Risk can be managed and reduced by understanding the potential failure modes of an analyzer, its sample conditioning system, and the process. They are observational, model-based, and functional. These three diagnostic types are differentiated by the means through which the evaluated data is collected.
Three Diagnostic Models Approach
Observational Diagnostics extract useful information through the evaluation of sensor data. For the Model 888 temperature, pressure, and flow sensor data are monitored.
Model-based Diagnostics extract useful information from the output of predictive models.
Elemental sulfur vapor is a strong ultraviolet (UV) absorber. H2S and SO2 are the two primary components of interest in tail gas analysis. The UV absorption spectrum of elemental sulfur vapor overlaps the UV absorptions of both H2S and SO2. The Model 888 firmware contains an elemental sulfur calibration model and the calculated sulfur vapor concentration is used as a diagnostic evaluation of the demister performance.
Functional Diagnostics are based on internal and external challenges to various subsystems.
Measuring a zero reference gas, is one of the basic functions of all process photometers including the Model 888. The zero reference is the signal reaching the detector when the sample cell is filled with a non-absorbing gas. The Model 888 self-adjusts for changes in the signal intensity by periodically measuring the zero gas. If the signal intensity continues to decrease over time, so does the signal-to-noise ratio and the reported concentrations for the H2S and SO2 analytes become less accurate. The zero reference eventually becomes too small and maintenance must be performed.