Redundant chemical sensors for calibration-impossible applications

Multiple chemical sensors are used to measure the same analyte simultaneous ly to determine whether the redundant signals can improve the long-term acc uracy and circumvent the need for periodic calibrations. A specific marine chemistry application was investigated where six glass pH electrodes were p laced in a synthetic seawater solution for nearly 2 months without recalibr ation. The pH accuracy was evaluated by comparison with spectrophotometric pH measurements. The standard deviation, t-test and principal-component ana lysis were used to evaluate the redundant signals. The average signal stand ard deviation was useful for determining the onset of drift. whereas, the p rincipal-component analysis readily identified specific sensors that were d rifting. The sensor signals, shown through t-tests to be outliers, were eli minated From the data set, resulting in a significant improvement in measur ement accuracy. After 56 days, the signals from non-drifting and drifting s ensors resulted in a pH accuracy of +/- 0.012 and +/- 0.0401 respectively. over a threefold improvement. The residual +/- 0.012 inaccuracy was limited by the performance of the remaining sensors, which appeared to drift with similar magnitude and could therefore not be statistically separated. These results indicate that redundant sensors coupled with a principal-component analysis are a potential alternative for situations where calibrations are not feasible. (C) 2001 Elsevier Science B.V. All rights reserved.