The two main technologies used today in the design of combustible gas safety monitoring systems are infrared gas detection and catalytic bead gas detection. Both technologies reliably detect gas at or below the lower explosive limit (LEL), from 0 to 100 percent LEL.
Each one of these sensing technologies has specific advantages, depending on the application. A thorough analysis of the application’s unique field environment is needed to ensure the proper sensing technology selection and its optimal performance, safety, reliability, and cost-effectiveness. A quick decision, of course, can lead to poor detector choices as well as safety, performance, maintenance, and life-cycle cost consequences.
Catalytic Sensing Is Longtime Workhorse
With over 50 years of proven field performance, catalytic bead (or pellistor) detectors are based on a highly responsive technology. They are single-point detectors for combustible gas detection applications, and are based upon the simple principle that combustible gas produces heat as it oxidizes and the sensor converts the temperature change via a standard Wheatstone-bridge-type temperature transducer to a sensor signal.
The secret of a catalytic detector’s accuracy, longevity, and reliability is in the design of the bead and catalyst system. It is critical to maintain an abundance of active sites as some may become poisoned in service.
The major advantages of catalytic detectors are:
- Simplicity of operation
- Easy installation, calibration, and use
- Long life and low life-cycle cost
- Proven technology with exceptional reliability and predictability
- Ideal for multi-gas applications
- Ease of individual calibration to gases such as hydrogen.
There are two primary limiting factors in catalytic detector technology:
- Catalysts can become poisoned or inactive due to contamination
- The only means of identifying detector sensitivity loss is by checking with the appropriate gas on a routine basis and recalibrating the sensor as required.
Infrared (IR) Gas Detectors Offer Fail-to-Safe Operation
Infrared gas detectors rely on two wavelengths in order to function: one at the gas-absorbing wavelength and the other at a wavelength not absorbed by gas. If a gas intervenes between the source and the detector, the level of radiation falling on the detector is reduced and can be continuously monitored. Gas concentration is determined by comparing the relative values between the two wavelengths.
The major advantages of IR gas detectors are:
- Fail-to-safe operation
- Immunity to contamination and poisoning
- Ability to operate in continuous presence of gas
- Ability to operate in the absence of oxygen or in enriched oxygen.
There are limiting factors in IR technology:
- The gas to be measured must be infrared-active
- Gases that do not absorb IR energy (such as hydrogen) are not detectable
- It does not perform well for multiple-gas applications
- IR source cannot be replaced in the field.
Site Location and Experience Play Roles in Detector Choice
A key advantage of both IR point detectors and catalytic detectors is that they have demonstrated long life performance, including in severe environments. In the harsh environments of refineries, IR detectors offer fail-to-safe operation but still should be checked with gas periodically to verify that gas is free to enter the optical path.
Experience has shown that users of both IR and catalytic technologies do prefer to check the detectors with gas, and, as such, perhaps there is no significant difference in the overall maintenance requirements. In climates with low- and high-temperature extremes, very humid conditions, or hot or vibrating machinery, catalytic detectors are the best choice.
There is clearly a requirement for both IR and catalytic detectors. While IR detectors offer enhanced reliability due to their fail-to-safe style, immunity to poisons, and ability to function without oxygen, catalytic detectors offer application flexibility, simple maintenance, and low replacement costs. Both technologies are reliable, fast detecting, and accurate.
When making a selection in gas detectors, consider the field environment and variables in detector design from manufacturer to manufacturer. Life-cycle cost assumptions will not hold true in all environments. The same can be said for detector mean-time-to-repair or failure data among various manufacturers.
Careful analysis of detectors, suppliers, and field experience will help in selecting the best detector for your application.
Shankar Baliga is the manager for research and development at General Monitors in Lake Forest, Calif. He is responsible for the development of new sensing technologies for gas and flame detection. He is a senior member of ISA, SPIE, and IEEE and a voting member on the ISA 12.13 committee for combustible gas detection instruments. Dr. Baliga received a Ph.D. in physics from Ohio State University. General Monitors manufactures gas detectors, flame detectors, and fire and gas systems that protect from combustible, toxic, and flammable environments.