Fixed and portable gas detectors protect people, equipment and facilities in hazardous environments from combustible and toxic gases. They are designed with various sensing technologies to support a wide range of applications. For safety and health managers, the question is which type of detector (fixed or portable) provides the right level of protection.
Nobody can afford to place a fixed gas detection system in every manhole or other confined spaces, nor can you afford the man-hours to station personnel with portable gas detectors throughout a large industrial facility. Somewhere in between, the situation is less obvious: Just where and when do I use fixed, and where and when do I use portables? This article will provide guidance on the strengths and appropriate use of each type of system.
Fixed systems offer a wider variety of sensing technologies than portables. Many times, however, fixed and portables systems rely on electrochemical sensors. As a general statement, fixed gas detectors are primarily intended to protect property, capital equipment and lives. Portable gas detectors are used primarily to protect people and then property. The question for many safety professionals is which type of detector best protects employees in specific applications?
Catalytic bead. This technology is well-known for its reliability in detecting combustible gases or vapors. This sensor consists of two ceramic beads formed onto two platinum filaments, one being the active and the other being the inert reference. An electrical current flows through the wire, heating the beads hot enough to combust flammable gas or vapors. If a gas is present, it burns at the active electrode only. This causes a change in the electric current, which can be reported electronically to a display or controller. These sensors may not respond well in oxygen-deficient areas. Portable and fixed detectors use this technology.
Electrochemical. Many popular gas detectors rely on electrochemical sensors or cells. They are reliable, accurate, low in cost, react quickly and are easily replaced. They are basically batteries fueled by a specific gas and, like batteries, they have a set life, usually about two to three years. Care should be taken to note which other gases may be present as some cells may also respond to interfering gases. Electrochemical cells are available for most of the commonly encountered gases. They are frequently used in portable and fixed systems.
Metal oxide semiconductor. Metal oxide semiconductor (MOS) detectors use a silicon chip to detect a variety of gases, most often [H.sub.2]S. In an MOS sensor, semiconductor film is exposed to a gas, and the surface of the sensor is affected in such a way that the resistance of the semiconductor is temporarily changed. This change is reported electronically. MOS sensors are very reliable and have a long life, often up to 10 years. This technology requires more electrical power than a portable can usually provide and lends itself best to fixed systems.
Infrared. An infrared (IR) system uses specific wavelengths of IR light to detect a gas. The beam's intensity is reduced as it is absorbed by the gas passing through it. The system's electronics measure the beam's intensity loss, which is proportional to the amount of gas present. These detectors can be set up to be somewhat discriminatory and work well in oxygen-deficient atmospheres and fail-safe environments. Due to power requirements, this technology is not often used in portable instruments.
Ultraviolet. This technology is based on a sensor that detects ultraviolet (UV) emissions. This sensor is most often used to detect flames, all of which conveniently emit UV. It can be used much the same as IR to detect gases. Its use, however, is largely limited to analytical instruments (photoionizing detectors).
The Basic Fixed System
A typical fixed system can have from one to hundreds of detectors hard wired back to a stand-alone wall mount controller or to a programmable logic controller (PLC) in a control room (Figure 1), which may be part of a larger distributed control system (DCS). …