Testing a Tunable Diode Laser Absorption Spectroscopy Oxygen Analyzer

Chemical processing systems can be controlled using a wide variety of instrumentation. Often pressure and temperature are key parameters that are monitored and controlled, and this instrumentation is extremely mature technology. In some cases, chemical concentrations are a key process parameter that must be monitored and used as an input to the system controller. Chemical concentrations may be monitored indirectly through pH or conductivity. Direct measurements are often made via electrochemical cells, but these tend to have slow response times. For chemical processes that operate in batch mode, particularly ones that have relatively short cycle times, chemical sensors that have fast response and recovery times are needed, and electrochemical sensors may not be suitable. This paper describes such an application where the chemical concentration of oxygen is an important process and safety parameter. A Tunable Diode Laser Absorption Spectroscopy (TDLAS) oxygen sensor was selected for this application because of the short response and recovery time. When the oxygen sensor was received, it was set up in the laboratory and tested to verify measurement accuracy and data communication with the sensor. During this testing, we discovered that the sensor produced incorrect output over a portion of the system operating pressure range. The root cause of the error was traced to how the firmware (sensor operating software) handled the measured pressure input to produce the oxygen concentration output. The paper discusses the laboratory testing setup, and the testing that was executed. A structured test program was executed that enabled us to understand the nature of the incorrect measurement, and trace the source of the error to the sensor operating software.y