Generally speaking, gas detectors are mainly divided into two categories, stationary gas detectors or portable gas detectors. Both types use various gas sensor technologies to detect gas or vapor hazards. Both types have their own advantages and disadvantages as well as subcategories. Therefore, it is extremely important to understand the nature of the gas or vapor hazard you are trying to mitigate in order to determine the most appropriate detection solution. Risk assessment is always the starting point. Mitigation will take several forms – gas detection is one of them. At this point, it is usually best to let the gas testing company find a suitable solution.
Fixed Gas Detectors
Permanently installed on-location, fixed gas detectors are used to continuously monitor environments. Fixed detectors work in a similar manner to fire and smoke alarms. Detectors specific to a target gas are deployed on site which are connected to a control panel. The controller monitors the detectors and sets alarm conditions based on gas levels. At a basic level audible and visual alarms are triggered to alert personnel. More advanced installations may alert other monitoring systems, activate shut down systems, log and trend data, connect to cloud-based systems and more. This is one of the main benefits of fixed over portable gas detectors.
Fixed detectors will monitor a limited area/volume around the detector, typically a 5M radius. This means placement needs to be carefully considered for best performance. Detectors may be diffusion types or pump (aspirated). The right choice is application dependant.
The range of measurement for the detector needs to be considered and that alarm levels meet HSE/COSHH/DSEAR requirements. In some cases, detectors may operate as stand-alone devices with their own in-built alarms and audible visual displays. Fixed detectors can also be either classified and approved for use in ATEX-zoned hazardous areas or what are termed safe areas. That choice will usually be based on a gas hazard survey and or a DSEAR report depending on the site and application. There is also a distinction between fixed detectors that are intended for life safety applications and those that are used for air quality monitoring. The latter will generally not be suitable or carry approvals for life safety application.
Fixed gas detectors are appropriate for use where the gas hazard is known, and detectors can be fitted strategically to detect leaks or emissions. In these situations, fixed systems meet BATNEEC requirements as a mitigation solution (Best Available Technology Not Entailing Excessive Cost). Consider also that areas not usually occupied can be more of a problem than areas that are normally occupied. There is always a duty of care to provide as safe an environment as possible for personnel.
Portable Gas Detectors
Portable gas detectors are designed to be carried or worn clipped to an item of clothing. They form part of a person’s PPE. This category of detector is a life safety device designed to provide alarms to the wearer when target gas levels exceed safety limits. They should not be mixed up with handheld gas leak detectors or analysers which are not intended as life safety devices. They can screen for hazardous species that may enter an operator’s breathing zone or be used for confined space entry testing. Portable gas detectors can be just for a single gas or may monitor for up to seven gas types. They may be diffusion-based or be pumped depending on the application. Portable gas detectors are intended for use where it is impractical to fit a fixed system.
Deciding Between Fixed and Portable Gas Detectors
As a general rule: If it is at all practical a fixed system is always preferable to a portable gas detector.
Portable gas detectors may be required to verify alarms from the fixed system or on large sites to provide supplemental cover. If fixed systems are impractical (e.g. no one is fitting fixed gas detection systems in sewers) then use portables.
It is important to involve gas detection specialists at an early stage. You may identify through risk assessment a gas hazard. That is the safety requirement for any owner/operator. Once identified understanding the hazards of your application is critical to choosing between a fixed and/or a portable gas detector. A gas hazard overview will consider:
How are gases stored?
How are gases transmitted/delivered?
How and where are gases used?
By products of gas usage?
Nature of the gas hazard and gas characteristics?
Solutions provided need to consider the best monitoring technology for the gas type and the environment in which it must operate. Suitable maintenance regimes and Safe Operating Procedures (SOP’s) must be established. People must know how to respond to alarms.