Dual Mode NOx Sensor: Measuring Both the Accumulated Amount and Instantaneous Level at Low Concentrations

The accumulating-type (or integrating-type) NO(x) sensor principle offers two operation modes to measure low levels of NO(x): The direct signal gives the total amount dosed over a time interval and its derivative the instantaneous concentration. With a linear sensor response, no baseline drift, and both response times and recovery times in the range of the gas exchange time of the test bench (5 to 7 s), the integrating sensor is well suited to reliably detect low levels of NO(x). Experimental results are presented demonstrating the sensor's integrating properties for the total amount detection and its sensitivity to both NO and to NO(2). We also show the correlation between the derivative of the sensor signal and the known gas concentration. The long-term detection of NO(x) in the sub-ppm range (e.g., for air quality measurements) is discussed. Additionally, a self-adaption of the measurement range taking advantage of the temperature dependency of the sensitivity is addressed.