Aspectos básicos de la detección de gases

The evolution of gas detection has changed considerably over the years. New, innovative ideas from canaries to portable monitoring equipment provides workers with continuous precise gas monitoring. Gas detection equipment can be broken down into the monitoring of gas using sensors and gas path technology, the user interface that informs people or equipment of any necessary action, and the supporting power management system that keeps it all charged up and working. To the package we can now add a fourth consideration – communications and recording technology.

Types of sensors

Photo-ionisation detection (PID)

PID technology is generally considered the technology of choice for monitoring exposure to toxic levels of VOCs. The sensors include a lamp as a source of high-energy ultraviolet (UV) light. The UV light’s energy excites the neutrally charged VOC (Volatile Organic Compounds) molecules, by removing an electron to leave it charged. A current then flows between two charged plates within the sensor, and the gas concentration is proportional to that current.

Electroquímica

Electrochemical sensors measure gas which enters through a small hole in the face of the cell, pass through a PTFE moisture and oil filter and onto an electrode via an electrochemical solution. Sensor ranges and sensitivities can be varied in design by using different size holes, with larger holes providing higher sensitivity and resolution, and smaller holes reduced sensitivity and resolution but increased range. The gas type measured is chosen by selection of the electrode material, selection of the electrolyte and sometimes use of filters to block unwanted gas types.

Catalytic Beads (Pellistor)

Pellistor sensors consist of two matched wire coils, each encased in ceramic beads. Current is passed through the resistance coils, heating them to approximately 230˚C. One of the beads contains a catalyst material, so when a mixture of air and flammable gas enters the sensor, it contacts the beads and burns near the one containing the catalyst. This results in a temperature difference between this active and the other ‘reference’ bead.  The temperature difference causes a difference in resistance, which is measured; the amount of gas present is directly proportional to the resistance change, so gas concentration as a percentage of its lower explosive limit (% LEL*) can be accurately determined. Pellistor sensors are widely used throughout industry including on oil rigs, at refineries, and in underground construction environments such as mines, and tunnels.

Infrared Sensors

Infrared emitters within the sensor each generate beams of IR light. Each beam passes through a sample of atmosphere and is measured by a photo-receiver. A “measuring” beam, with a frequency of around 3.3μm, is absorbed by hydrocarbon gas molecules, so the beam intensity is reduced if there is an appropriate concentration of a gas with C-H bonds present. A “reference” beam (usually 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 beams measured by the photo-receiver.

Molecular Property Spectrometer™ (MPS™)

MPS™ sensors represent the new generation of flammable gas detectors. MPS™ can quickly detect many gas types and identify over 15 characterised flammable gases at once. Until recently, anyone who needed to monitor flammable gases had to select either a traditional flammable gas detector containing a pellistor sensor calibrated for a specific gas, or containing an infra-red (IR) sensor which also varies in output according to the flammable gas being measured, and hence needs to be calibrated for each gas. While these remain beneficial solutions, each has environments where they can be used and environments to avoid. For example, both pellistors and infrared sensors require regular calibration and the catalytic pellistor sensors also need frequent bump testing to ensure they have not been damaged by contaminants containing permanent poisons (known as ‘sensor poisoning’ agents) or by harsh conditions. In some environments, sensors must be changed frequently, which is costly in terms of both money and downtime, and product availability. IR technology cannot detect hydrogen – which has no IR signature, and both IR and pellistor detectors sometimes incidentally detect other (i.e., non-calibrated) gases, giving inaccurate readings that may trigger false alarms or concern operators. The solution is the MPS sensor which detects both hydrogen and other flammable gases, identifies them, and applies the right calibration for each gas or constituent gas of any mixture it monitors.

Some instruments use a pump to supply air or gas samples to the sensor.

Types of Detection

Fixed

Fixed gas detectors are permanent fixtures that stay mounted in one location. They can be set up in single-detector configurations, in small and large multiple-detector configurations and in an addressable ‘daisy chained’ loop. Fixed gas detectors are generally installed anywhere there is a risk to plant, buildings or installations, and can detect slow build ups or major leaks to give an early or automated warning of gas leaking from a particular source. They are often set up to trigger other safety measures, so they can open vents, start fans, close valves or even shut processes down automatically once they detect a problem. Quite often they are set up to warn a control room or security personnel of a potentially dangerous gas leak, so executive action can be taken by people. They can also set off alarms to begin an evacuation. On the other hand, fixed gas detectors are usually not designed to prevent a worker coming into contact with the gas, though some systems do have a component of area coverage to their design. Portable gas detectors and the best way to protect individuals at risk of coming into contact with toxic or flammable gas build ups or releases.

Each fixed gas detector must communicate with a control panel. The control panel is the hub of the fixed gas detection system, which compares the quantities of gas with pre-set levels and provides various options for input and output functions. The gas control panels are normally located in a safe area but can be installed in hazardous zones if appropriately housed. They communicate with gas detection sensor heads or transmitters and can be networked to a central point so that multiple control panels/systems can be monitored remotely. There are multiple methods of communicating with fixed gas detectors. The most common is analogue, but there is a growing demand for digital and wireless communications. There are also various features available via the detector to improve efficiency and reduce the time spent by personnel in potentially hazardous locations, thereby reducing risk to people.

Portable

Portable gas detectors are personal protection devices that continuously monitor the user’s breathing zone. Because they are generally small, these handheld, lightweight and robust devices are carried on the person and constructed to be ergonomic and worn unobtrusively. They are also sometimes used to check confined spaces such as tanks where the type of gas risk is known, before someone enters the space. They are intended for monitoring at close range and are usually not suitable for long term continuous monitoring of larger spaces. Portable gas detectors are the safest proven way to protect individual workers as they move around.

Portable detectors store information on gas exposure throughout the duration of a shift, as well as events such as alarms or near misses. This data can be transmitted to a cloud-based portal to allow for numerous benefits such as improved operational efficiency and safety compliance, as well as providing a robust and flexible mechanism to deliver valuable actionable insights. Data solutions offer tangible benefits to all sizes of portable fleet, whether gas detectors are being used onsite, offsite or both. Portable gas detectors typically cost less than fixed systems and most are battery powered. On the other hand, each user must be properly trained to operate their portable detector. In addition, portable detectors are not typically connected directly to other safety systems. If the detector raises an alarm, the user is therefore required to take action on their own to mitigate any risk to themselves or others.