Infrared detection technology (IR) is used within a range of applications including agriculture, oil and gas extraction, waste management, utilities and food and beverage production, to detect specific gases that absorb IR light at characteristic wavelengths. An infrared light beam passes through a gas cloud and onto collection optics where it is split and sent through filters onto infrared sensors.
Infrared emitters within the sensor generate beams of IR light that are measured by photo-receivers. Hydrocarbon gas molecules absorb light at 3.3 microns, Carbon dioxide molecules at 4.25 microns and other molecules at different wavelengths, so the beam intensity is reduced if there is an appropriate concentration of absorbing gas present. A “reference” beam (around 3.0μm) is not absorbed by gas, so arrives at the receiver at full strength. The %LEL of gas present is determined by the ratio of the absorbed and reference beams measured by photo-receivers.
How do IR Beam Sensors Works?
The Infrared beam sensor uses near identical infrared technology, but where the transmitter and receiver are separated by a distance. When a gas passes between the two and is absorbed by the IR light, the ‘beam is broken’ and the receiver will let you know. Typically, infrared open path detectors have a single gas detection beam 10m to 200m in length.
Advantages of Infrared Beam Sensors
- Infrared beam detectors do not need any contact with the gas to be detected. They don’t need the gas to come to them
- The IR sensors have a quick response. Any target gas crossing the beam is detected
- One beam detector can cover an area, potentially replacing many fixed-point detectors
- They are considered secure due to the point-to-point detection principle
- All the normal pros and cons of IR sensors apply, including fail to safe, no poisoning, long lasting
Disadvantages of Infrared Beam Sensors
- If it is very foggy, that counts as a beam interruption and gas cannot be detected until the fog clears
- Beam detectors can sometimes be quite costly since extra measures need to be designed in to avoid sunlight interaction or excessive vibration affecting the receiver and causing reading inaccuracies
- Cannot detect hydrogen
Why have beam detection?
When detecting gases, it is usual to build a gas detector, install it in a relevant place and wait for the gas to come to it to be detected. Sometimes, that is impractical due to a need to keep some working areas uncluttered for safety reasons, or where the gas needs to be detected close to a leak because the delay in it reaching a detection point would be unacceptable for a critical safety purpose. Under these circumstances having a gas detection system that can be pointed through the region of risk is often a good option.
Sometimes it is thought better to cover a whole enclosed volume with beam IR detectors instead of using many fixed-point detectors. The same applies with hand-held portable laser methane detectors.
A typical installation may be installing 2 beams across the top of several turbines in a power station instead of many fixed-point detector heads.
Here 2 beam detectors are being used instead of 23 fixed point gas detector heads to allow similar coverage. Typically beam detectors are about 6 times the cost of fixed-point detectors to manufacture, making system cost differences marginal. It has been known for some installations e.g., large FPSO floating refineries, to have their operational areas designed around their beam detector gas detection systems.
When detecting and monitoring methane leaks and emissions using portable handheld equipment, it is preferable to use laser IR detection methods. This helps save time as multiple areas can be analysed from one spot and often without having to access a hazardous area, improving worker safety, associated risk assessments and work permit paperwork.