利用非线性模型提高加长型热脉冲双探针土壤热参数测定精度

热脉冲双探针技术被广泛应用于土壤热参数和含水率的测定。然而长度为2.8 cm的常规热脉冲双探针空间测试范围太小,加长探针可增加测定结果的代表性,但容易出现探针倾斜(探针间距改变),进而影响测量精度。设计制作了长度为10 cm的热脉冲双探针,通过室内土柱试验测定了四种倾斜方式(共面外倾、非共面外倾、共面内倾以及非共面内倾)下的土壤热参数和含水率,并利用线性和非线性修正模型对探针倾斜引起的测定误差进行了校正。结果表明:对于加长型双探针,非线性模型对探针间距的原位校正效果明显优于线性模型;非线性模型校正后的土壤容积热容和含水率准确度大幅提高。有效解决了加长型热脉冲双探针倾斜引起的测量误差,为探针的广泛应用提供了理论基础。

Using Non-linear Model to Improve Measurement Accuracy of Soil Thermal Parameters with Extended Dual Probe Heat Pulse Sensor

Objective] The technique of heat-pulse dual probe has been widely used for measuring soil thermal parameters and water content. However, being only 2.8 cm long, the conventional dual probes are very limited in monitoring range. Using lengthened probes may improve representativeness of the measurement, but long probes tend to get deflected, causing deviation of the distance between two probes, thus affecting measurement accuracy. Once the probe gets deflected by 1°the measurement of soil thermal diffusivity and volume heat capacity may deviate by 10%. By measuring temperatures at different points of the probe, spacing error caused by the deflection may be corrected with an in-situ spacing correction linear model or non-linear model. The linear model is preferred because it only requires the probe to have two thermistors rather than three thermistors as the non-linear model does, thus making it easier to assemble and lower in cost because it needs less data acquisition interfaces. However, as to which one, linear or non-linear, is more suitable for long probes, more work should be done.Method] In this study, 10 cm long probes were designed and used, because 1) in measuring heat-pulses in large soil columns, the probe used by the single probe technique is 10 cm; 2) the thermo-TDR probes longer than 10 cm can improve accuracy of the measurement of soil water contents; and 3) the minimum resolution of the distributed optic fibre temperature measurement is about 10 cm. With development of the technique of thermo-TDR and thermo-distributed temperature sensing with heated fiber optics, the use of 10 cm long probes in this study may better embody the advantages of the above-described technique and cherish a bright future in applying the technique. In the indoor soil column experiment, soil thermal properties and water contents were measured with probes that might deflect off in four ways(coplanar extraversion, non-coplanar- extraversion, coplanar-introversion, non-coplanar-introversion), and errors of the measurements resulting from deflection-caused changes in needle spacing were corrected with the linear or the non-linear model, separately.Result] In using the extended dual probes, the non-linear model performed better in correcting probe spacing in-situ than the linear model (relative error of the former was -8.30 and of the latter 43.90%). Correction with the non-liner model improved accuracy of the measurement of soil volumetric heat capacities and water contents by a large margin, controlling relative errors within 10%. It is worth noting that the main factor affecting the nonlinear model correcting probe spacing is that the corrected spacing acquired by the non-linear model is determined only determined by t_m(the time when the temperature response curve reaches the highest value), while t_m is determined jointly by spacing and soil moisture content. Therefore, it is expected to further improve the accuracy of probe spacing correction in using the non-linear model by modifying t_m value in the nonlinear model to reflect changes in probe spacing and water content.Conclusion] This study has effectively solved the problem of measurement errors caused by probe deflection of the extended dual thermal pulse probe, and hence provided a theoretical basis for the wide application of the probe. Compared to the linear model, the non-linear one can effectively minimize spacing errors. With the development of thermo-TDR and thermo-distributed temperature sensing with heated fiber optics, the use of 10 cm-long dual probes may embody advantages of the above-described measuring technique and has broad application prospects.