Process analyzers measure the concentration of specific substances for the purpose of measuring and/or controlling those concentrations in a process stream. Safety analyzers detect the presence of dangerous concentrations of specific substances to warn personnel of threats to life or health. While there is virtually no end to the different types of process analyzers in existence, with new analyzer types invented to meet the needs of process industries, safety analyzers are rather restricted to those substances known to pose health hazards to human beings.
Safety analyzers are designed for fast response, rugged construction, and ease of portability. As such, they are usually not as accurate or as sensitive to slight changes in concentration as process analyzers. The sensing technologies used in safety analyzers are often very different from those used in process analyzers. You will never, for example, see an NDIR instrument with a Luft-style “microphone” detector used for portable safety applications. The high accuracy of a Luft-style NDIR instrument is not necessary for safety, and the bulk (and fragility) of such an instrument makes it completely impractical as a portable device.
An important industrial application of safety gas analyzer technology is testing the safety of the atmosphere inside a confined space, such as an industrial vessel that has been emptied in preparation for workers to enter and perform maintenance work. Virtually all pressure vessels are classified as confined spaces from the perspective of internal maintenance work, and many are explicitly labeled as such to remind workers of the danger:
This photograph shows a hand-held safety gas monitor, used to detect four different gases with known hazard levels (oxygen, carbon monoxide, combustibles, and hydrogen sulfide). A technician working in hazardous environments would wear one of these at all times, listening for the audible warning tone generated by the device if any of its pre-set limits is exceeded:
Most portable gas analyzers such as this employ electrochemical sensing “cells” generating a small electric current when exposed to the gas of interest. Such technologies may not always be the most accurate or the most sensitive, but their characteristics are well suited for portable applications where they will be exposed to vibration and must operate on battery power.
A close-up photograph taken of the monitor’s alarm thresholds reveals the relative concentrations of four gases monitored by the device. Two distinct alarm levels – “low” (alert) and “high” (danger) – exist to warn the user of threats:
The two most dangerous gases detected by this device – hydrogen sulfide (H2S) and carbon monoxide (CO) – are measured in units of parts per million, or ppm. The next two detected gases – oxygen (O2) and combustibles (“LEL”) – are measured in units of percent (which may be thought of as parts per hundred).
Like all safety-related devices, portable gas analyzer require regular “proof-testing” and calibration. In order to accurately check the response of a gas analyzer, it must be exposed to gases of known concentrations. Special mixtures of “test gas” for safety analyzers are available from chemical supply companies, the next photograph showing the certified concentrations of different gases contained inside the pressurized test cylinder:
Note the limited shelf life of this span gas: only one year between its date of manufacture (November 2007) and its expiration (November 2008). Relatively short expiration periods are common with chemical standards used for analytical calibration, owing to slow chemical reactions between the constituent compounds over time altering the composition of the mixture. In this test gas cylinder, for example, the relatively high concentration of oxygen gas (18%) will over time oxidize the hydrogen sulfide, methane, and carbon monoxide gases, resulting in decreased concentrations of all these gases, with the production of water vapor and other oxides (e.g. CO2) inside the gas cylinder.
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