Choosing the right system
October 1st 2007

With an array of portable gas detection systems available, making the right choice for both the user and the application can appear difficult, explains Ian White, product specialist, Draeger Safety

The manufacturing process, the materials used, the cleaning methods and airflow can all affect the quality of air in workplace environments. In particular, the creation of gases and vapours can turn a previously safe atmosphere into a toxic and possibly lethal area. In addition, the need to monitor oxygen levels for deficiency can be as important as the need to monitor and control potentially explosive or flammable environments.

Lethal mix While the seriousness of the hazard is dependent upon the nature of the substance and/or the application concerned, the dangers can be dramatically increased if a combination of hazards exists. Toxic gases or vapours can poison or suffocate personnel, while work in progress, such as cleaning, welding and painting can produce dangerous fumes. It is also possible that the toxic gases and vapours that are created during previous work might still be present upon re-entry at a later date.

In accordance with CoSHH as well as European legislation, employers must ensure that any hazardous substances that are used in or created by workplace activities are both monitored and controlled. To ensure absolute safety, employees should also perform a risk assessment to ascertain the hazards that may be present and then continue to monitor the workplace area before entering a potential hazardous environment.

A typical example can be found in the brewing industry where increased levels of nitrogen (used to make a more effervescent beer) could lead to oxygen depletion and, as a result, asphyxiation. In steelworks, leaks of carbon monoxide, which is used as a process gas, can create a potentially hazardous environment. Even those working in the food industry can suffer from the effects of disinfectants containing hydrogen peroxide or, in refrigeration and cold storage areas, unhealthy levels of ammonia. It is also possible that the toxic and/or flammable gases and vapours that are created during previous work might still be present upon re-entry at a later date. In addition, an atmosphere containing less than 21% oxygen is known as being oxygen deficient and can be life-threatening.

Risk assessment Under CoSHH legislation any organisation with more than 5 employees should now be carrying out a workplace risk assessment if they are using asphyxiant gases. As some of these asphyxiants, such as nitrogen, helium and argon are colourless and odourless, it is difficult to prevent or adequately control exposure without measuring the oxygen content of air. Designed to continuously monitor toxic gas or oxygen concentrations within the workplace environment, it is essential that portable gas detection systems are able to detect this type of hazard.

Irrespective of whether there are single or multiple gas hazards to consider, there is more to the selection of portable gas monitors than hazard detection. The application must also be carefully considered. For instance, if two hands are required to carry out a task, can the system be worn around the neck? If gloves are worn and a handheld system is being used, can the instrument still be operated easily and efficiently? Is data logging required? Can the display be seen in poor light? Will it work in hot and humid atmospheres? There are a number of other questions that need to be asked:

Which alarms are available? The better units can combine vibrational with visual and two-tone audible alarms, which are activated as soon as the threshold levels are reached.

How easy is it to configure, calibrate and download an event logger? These functions can be easily and quickly carried out via an infra-red (IR) interface – important if records need to be kept.

Do the sensors have a short reaction time and how often is maintenance required? The latest electrochemical sensors respond immediately to any gas hazard and can offer maintenance free operation for up to five years.

Which alarms are available? The better units will combine vibrational with visual and two-tone audible alarms, which are activated as soon as the threshold levels are reached.

Will the unit still work reliably even if it is accidentally placed in a jacket pocket? Units with gas inlets on both the top and front ensure that it will still provide a reliable warning against gas hazards.

Is the unit small and, if so, is the performance affected? Units can be as small as a mobile phone. By using the latest, miniaturised sensors, gas detectors can retain their powerful functionality and still incorporate two-button control panels, a large liquid crystal display and easy menu guidance.

What about explosive hazards? For improved safety when facing unknown hazards, some units incorporate a catalytic Ex sensor which, when calibrated to methane and other substances, responds quickly to explosive gases and immediately warns the user.

Regular air sampling will ensure that effective control measures are introduced as and when necessary to eliminate risks to health and safety for all employees.

However, as there is no one simple way to specifically detect all gases, a variety of techniques are usually required to ensure a complete monitoring programme:

Short term tubes Short term tubes provide on the spot gas measurement and are suitable for monitoring personal exposure, spot check measurements, leak checks and confined space investigation.

Short term pumps Used in conjunction with the tubes, hand bellow pumps enable rapid measurements to be taken and provide optimum volume and flow specifications. Incorporating automatic stroke counters and a clear end of stroke indicator, pumps require no special tools and give accurate and reproducible results.

Diffusion tubes Providing up to eight hours of use, diffusion tubes give an accurate indication of exposure by allowing contaminant molecules to move into the tube without the aid of a pump.

Sampling tubes Requiring laboratory analysis, passive sampling tubes provide short and long term collection of toxic substances.

Available in activated charcoal and silica-gel versions, they are often worn on the breast pocket.

Personal single gas monitors Simple to use, these instruments can be handheld or clipped to the user via a crocodile clip, and can monitor a broad range of gases and are often available with an additional neckstrap for optimum portability. Offering an immediate visual display of gas concentrations, they can also provide audible, vibrational and visual alarms when danger levels are reached.

Multi-gas measurement These high performance instruments can be used to measure a wide variety of gases in virtually any application such as indoor air quality for confined space entry or the monitoring of landfill gas. The more modern units feature 360 degree visual and acoustic alarms and can provide continuous detection of up to five gases simultaneously What makes this type of instrument particularly easy to use is the fact that each sensor is pre-calibrated and is recognised automatically by the instrument. This means that it can be reconfigured simply by changing a sensor and without requiring additional service or maintenance.

In addition, the measuring range of these sensors can be changed to any other gas detected by that sensor at the push of a button and without needing recalibration. As a result, the accuracy and range of the monitored substance is substantially increased.

In addition, instruments such as this can simplify the search for leaks in flanges, shut off devices and valves by the addition of a flexible gooseneck probe. When in tracking mode, some systems also feature an increasing or decreasing rate of beeps in accordance with the gas concentration detected.

More articles from Draeger Safety UK Limited: