On-Site Sensor Calibration Procedure for Quality Assurance of Barometric Process Separation (BaPS) Measurements

Barometric process separation (BaPS) is an automated laboratory system for the simultaneous measurement of microbial respiration and gross nitrification rates in soil samples. To ensure optimal functioning, the sensor system, consisting of a pressure sensor, an O₂ sensor, a CO₂ concentration sensor, and two temperature probes, must be accurately calibrated. For the regular on-site quality control of the sensors, we developed easy, inexpensive, and flexible calibration procedures. The pressure sensor was calibrated by means of a differential manometer. The O₂ and CO₂ sensors were simultaneously calibrated through their exposure to a sequence of O₂ and CO₂ concentrations obtained by sequentially exchanging O₂/N₂ and CO₂/N₂ calibration gases. Linear regression models were best suited for describing the recorded calibration data. The accuracy of O₂ and CO₂ calibration was mainly affected by the accuracy of the utilized gas mixtures. Because the applied measuring method is based on the O₂ conductivity of ZrO₂, the O₂ sensor is particularly susceptible to aging and to consequent signal shifts. Sensor signals were characterized by high temporal stability over the years. Deviations in the calibration parameters affected the measured gross nitrification rate by up to 12.5% and affected the respiration rate by up to 5%. Overall, the proposed calibration procedures are valuable tools for ensuring the quality of BaPS measurements and for promptly identifying sensor malfunctions.