Application of Spacing Correction Method of Heat Pulse Probe in Field Measurements
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the National Key Research and Development Program of China (No. 2016YFD0800102) and the National Natural Science Foundation of China (No. 41371231)

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    Abstract:

    【Objective】Soil thermal characteristics, including thermal diffusivity, thermal capacity and thermal conductivity, are basic physical parameters reflecting changes in soil temperature, and maintenance and transport of heat in the research on soil physical processes. Heat pulse probing method is the most common one used nowadays to determine soil thermal properties. However in using the method, spacing between probes is a major factor affecting accuracy of the results. In measuring, especially in the field, when probes are inserted into the soil, spacing between probes may vary with penetration resistance of the soil. In addition, alternation of soil freezing and thawing, movement of soil organisms, and growth of plant roots may all cause expansion and contraction of the soil, thus forcing the tips of the probes to deviate or squint and hence change in spacing. It is therefore, essential to perform in situ calibration of probe spacing. 【Method】To that end, Liu et al. proposed a method to calibrate probe spacing. They found that once the heating probe or temperature probe squinted by 1o, the thermal conductivity and thermal capacity measured would deviate by more than 10%. So Liu et al. put two thermistors on to the temperature probe in different positions, and figured out a theoretical formula to calibrate the spacing. The formula was very sensitive to the maximum temperature difference (ΔTm) and the time when the maximum temperature difference appeared (tm). In this study, the calibration method was applied to field measurement and monitoring of changes in probe spacing. Temperature probes were installed at 3 cm, 8 cm and 13 cm deep in the soil, separately, to record heat pulses from the heat pulse probe during summer from May 19th to July 3rd, 2015 and the winter from February 11th to March 28th. 2016. 【Result】Results show that firstly, probe spacing did not have any impact on the measurement of thermal conductivity, which suggests that the soil is homogeneous in texture, and when comparison was made of the measurements of thermal capacity, it was found that the differences between the two thermistors was determined by accuracy of the probe spacing; and secondly, the measurements of soil thermal properties with the probe spacing calibrated method were more accurate and reliable than those without any spacing calibration. However in the winter, when soil temperature was under 0℃, the soil would freeze and thaw alternately, causing the probe tips to deviate. Besides when the heating probe heated, the heat would be divided into two portions: one to melt the ice and the other to warm up the soil. Consequently, the measurements were not so accurate and could be corrected with the spacing calibration method. Besides, the freezing and thawing processes of the soil in winter also made it hard to use heat pulse probes to determine soil thermal properties. It was also found that the measurement with the probes embedded in the surface soil layer was more sensitive to variation of probe spacing. So far as of yet, it is still very hard to explain why, because of various unknown causes. Nevertheless soil thermal properties in the surface layer mean extremely significant to soil water movement, measurement of ground heat flux and so on. Therefore, more efforts should be done in studying accuracy of the heat pulse probe method and practicability of the spacing calibration method. 【Conclusion】To sum up, in using the modified dual heat pulse probe method to monitor soil thermal properties in the field in summer and winter, it is found that probe spacing varies more in the surface soil layer than in the others, and the use of the probe spacing calibration method can significantly minimize deviation of the measurements of volumetric heat capacity, and the spacing calibration method is applicable to field measurement. However, due to the presence of frozen soil in the surface soil layer in winter, it is hard to determine accurately soil thermal properties with the heat pulse probe method and to use the probe spacing calibration method, too.

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QIAO Zhao Yu, YUAN Xiaohui, LIU Gang. Application of Spacing Correction Method of Heat Pulse Probe in Field Measurements[J]. Acta Pedologica Sinica,2018,55(1):122-130.

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History
  • Received:May 25,2017
  • Revised:August 07,2017
  • Adopted:August 15,2017
  • Online: October 30,2017
  • Published: