission has announced a voluntary recall of Dell notebook computer batteries involving approximately 2.7 million units within the US and 1.4 million outside the US.

Manufactured by Sony, the Dell-branded lithium-ion batteries in the notebook computers can overheat and pose a fire hazard. Fortunately, there have been no injuries reported, but Dell has received six reports of the batteries overheating and damaging furniture and personal effects.

The batteries affected by the recall were sold either separately for or with the following Dell notebook computers:

• Latitude - D410, D500, D505, D510, D520, D600, D610, D620, D800, D810
• Inspiron - 6000, 8500, 8600, 9100, 9200, 9300, 500m, 510m, 600m, 6400, E1505, 700m, 710m, 9400, E1705
• Dell Precision - M20, M60, M70 and M90 mobile workstations; and
• XPS, XPS Gen2, XPS M170 and XPS M1710.

Dell recommends that consumers stop using the recalled batteries immediately and contact the company to receive a replacement. Note that you can continue to use the computer. Simply remove the battery (with the system off, of course) and power the computer using the AC adapter and power cord.

For more information, contact Dell at 1.866.342.0011 or visit www.dellbatteryprogram.com. View this recall online.

2. Black & Decker Cordless Electric Lawnmowers

In 2002, the US Consumer Product Safety Commission announced a voluntary recall of 140,000 Black & Decker and Craftsman-brand lawnmowers. That recall has now been expanded.

In the original recall, there were 11 repots of an electrical component in the lawnmower overheating, resulting in one minor hand burn and property damage. Since the original recall, Black & Decker has received an additional 10 reports of electrical components in the lawnmower overheating.

The products affected by the recall are:

• Black & Decker cordless electric lawnmower, model number CMM1000 or CMM1000R, labeled as Type 1 through Type 2
• Craftsman-brand cordless electric lawnmowers model number 900.370520

All consumers - including those who had their mowers repaired in the previous recall - should contact either Black & Decker at 1.866.229.5570 or Craftsman at 1.888.375.9741 for more information or visit www.blackanddecker.com. View this recall online.

Click on the links more information on Safety Compliance or OSHA Compliance.

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