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Air Monitor Sensor Selection

Quick Tips #293

Introduction

Air monitors may be one of the most important pieces of life-saving equipment in your company's inventory; unfortunately, they are also sometimes the most neglected. This document is intended to be a guide to understanding how the sensor technology works in most portable gas monitors and what you can do to ensure that your air monitor is working properly when someone's life depends on it. This is intended only as a general air monitor sensor selection guide and may not apply to the sensors in your specific monitor. You should always follow specific manufacturer's instructions for care and use of your air monitors.

Background

Air monitors can be purchased with many combinations of sensors. Single-gas air monitors are available with one sensor for detecting individual gases or with combinations of different sensors to detect gases like oxygen, chlorine, carbon monoxide, hydrogen sulfide, ammonia, methane and many others. This air monitor sensor selection guide focuses on the four most commonly found sensors: oxygen, combustible gas (LEL sensor), carbon monoxide and hydrogen sulfide.

Sensor Construction

Most oxygen and toxic sensors (such as carbon monoxide and hydrogen sulfide) are electrochemical sensors. A chemical reaction takes place inside the sensor causing electrons to flow in one direction or another. The electron flow constitutes an electrical current, which is measured and displayed as a value—typically parts per million (ppm) for toxic gases and % volume for oxygen.

An oxygen sensor is considered a consumable sensor, meaning that something inside the sensor is used up when exposed to oxygen. For example, some oxygen sensors use lead. When the reaction takes place, the lead is converted to lead oxide. Eventually, the sensor consumes all the lead inside, and the sensor will fail and must be replaced. An oxygen sensor is always working in a gas detector circuit—even if your monitor is turned off. Anytime the oxygen sensor is stored in air, your sensor life is steadily decreasing. Some manufacturers suggest temporarily removing the sensor from the unit or storing the unit in an airtight container. These suggestions will only slightly extend the life of the sensor, and if you remove a sensor, there can be a 15-minute to four-hour or more warm-up period required before you can use the sensor again. Oxygen sensors are usually warranted for one or two years, but a typical oxygen sensor should last around two years.

A toxic sensor is nonconsumptive; nothing is consumed when it is exposed to the target gas. In general, a toxic sensor contains a sensing electrode, a counter electrode, a reference electrode and a reservoir of an acid electrolyte (usually sulfuric or phosphoric). Gas diffusing into the sensor reacts with the sensing electrode and is oxidized or reduced, depending on the sensor. This reaction causes the electrical potential of the sensing electrode to rise or fall with respect to the counter electrode. The current generated is proportional to the gas present and is displayed as a numerical value. Theoretically, a toxic sensor should last indefinitely, but factors such as leakage and contamination limit the life of these sensors. Most manufacturers warranty their toxic sensors for two years. However, they can last up to four or more years. Toxic sensors contain an acid electrolyte that can damage your instrument if they leak. Waiting to replace an old sensor can lead to costly repairs.

Combustible sensors (LEL) are different from the other sensors as they are a solid-state catalytic sensor, not a wet electrochemical. They are designed around two porous ceramic beads surrounding wire that has been wound into a coil. Each bead contains a catalyst system, one to make the first bead active, and the other to make the second bead inert so it can be used as a reference. During operation, they draw current to stay at an elevated temperature. When the active bead comes in contact with a combustible gas, the gas begins to burn causing the temperature of the bead to increase. The reference beads temperature does not change, since it is not capable of burning the gas. The heating of the active bead causes an imbalance in the circuit, which is interpreted as a reading (usually % LEL) on the display.

Most LEL sensors are guaranteed one or two years, but they can last up to four or more years. Many factors can shorten the life of an LEL sensor. A strong shock, such as dropping an instrument, could break the fine wire inside the sensor resulting in an immediate failure of the sensor. If an LEL sensor encounters a fuel-rich but oxygen-deficient environment, tars, carbon and unburned fuel can build up on the active bead, which can lead to sensor failure.

Certain types of materials will also poison the sensor, rendering it ineffective. These poisons include tetra-ethyl lead (found in leaded gasoline) volatile silicone oils and silicone (RTV) products that off-gas during curing and halogenated hydrocarbons (freon, methylene chloride, etc) and very high concentrations of hydrogen sulfide or other sulfur-containing gases.

When a sensor becomes poisoned, the first gas that it loses sensitivity to is methane. This means that a poisoned sensor may exhibit a loss of sensitivity when exposed to methane but can respond normally to other combustible gases. This is particularly important if you are calibrating with pentane or another calibration gas other than methane. Many manufacturers are making equivalent gases that are methane-based to test sensors for poisoning while still allowing a sensor to be calibrated to a different sensitivity scale than methane.

Conclusion

There is a risk of injury, illness or even death from oxygen deficiency or toxic/combustible gases in many work environments. The gas detection technology in air monitors can minimize these risks. The only way to ensure that your air monitors are functioning properly is to follow your manufacturer's maintenance recommendations, including regular calibration, sensor replacement and bump testing your unit.

Commonly Asked Questions

Q.   What is bump testing?
A.   Bump testing is exposing your monitor to a known concentration of a gas to ensure that your monitor is functioning correctly. Most manufacturers suggest performing a bump test daily or before each use. If the readings are within an acceptable range of the actual value, it is not necessary to make a calibration adjustment at that time. Note: bump testing does not take the place of routine calibration. Refer to your owner's manual for specific procedures.
 
Q.   If I calibrate my LEL sensor with methane, will it still detect pentane?
A.   A typical LEL sensor is nonspecific and will respond to all combustible gases. The problem is that the amount of heat produced on the active bead in the LEL sensor is different for different combustible gases. If the gas you are detecting is different from the gas you used for calibration, your monitor will give % LEL readings on the display based on your calibration gas. These readings may be higher or lower than actual. In the case of pentane, your display will be lower than the actual levels because the sensor was calibrated with methane. Manufacturers provide LEL correlation factors with their units, so you can calculate the LEL of the sampled gas based on your calibration gas. If you can, it is always best to calibrate your monitor with the gas that you expect to find. If you cannot calibrate to the specific gas, calibrate your instrument with a gas that responds the closest to it and make sure that you have the alarm set at 10% LEL or less. At 10% LEL or below, the differences due to sensor response are minimal.
 
Q.   Are there temperature limitations for air monitors?
A.   Yes, there are temperature limitations for air monitors. At high or low temperatures, sensors will not respond correctly and displays and batteries can fail. Operating temperatures vary by instrument. Always follow the manufacturer's operating temperature guidelines.

Sources

www.osha.gov

International Safety Equipment Association
1901 North Moore Street
Arlington, VA 22209-1762
P: (703) 525-1695

(Rev. 1/2012)

 

Find even more information you can use to help make informed decisions about the regulatory issues you face in your workplace every day. View all Quick Tips Technical Resources at www.grainger.com/quicktips.

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Please Note:
The content in this newsletter is intended for general information purposes only. This publication is not a substitute for review of the applicable government regulations and standards, and should not be construed as legal advice or opinion. Readers with specific compliance questions should refer to the cited regulation or consult with an attorney.

©2012 W.W. Grainger, Inc.

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