When choosing a gas detector, the single most important factor to consider is the sensor and its capabilities. Last week, we compared the pros and cons of combustible gas sensors. This week, we'll look at toxic sensors.

The Two Kinds of Toxic Sensors

When choosing a toxic sensor, you have two options: a wet chem toxic sensor and an MOS toxic broad range gas sensor. Let's look at each option:

1. Electrochemical (wet chem) toxic sensors react to specific chemical substances such as chlorine, ammonia, carbon monoxide, carbon dioxide, nitrogen dioxide, nitric oxide, hydrogen cyanide, hydrogen sulfide, sulfur dioxide and hydrochloric acid. Read the technical information supplied by the detector's manufacturer to learn what sensors are available for the unit.

The electrochemical sensor housing contains two, sometimes three, electrodes sitting in a liquid solution, either a base or alkali, depending on what the sensor is "looking for." The housing is covered by a Teflon membrane, which keeps the fluid in the housing but allows in air. As air molecules enter through the thin Teflon membrane, the fluid reacts with a specific substance if it's present.

When the detector is working, a small electrical current passes between the two electrodes. Any change in the fluid's density caused by a reaction to the substance in the air will affect the density of the fluid and change the amount of current passing between the two electrodes. The current then passes through a temperature compensating circuit and the electron flow is read as a specific amount of the substance.

The sensor's ability to detect specific types of gases is based on:

• The choice of membrane;
• The number of electrodes;
• The alloy of the electrodes (gold, lead, etc.); and
• The type of electrolyte fluid.

Pros

• Very good linearity, which makes them very accurate for the substance to which they'll react.
• Can measure either large or small quantities.

Cons

• Have a typical life span of approximately one year.
• Fluid can freeze when left in environments having temperatures lower than 0°C.
• Adversely affected by altitude. (Air pressure at sea level (14.73 psi absolute) is the force required to induce the air into the sensor. An increase in altitude means less force is available to push the air into the sensor, thus reducing the accuracy of the reading.)
• Some substances, such as moisture, affect the sensor by changing the make-up of the fluid. This reduces the amount of electrical resistance and impacts the reading. Check the manufacturer's instructions to see which substances will affect the sensor.
• May generate readings that are abnormal or don't make sense. (Note: This problem can be minimized if you know the hazards in your workplace, have a basic understanding of chemistry, know what interference gases adversely affect your unit and follow strict testing protocols.)

2. Metallic oxide semiconductor (MOS) toxic broad range gas sensors are just one of many MOS sensors on the market. The MOS sensor specifically developed for detecting toxic gases is similar in concentration and operation to those used for the detection of combustible gases.

Pros

• Capable of reacting to a wide range of toxic gases including carbon monoxide, hydrogen sulfide, ammonia, styrene, toluene, gasoline and many other hydrocarbons and solvents.

Cons

• Cannot detect carbon dioxide or sulfur dioxide.
• Incapable of telling you what gas you have encountered or the concentration, only that the atmosphere may be hazardous to your health.

Last word on toxic gas sensors

It's important to note that you should never use an oxygen sensor to detect toxic gases. It is true that a toxic gas will displace the oxygen in a confined space. However, it takes 60,000 PPM of any gas to lower the oxygen from 20.9% (normal) to 19.5% (alarm point). More importantly, 60,000 PPM of any toxic gas will kill you.

Conclusion

There are several types of oxygen sensors available and we'll look at those next week.

SENSORS:
The Natural Sources

By Catherine Jones

Semi-aquatic electrosensor from the land Down Under

In the old days, miners used canaries to monitor air quality. If any toxic gases were present, the canaries died. The birds weren't used because they had any special sensory abilities; they just had a lesser capacity than humans to survive. So their deaths were a warning sign for miners to get out.

But many animals do have sensory abilities that might be worthwhile in some workplaces. For example, if you need to detect the earth's magnetic field, it would be handy to tap into the ring of iron oxide contained in a worker honey bee's abdomen.

Here are some other sensory abilities found in the wild:

• 30,000 lenses: Dragonfly
• Ability to hear in infrasound range: Elephant
• Detect changes in water pressure: Fish
• 100,000 nerve fibers (nearly 6 times the number of touch receptors in the human hand): Star-nosed mole
• Specialized electro-sensing receptors with thresholds as low as 0.005 uV/cm: Shark
• Electric senses that can detect 0.05 microvolts: Platypus
• Ability to detect temperature change as small as 0.002 to 0.003 degrees centigrade: Snake (pit vipers)

MEMBER REPLY
Type Typo

Last week's article on the recall of Black & Decker lawnmowers contained a typo. The products affected by the recall are:

• Black & Decker cordless electric lawnmower, model number CMM1000 or CMM1000R, labeled as Type 1 through Type 4 (not Type 1 through 2 as written last week); and
• Craftsman-brand cordless electric lawnmowers model number 900.370520

PRODUCT RECALL
More Computer Batteries Recalled for Fire Hazards

First Dell, now Apple. On August 24, 2006, Apple Computers Inc. announced a voluntary recall of its lithium-ion batteries used in Apple's iBook G4 and PowerBook G4 notebook computers, a recall of approximately 1.1 million units within the US and 700,000 million outside the US.

Manufactured by Sony, the rechargeable lithium-ion batteries in the notebook computers can overheat and pose a fire hazard. In the nine reports received, there have been two minor burn injuries as a result of the overheating.

The batteries affected by the recall are:

• 12-inch iBook G4, battery model #A1061, battery serial #ZZ338 through ZZ427, 3K429 through 3K611 and 6C510 through 6C626
• 12-inch PowerBook G4, battery model #A1079, battery serial #ZZ411 through ZZ427 and 3K428 through 3K611
• 15-inch PowerBook G4, battery model #A1078 and A1148, battery serial #3K425 through 3K601, 6N530 through 6N551, and  6N601

Note that you must remove the battery from the computer to view the model and serial numbers.

Apple Computers Inc. recommends that consumers stop using the recalled batteries immediately and contact the company to receive a replacement. You can continue to use the computer while waiting for a replacement battery. 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 Apple at 1.800.275.2273 or visit http://support.apple.com/batteryprogram. View this recall online.

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