Review of sensor technologies used in personal safety instruments

Gas detection and monitoring systems are used as safety devices to alert workers of the potential danger of poisoning by toxic gas exposure, suffocation due to lack of oxygen and fire or explosion caused by combustible gases. The sensors in any gas monitor are the heart of the instrument, and the foundation of gas detection.

Which sensing technology is most appropriate for my application?
Why are some sensor types used for certain gases, but not others?
What is the life span and shelf life of my sensor?

To help answer these questions and more, each sensor technology will be described in terms of general principles, common uses, advantages and disadvantages. A gas sensor selection guide compares all sensor types across common performance indicators to put it all in perspective.

Catalytic
The catalytic sensor, also referred to as the catalytic bead sensor, is commonly used to detect and measure combustible gases and vapors from 0-100% LEL (lower explosive limit). The sensor’s response to a combustible gas depends on the chemical composition, the molecular weight and vapor pressure of the gas. Also, a minimum oxygen concentration of 5-10% by volume in the mix of diffused gas is generally required for the sensor to operate properly. The catalytic sensor is less sensitive to temperature and humidity effects, offers repeatable performance and is relatively stable. It is, however, susceptible to poisoning or inhibition from some gases, which may decrease its sensitivity or damage the sensor beyond recovery. The catalytic sensor is used in both portable and fixed gas detection systems.

Thermal Conductivity
For many years, the thermal conductivity sensor has been used in instruments for measuring combustible gases above the % LEL range and for leak detection. The thermal conductivity sensor does not require oxygen to operate, and it is not susceptible to poisons. One drawback is that it cannot measure gases with thermal conductivities similar to the reference gas (i.e. Nitrogen). Thermal conductivity sensors are used primarily in portable gas leak detectors.

Non-Dispersive Infrared Absorption (NDIR)
The non-dispersive infrared sensor, commonly referred simply as the infrared sensor, can detect gases in inert atmospheres (little or no oxygen present), are not susceptible to poisons, and can be made very specific to a particular target gas. The limitation of NDIR technology for gas detection is dependent on the uniqueness of the absorption spectrum of a particular gas. NDIR sensors are also extremely stable, quick to respond to gas and can tolerate long calibration intervals. Infrared sensors are commonly used to detect methane, carbon dioxide and nitric oxides in both portable and fixed gas detection instrumentation.

Metal Oxide Sensors (MOS)
A variety of MOS sensors are available for the detection of combustible gases, chlorinated solvents and some toxic gases, such as carbon monoxide and hydrogen sulfide. MOS sensors, also referred to as solid-state, are inherently non-specific, and as a result are quite useful in applications where the atmospheric hazards are unknown. The output of the MOS sensors varies logarithmically with the gas concentration. This limits the accuracy of the sensor and the overall measuring range of the sensor. Changes in the oxygen concentration, humidity and temperature also affect the sensor performance. Although MOS sensors are relatively low cost, the stability and repeatability of the sensor are poor. Power consumption is high due to the heating of the element, which restricts the use of this sensor in portable devices. MOS sensors are commonly used in low cost, hard-wired fixed gas detection systems.

Electrochemical
Electrochemical sensors are widely used for the detection of toxic gases at the PPM level and for oxygen in levels of percent of volume (% vol). Toxic gas sensors are available for a wide range of gases, including sensors for carbon monoxide, hydrogen sulfide, sulfur dioxide, nitrogen dioxide, chlorine and many others. Although the sensors are designed to be specific to each gas, there are often some cross interferences with other gases present. Electrochemical sensors are usually small (typically ? 1 inch diameter) and require little power usage which is beneficial for portable gas monitors. The sensors can be used over a wide temperature range (-20? to +50?C is common), though for improved accuracy temperature compensation is often built into the instrument electronics. Overall, electrochemical sensors offer very good performance for the routine monitoring of toxic gases and percent of volume oxygen presence in both portable and fixed gas monitors.

Fiber Optic
In the last decade there have been considerable research efforts to develop fiber optic chemical sensors to take advantage of the low manufacturing costs associated with the technology. Fiber optic technology produces superior sensor performance than other technologies for certain compounds. For example, ammonia detection with any other technology is unstable and costly. Other advantages of fiber optic technology include small size and low cost, especially with the ability to reactivate the sensor. The major disadvantage is a limited temperature range capability, but heating devices can be incorporated into the electronics to counter this deficiency. Commercial availability is limited at this time, but fixed gas monitors using a fiber optic sensor for ammonia detection are currently.

Photoionization Detector (PID)
Photoionization detectors are often used in situations where high sensitivity (sub-PPM levels) and limited selectivity (broad-range coverage) is desired. PID’s are commonly used for detecting Volatile Organic Compounds (VOC’s) such as benzene/toluene/xylene, vinyl chloride and hexane, and provide quick response for this growing concern. Advantages of this technology include the fast response time and excellent shelf life (however sensor life is poor). Major disadvantages are that PID’s suffer from sensor drift and humidity effects, making calibration requirements more demanding than other common gas detectors. PID technology is most commonly found in portable instrumentation.

This summary of gas sensing technologies is not exhaustive, but hopefully is comprehensive for comparative purposes. Although some of the technology platforms date back fifty years, research and development efforts continually challenge and improve the performance of sensors used for gas detection. Also, new technologies emerge as applications require differing needs. While some applications require only a gross indication of detection, others may require the precision of specific gas measurement. Marrying the best-suited technology with a solid electronics design gives you the ideal match for the best personal protection against hazardous gases.

Authored by the Research and Development team of chemists and physicists of Industrial Scientific Corporation. The growing team of chemists and physicists develop new sensing solutions for gas monitoring, measuring and controlling. For more information contact Kay Mangieri, Director of Marketing by e-mail kmangieri@indsci.com or phone (412)490-1869 or (800)338-3287.