Gas detection
CALIBRATION Notwithstanding the safety and convenience considerations mentioned above, extractive gas analysers also offer advantages in calibration because they only require very small amounts of calibration gas. Operators of in-situ monitors would need to flood the stack or duct with calibration gas at the normal stack temperature, or rely on the instrument’s built-in calibration cells.
CONVENIENCE AND HEALTH & SAFETY Anyone involved with gas monitoring will tell you that the ideal measurement point is rarely easy to access. Typically, it will either be located deep within a processing plant, in a confined space, alongside hot equipment, or it will be high up on a stack and exposed to all forms of extreme weather. A major advantage of extractive gas analysers therefore, is that they can be located in a convenient location, so that access for the purposes of maintenance or calibration, is simple and safe.
MAINTENANCE In-situ analysers can either contain a measurement cell within a probe that is inserted into the stack or gas stream, or they may use a cross-stack optical method. Both of these methods risk contamination that may affect accuracy, or in the case of optical instruments prevent measurement completely when the instrument’s windows become obscured by particulates or other materials. Cross-stack instruments send an optical
signal such as a laser across the duct or stack and reflect the signal back to a detector. Alternatively, the detector may be located directly across from the signal generator. Alignment is therefore crucially important, and frequent maintenance may be necessary if the instrument’s windows are obscured. The
advantage of extractive analysers therefore, is that the sample can be filtered and conditioned to prevent contamination from affecting the readings.
FLEXIBILITY The range of measurement technologies that are suitable for in-situ measurements is limited in comparison with extractive techniques. For example, Signal Group’s extractive analysers include options such as FID, CLD, NDIR, PMG and TDLS. Extractive systems are also better able to
accommodate changes in a process, waste stream or combustion material. So, an application that was originally better suited to in-situ might need to change to be extractive. For example, if it becomes necessary to measure new parameters; it is easier and generally less expensive to add that capability to an extractive system; largely because the sample handling system is already in place. Extractive monitors therefore help to future- proof monitoring capability. In summary, given the overwhelming number
of advantages offered by extractive analysers in a wider variety of applications, it is hardly surprising that Signal Group continues to focus on extractive systems.
Signal Group
www.signal-group.com
Instrumentation Monthly January 2023
61
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