Design of an Automatic Frozen Soil Instrument Based on Resistance Change Caused by Water

The freezing-thawing state of soil directly affects the redistribution of soil energy and water, which is important to the state of climate. In addition, frozen soil observation has been widely used in climate monitoring, agricultural production, building planning, and railway construction and so on. As water changes resistance when it freezes, we designed an instrument that can determine the freezing–thawing state by measuring the resistance change between different spacers. The instrument designed in this paper is composed of three parts: frozen soil sensor, terminal and peripheral components. Pure water could be overcool when below zero, so we use tap water in the sensor. In order to determine the length of frozen soil, we set a series of measuring electrodes at an interval of 1cm. By measuring the change of electrical resistance caused by the phase change of non-purified water, the freezingthawing state of water layer between each pair of electrodes was detected and transformed into electrical resistance signals, after that, the collected AC conductivity signal was differentially sampled, and the RMS converter was used to convert it into DC signal. The input impedance is kept above megohm to ensure the minimum interference of the input signal. Frozen layer and frozen depth of soil were stored through mathematical transformation, and then the soil condition was obtained automatically. The automatic frozen soil observation instrument could not only connect with the computer terminal to form an independent observation system, but also be attached to the integrated hardware controller of the national surface meteorological observation station. Automatic data collection, quality control and upload of frozen soil observation were completed by ISOS. Its design inherits the observation principle and basic structure of the existing Danilin frozen soil instrument, and the overall structure design is scientific, safe and novel. From 2016 to 2019, comparative observation experiments were carried out in Juxian (Shandong province), Kenli (Shandong province), Kazuo (Liaoning province), Liaoyang (Liaoning province) and Manzhouli (Inner Mongolia). The test results show that the frozen depth variation of the automatic frozen soil observation is basically consistent with Danilin insturment. In addition, the freezing–thawing trend is almost the same. The consistency rate of frozen soil detestation between our device and the Danilin instrument is greater than 90%. It can satisfy the need of automatic observation work of frozen soil, and it can replace artificial observation of frozen soil layer and frozen soil depth.