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Automated and Continuous Redox Potential Measurements in Soil

^a Institute of Ecological Science, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, the Netherlands
^b Electronics Department, Faculty of Earth and Life Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, the Netherlands
^c Aquatic Ecology and Ecotoxicology, Faculty of Science, University of Amsterdam, Kruislaan 320, 1098 SM, Amsterdam, the Netherlands

View larger version (18K): [in a new window]   Fig. 1. Electrical layout of Hypnos. Its main characteristics are industrial temperature range components (–20 to +70°C); maximum of eight redox inputs; very low power consumption (input current typically = 1 pA); electrostatic discharge protected input of 2 kV; surface mount assembly techniques; ferroelectric nonvolatile random access memory (FRAM); water resistance: IP67. MUX, multiplexer; RTC, real-time clock.

View larger version (24K): [in a new window]   Fig. 2. Layout of the redox–temperature probe. (A) Three-dimensional view. (B) Layout of probe, without the sensors placed. The probe consists of a Perspex tube, with holes drilled in it for the temperature sensors. At the same depths, Pt wires are placed on the outside of the probe, slightly sunken into the tube ensuring minimal damage to the soil profile when the probe is pushed into the soil. The pointed tip is glued to the tube. All sensors and Pt wires are connected to the Hypnos datalogger with isolated computer cable (connecting wires not shown).

View larger version (24K): [in a new window]   Fig. 3. Overview of the setup with Hypnos in the field. The probes with redox and temperature (REDOX/TEMP) electrodes are placed at a distance of several meters from the datalogger. The datalogger has been placed on a 2-m-high stake to prevent direct contact with water during flooding of the area. The calomel reference electrode (REF) can be placed anywhere near the datalogger. This setup allows sampling without disturbance when taking data from the logger.

View larger version (31K): [in a new window]   Fig. 4. Results of two in situ measurements. The redox potential at three depths and the temperature at the top level are shown for the mesocosm experiment (A and B) and the salt marsh (C and D). Numbers indicate the depth of measurement (cm). Note the daily fluctuations and range of fluctuations at each depth over time.