.jpg)
Guide to Air Monitor Sensor Selection
Check out PE Facts to learn more about the importance of maintaining your air monitor!
Air monitors are very important pieces of equipment to your company and can often be life-saving; however, they can also often be overlooked. This guide is intended to help you understand how sensor technology works in most portable gas monitors, and what you can do to maintain your air monitor so that it is working properly in emergency situations. This is a general guide and may not apply to your sensors. You should always refer to manufacturer’s guides for specific instructions on operating your air monitor.
Background
There are many different combinations of sensors that may come with an air monitor. Single-gas air monitors have one sensor for detecting specific individual gases or they may have a combination of sensors to check for a variety of gases such as oxygen, chlorine, carbon monoxide, hydrogen sulfide, ammonia, methane and many others. The four sensors found most frequently are Oxygen, Combustible Gas (LEL sensor), Carbon Monoxide and Hydrogen Sulfide.
Sensor Construction
Oxygen and toxic sensors such as Carbon Monoxide and Hydrogen Sulfide are, for the most part, electrochemical sensors. This means that a chemical reaction takes place inside the sensor causing an electrical current due to the flow of electrons. The electrical current is typically measured and displayed as a value in parts per million (ppm) for toxics and % volume for oxygen.
An oxygen sensor is a consumable sensor, which means that it uses up something inside of the sensor when oxygen is present. In some oxygen sensors lead is converted to lead oxide when the chemical reaction takes place. Eventually all the lead will be used up and the sensor must be replaced. Even if the gas detector is turned off, the oxygen sensor is working. If the oxygen sensor is in air it is automatically detecting the level of oxygen and, therefore, its life is steadily decreasing. Some manufacturers recommend disconnecting the sensor from the unit for a short period of time or storing it in an airtight container. However, if you remove the sensor, it may take anywhere from 15 minutes to 4 hours to warm up and give reliable readings again. These suggestions may only slightly extend the life of an oxygen sensor. They are typically warranted for one or two years, but most oxygen sensors last around two years.
A toxic sensor is non-consumptive, meaning that nothing inside the sensor is used up when it is exposed to its target gas. A toxic sensor generally has a sensing electrode, a reference electrode, a counter electrode and a reservoir of an acid electrolyte (usually sulfuric or phosphoric). The sensing electrode detects gas coming into the sensor and, depending on the sensor, either oxidizes or reduces it. This reaction causes a rise or fall in the electrical potential of the sensing electrode in regards to the counter electrode. The current that is created is in proportion to the gas level and generates a numerical value. Supposedly toxic sensors should last forever, but damage from contamination or leakage reduces the lifespan of these sensors. Most toxic sensors come with a two year warranty, but they typically last for over four years. Toxic sensors hold an acid electrolyte that can harm your instrument if there is a leak; waiting to replace an old sensor can lead to costly repairs.
Combustible sensors (LEL) are different from other sensors because they are not wet electrochemical reactors; rather they are solid state catalytic sensors. They consist of two porous ceramic beads surrounding a coiled wire. Each bead includes a catalyst system, one to make the first bead active and the other to make the second bead inert in order to use it as a source of reference. While operating they draw a current to achieve an elevated temperature. This rise in temperature is due to the active bead burning any combustible gas it comes into contact with. The temperature of the reference bead never changes since it is inert and cannot conduct heat. The heating of the active bead causes an imbalance in the circuit which is interpreted and displayed as a reading (usually % LEL).
LEL sensors are usually guaranteed to last one or two years, but they can last over four years. There are numerous factors that may decrease the lifespan of an LEL sensor. Dropping the sensor could easily cause the fine wire inside to break. A fuel rich but oxygen deficient environment can lead to sensor failure due to a build up of carbon, tar and unburned fuel on the active bead. Tetra-ethyl lead (found in leaded gasoline), volatile silicone oils and silicone (RTV) products that off-gas during curing, halogenated hydrocarbons (Freon, methylene chloride etc) and very high concentrations of high hydrogen sulfide or other sulfur containing gases are poisonous to sensors and can cause them to break. When a sensor becomes poisoned, the first gas that it loses sensitivity to is methane. In other words, a sensor may not be able to detect methane, but it can still reliably identify levels of other combustible gases. This is important to keep in mind when calibrating with pentane or gases other than methane. Some manufacturers are now offering methane-based "equivalent gases" to test sensors for poisoning while still allowing the sensor to be calibrated to a sensitivity scale other than methane.
Conclusion
In many work environments, there is a definite risk of illness, injury or even death from oxygen deficiency or toxic gases. However, these risks can be seriously reduced by using the gas detection technology in air monitors. In order to make sure that your air monitors and sensors are working properly you should always follow manufacturer's instructions regarding regular calibration, sensor replacement and "bump testing" your unit.
Commonly Asked Questions
Q. What is bump testing?
A. Bump testing is when you expose your monitor to a specific concentration of a gas to test if it is functioning properly. Most manufacturers recommend performing a bump test daily or before each use; if the readings are within an acceptable range of the actual concentration, it is not necessary to make a calibration adjustment at that time. Note: this does not take the place of routine calibration. Refer to your owner’s manual for specific instructions.
Q. Will my LEL sensor still detect pentane if I calibrate it with methane?
A. Most LEL sensors are non-specific and will respond to all combustible gases. The difference is in the amount of heat produced on the active bead in the LEL sensor for various 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. Therefore, these readings may be higher or lower than they should be; in the case of pentane, your reading will be lower than the actual levels if 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. It is best to calibrate your monitor with the gas that you expect to find whenever possible. If you cannot calibrate to the specific gas, calibrate your instrument with a gas that reacts the closest to it. Also, make sure that you have the alarm set at 10% LEL or less because the differences due to sensor response at this level are minimal.
Q. Do air monitors have temperature limitations?
A. Yes, air monitors do have temperature limitations. At high or low temperatures, sensors will not react properly and displays and batteries can fail. Operating temperatures vary by instrument; always follow the manufacturer’s guidelines for the ideal operating temperature.
Please Note: The information contained in this publication 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 questions should refer to the cited regulation or consult with an attorney.
