Gas detectors are usually categorised as either being fixed, portable or transportable. All of these types have a range of methods of detecting gas or vapour hazards and have their own strengths and weaknesses. Selecting the correct solution depends on the circumstance of operation, topography of the site or application and the nature of the gas hazard. For example fixed systems are impractical for sewer workers operating in confined spaces. This type of application suits a portable personal gas detector. Similarly an indoor process in a lab or production facility requires 24 hour monitoring, something a portable detector cannot provide. In some instances a combination of both fixed and portable provides the best solution so correct assessment of the hazard and best available technology to monitor is vital.