What is Fixed Gas Detection?

Fixed gas detection systems are installed with the aim of providing continuous, area and personnel protection around the clock. These detection systems are designed to alert workers of the potential danger of poisoning by toxic gas exposure, asphyxiation due to lack of oxygen or explosion caused by combustible gases.

Which Gas Hazards Pose a Risk? 

A gas hazard can be found in three forms; oxygen depletion, toxicity, and explosive gases.

Oxygen Depletion can occur where there is potential for high concentrations of gas other than ambient air. Even non-explosive or non-toxic gas can displace enough ambient air to reduce the oxygen level to a level where is it unsafe for an operator to enter. A particular risk is in areas where there are gas storage tanks housed in a confined space where access is rare but required, i.e. boiler room.

Toxic Gases can come in many forms, a risk assessment will identify all toxic gases that may occur, every toxic gas has an exposure limit regulated by the HSE with the EH40 document. In an area in which there is potential for toxic gases to be released into atmosphere, there should be a specific sensor to monitor that gas and should alarm before the concentration exceeds the exposure limit.

Common Misunderstanding is to use an oxygen sensor to monitor toxic gases on the assumption that it will alarm when oxygen is displaced. A typical oxygen sensor will offer a resolution to 0.1% volume. This will alarm when there is a concentration of 0.1% volume or 1,000ppm. This is not a suitable technique for toxic gas detection as many any gases are toxic in concentrations below 1,000ppm.

Explosive Gases are monitored using either IR (infrared) or Catalytic sensors. Many gases at the right concentration can be explosive, therefore both technologies can detect a number of explosive gases, however, they offer different sensitivities for different gases. A risk assessment must identify all potentially explosive gases then the gas with the lowest LEL (lower explosive limit) must be selected as your target gas; this is often Methane or Pentane. Typically the fixed system should alarm at 50% of the LEL. Methane is explosive at concentrations of 5% vol. therefore the LEL should be set to 2.5% vol. which equates to 50% of its LEL.

Benefits of Fixed Gas Detection


A fixed gas detection system protects you and your assets. In addition, it also protects your workforce against asphyxiation, poisoning and explosions/fire.

Fixed gas detection systems connect to building management systems as well as other active safety systems. Relays may be activated to perform a number of functions including activating visual and audible alarms, closing electronic valves and opening/closing ventilation and exhaust fans. This type of automatic system alerts workers in hazardous situations and saves lives!

Continuous Monitoring

Fixed gas detectors provide monitoring 24 hours a day. All areas even when the majority of the time it is unmanned must be monitored to warn personnel of dangerous environments before they enter. Having fixed detection will reduce safety risks on site creating a safer environment, often satisfying insurance policy criteria.

A Return on Investment

Although it is not a replacement for personal gas detection, a fixed gas detection system can reduce the time in which it takes to assess, manually, a potentially explosive area often reducing man hours and ultimately cost. The role of the fixed gas detection system can prevent explosions which would cause damage to the plant or endanger lives which could come at a cost in terms of expensive insurance payouts. 

Reduced Maintenance

Fixed gas detection systems typically have a long-life and require minimal maintenance. They are often easier and cheaper to maintain than portable gas detection as they are less susceptible to loss and damage. The recommended service and maintenance schedule of a fixed gas detection system is calibration every 6 months with low sensor replacement frequency and cost.

Little training is required to operate a fixed gas detection system and its use is less likely to be affected by misuse or ignorance.

Other Considerations

Every fixed gas detection system is different - A site survey by a qualified installation engineer is highly recommended as there must be consideration for the following;

Gas Hazard Behaviour

The gas hazard must be assessed and the system specified appropriately. All gases have different molecular weights and will behave in different ways. The solution for gases like carbon monoxide and gases slightly heavier than air such as nitric oxide (NO) and hydrogen sulphide (H2S), is to mount the fixed detectors at a height as close as possible to where personnel will be exposed (the typical breathing area is 1.5 to 1.8metres). Carbon dioxide which is heavier will tend to sink to floor level so this is where the detectors should be.

When monitoring oxygen depletion, it is necessary to consider what might be displacing it. If combustion gases are consuming oxygen, the whole volume of air would gradually become depleted.


The location in which you are monitoring should be assessed based on envirinmental conditions such as temperature, humidity, wind or air flow. For example, if you are installing fixed point detection sensors, these rely on the gas passing across the sensor, to be detected. Therefore if there is a known, consistent direction of the air flow, the gas detector must be positioned “down wind” of the potential leak source. 


What do you want the fixed gas detection system to tell you? In cases where you are monitoring for explosive gases or oxygen depletion, it is to tell you that there has been a gas leak. As a result, a sound or beacon alarm will provide you with a “SAFE – NOT SAFE” indication controlled by the relay output of the controller.

In cases where you are monitoring toxic gases with average exposure limits STEL (Short-term exposure limit over 15 mins) or TWA (Time weighted average exposure limit over an 8 hour period), you will need data acquisition software to calculate these average over these two periods and ignore safe peaks on concentrations.

Closed Loop

You are able to use fixed gas detection systems to monitor and control an environment. The outputs of the control panel can activate emergency extraction, for example, to ventilate a confined space to reduce the unwanted gases. In some cases, fixed gas detection systems activate to shut down processes as a safety precaution to allow for corrective maintenance.

Choosing the Right Sensor

It is important to know which sensor is best for which gas and the application.

Detects: Explosive gases in the LEL range only
Cost: Low cost £300-£500
Service: Must be calibrated every 6 months
Life Span: 2-3 Years
Things to consider: If exposed to high concentrations it can burn out the sensor and will need to be replaced. If exposed to a silicone based product it can coat the sensor desensitising it to the target gas and will false zero.
Detects: Typically CO2 and Explosive gases in the ppm, LEL and %Vol range.
Cost: Low cost £500-£800
Service: Must be calibrated every 6 months
Life Span: 10+ Years
Things to consider: Very stable sensor with a long lifespan but is more expensive, they can detect multiple gases but only tuneable to one gas at a time
Detects: Typically O2, CO, H2S, NO, NO2, SO2, NH3.
Cost: Low cost £300-£500
Service: Must be calibrated every 6 months
Life Span: 2 Years
Things to consider: Overexposure to the gas can reduce the lifespan of the sensor. Ideally, it needs to be calibrated more frequently as some gases can be cross-sensitive.

To find out more about the fixed gas detection range that a1-cbiss can offer, contact us below.

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