Improvements Of The Heat Probe Method In Measuring Soil Water And Thermal Properties

It is very important to synchronize different physical properties of soil.Soil water and heat are very important in agriculture and climate.Improving the accuracy of thermal properties and water content measurement can also be useful for the research of other physical parameters and physical processes.Heat probe method is widely used in soil water and thermal properties monitoring.However,there are still many factors that limit the accuracy of heat probe method.Heat probe method includes the dual probe method and single probe method.In order to improve the accuracy of the heat probe method,we carried out the experiments both on dual probe method and single probe method.Dual probe sensor has two probes,one is the temperature probe and the other is the heating probe.The heating probe will release pulse heat when measuring.Therefore,the dual probe method is often called the dual probe heat pulse method.The spacing between the temperature probe and the heating probe is the probe spacing,r,which is an important parameter that can influence the measurement of heat capacity.For the study of the dual probe heat pulse method,we focused on the probe spacing correcting:(1)The linear model of the in situ probe-spacing-correcting method based on that the probe is linearly deflected can correct the probe spacing in situ.To evaluate the linear model of the in situ probe-spacing-correcting method,we did different deflections on the probes and use the linear model to correct the changed probe spacing.Then we compared the heat capacity values and errors calculated with and without correcting.Results showed that the linear model can largely reduce the heat capacity errors caused by the probe deflection.For various deflection conditions,including the nonlinear deflection,the linear model can reduce the measuring errors to a certain extent.But errors were larger in the saturated sample than in the dry samples.(2)Thermo-time domain reflectometry(thermo-TDR)technology combines heat pulse technology and TDR technology and is also limited by these two technologies.Heat pulse technology requires short probes to avoid probe deflection and TDR technology requires long probes to increase the accuracy of measured water content.The design conflicts of the two technologies limit the use of the thermo-TDR method.We introduce the in situ probe-spacing-correcting method into the thermo-TDR sensor to make the in situ probe-spacing-correcting thermo-TDR sensor.Both the heat pulse experiment and TDR experiment were conducted to test the application of the new in situ probe-spacing-correcting thermo-TDR sensor.For the heat pulse results,linear model and nonlinear model of the spacing correcting method can significantly reduce the water content errors caused by spacing changes.R2 for the average results of the thermistors in one temperature probe for the linear and nonlinear models compared with the gravimetrice volumn water conetents were 0.902(RMSE = 0.039 m3 M-3,RMSE,Root-mean-square error)and 0.981(RMSE = 0.013 m3 M-3),respectively.The nonlinear model is more accurate than linear model.In situ spacing correcting method can significantly reduce the measurement error of the water content caused by spacing changes.Even probe spacing changes,one can obtain accurate water content.For TDR experiments,the RMSE for 4 cm and 6 cm probes were 0.033 m3 m-3 and 0.022 m3 m-3,respectively,indicating that the 6-cm long TDR probe was more accurate to measure water content than the 4-cm long TDR probe.Using TDR technology to measure soil water content,results for long probes are more accurate than those for short ones.The new thermo-TDR sensor can accurately measure soil water content.(3)Although the nonlinear model of the probe spacing method performs better in water content measurement,the linear model is still in need.Nonlinear model is more accurate by using more thermistors in one temperature probe than the linear model.For the same number of sensors,sensors for nonlinear model will take up more datalogger channels.When there are three thermistors in one temperature probe,the linear or nonlinear model can be selected according to the actual situation.If users want to choose two thermistors in three to use the linear model,we suggest to choose thermistors close to the bottom of the probe,because deflections for the bottom part of a probe is more close to the linear deflection.(4)The in situ probe-spacing-correcting method is based on the assumption that the soil is homogenous.But soil in the field is more complicated.We set up two groups of sensors to evaluate the use of the probe spacing correcting method in the field.Results showed that the probe-spacing-correcting method can reduce errors of the measured volumetric heat capacity caused by probe spacing changing and show the trends changing with time.It can also show the general trends of the probe spacing changing with time.Single probe uses one probe that can release heat and measure the temperature.For the single probe method,we carried out experiments using single probes with different filling materials and different diameters(1.27 mm,2.00 mm,3.18 mm,3.88 mm)to measure the thermal conductivity and the thermal contact conductivity of agar and two soil samples.It is found that the result calculated with four fitting parameters is more accurate than that with two fitting parameters.The alloy filled probes are much closer to the assumption of the perfect conductor than the conventional epoxy filled probes,and the results for the alloy filled probes are more accurate.The relationship between the thermal conductivity and the thermal contact conductivity and the probe diameter is more complex which is the synthesis result of the probe characteristics and the particle configuration.Probe with 1.27-mm-o.d.is less affected by thermal contact resistance in this study(1.27 mm,2.00 mm,3.18 mm).Alloy filled probes with1.27-mm-o.d.can provide accurate results.