Simultaneous sensor and gravimetric measurements
Protocol Step: Simultaneous sensor and gravimetric measurements
Overall Step Status: Incomplete
A template for the data entry corresponding to all the sensor measurements during calibration is given in the ‘Calibration’ sheet of the example file ‘Template_tables.xls’ (given in Step 7: ‘Data tabulation and calibration relationships for sensors’).
Task 1 (Not Complete) : Properly mix the soil + water mixture inside the first container of the first soil type with an appropriate tool ensuring that there is minimal loss of soil due to the insertion of the tool. This is done to prevent the development of soil moisture gradients within the container. As a general rule, mix any soil carefully before taking a reading, ensuring minimal loss of weight due to accidentally removed soil particles.
Task 2 (Not Complete) : Weigh the container (container + soil + water). Make a record of the weight. This leads to a measure of the actual weighted VWC inside the container.
Task 3 (Not Complete) : Insert a sensor with a higher accuracy, which can act as a secondary standard for calibration. In this calibration setup, the more accurate impedence based soil moisture sensor, ThetaProbe ML2x (Delta-T Devices, 1999) was used to get a sensor based accurate measurement of VWC. Make a record of the measured VWC. Remove the secondary standard sensor.
Task 4 (Not Complete) : Insert the soil moisture sensor (which needs to be calibrated) into the soil and take a reading of the measured raw sensor output as well as the factory calibrated VWC value (if it exists). Make a record of the reading.
Task 5 (Not Complete) : Wait for 30 minutes and repeat the measurement and record the reading. For automated recording, multiple readings can be averaged and a standard deviation/error can be computed, as is given in 'Template_Tables.xls’. The interval between readings is recommended due to the process of stabilization of the soil + water matrix around the sensor material. Also, it leads to an estimation of sensor precision due to the multiple measurements with the same conditions of measurement. The duration (30 minutes) is an indicative recommendation based on the time to stabilize soil moisture readings in local soils for the instruments used, and may vary for different soil + sensor combinations.
Task 6 (Not Complete) : Remove the soil moisture sensor being calibrated, re-insert the impedence based ThetaProbe soil moisture sensor, take another reading and make a record of it. This additional reading averages out pre- and post-measurement conditions of soil packing which may have changed due to the insertion of the instruments. Remove the sensor.
Task 7 (Not Complete) : Wipe each sensor after removal from a container with a brush to return the soil to the container. This is done to prevent any loss of soil, and to ensure that any reductions in weight are only due to the loss of evaporated water and not soil.
Task 8 (Not Complete) : Weigh the container again. Weights are taken both before and after all the measurements to average out any minor changes in weight from losses in water.
Tools needed: appropriate tool for mixing soil + water mixture
Tables of Data
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