CN101799436A - Instrument for measuring soil moisture and conductivity based on phase detection and measuring method thereof - Google Patents
Instrument for measuring soil moisture and conductivity based on phase detection and measuring method thereof Download PDFInfo
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Abstract
本发明提供了一种基于相位检测的土壤水分、电导率测量仪，包括：信号发生器，环行器，延时器，相位检测器，检波器及微处理器。本发明使用单一频率的正弦信号代替阶跃信号，使用相位检测器将入射信号和反射信号的相位差转换为与之成正比的直流电压信号，通过测量直流电压信号即可得到电磁波在探头上的传播时间，检波器将参考信号和反射信号进行整流，分别获取其电压幅值并输出给模数转换器，通过标定可进一步得到土壤含水量和电导率。由于没有采用昂贵的快速阶跃信号发生器和高频采样示波器技术，大大降低了成本和技术难度，其技术性能与国外同类仪器相当，但价格仅为国外产品的50％。此外，还提供了使用该测量仪测量土壤含水量和/或电导率的方法。
The invention provides a soil moisture and electrical conductivity measuring instrument based on phase detection, which includes: a signal generator, a circulator, a time delay device, a phase detector, a wave detector and a microprocessor. The present invention uses a single-frequency sinusoidal signal instead of a step signal, uses a phase detector to convert the phase difference between the incident signal and the reflected signal into a direct-current voltage signal proportional to it, and obtains the electromagnetic wave on the probe by measuring the direct-current voltage signal During the propagation time, the detector rectifies the reference signal and the reflected signal, obtains their voltage amplitudes and outputs them to the analog-to-digital converter, and the soil moisture content and conductivity can be further obtained through calibration. Because it does not use expensive fast step signal generator and high-frequency sampling oscilloscope technology, the cost and technical difficulty are greatly reduced. Its technical performance is comparable to that of foreign similar instruments, but the price is only 50% of foreign products. In addition, methods of measuring soil moisture content and/or electrical conductivity using the meter are provided.
Description
Technical field
The present invention relates to field of measuring technique, particularly a kind of soil moisture, conductance instrument and measuring method thereof based on phase detection principle.
Background technology
The result of study of pedology shows in recent years, and this parameter of soil conductivity itself has comprised the abundant information of reflection soil quality and physical property.For example, salinity, moisture and the content of organic matter in the soil, soil compaction, quality structure and porosity etc. all affect the change of soil conductivity to some extent.In above factors, soil salt and water percentage obviously greater than other each factors, are estimated the growing environment of crops by means of the measured soil conductivity to the influence of conductivity, are one of focuses of the meticulous farming research of current developed country.Though the method for traditional experimental determination soil conductivity is accurate, process is loaded down with trivial details, makes troubles to engineering practice.Because the concentration and the conductivity of solute are linear in the soil liquid, and under the certain condition of water cut, there is linear relationship in the solution conductivity that the soil mass electricity is led with soil, therefore can directly determine the content and the migration of solute with the bulk conductivity of soil, but under the bigger situation of change of moisture content, soil mass conductivity and water cut are closely related, the direct content of indicating solute of leading with the body electricity of soil is just very difficult, soil moisture content and the soil mass conductance measured are combined, by the body electricity lead and solution conductivity between relation determine that the migration of material has very big meaning.
Time-domain reflectomer (TDR) is a kind of method of measuring soil moisture content that newly-developed gets up, its main advantages is can not destroy the undisturbed soil structure in the testing soil moisture process, easy and simple to handle, and can directly read soil moisture content, be convenient to the original position dynamic monitoring, accomplish message conversion and reach the purpose of automatic data collection, thereby accepted very soon by people.Topp and Davis were used for Research of Measurement of Moisture in Soil with the TDR technology first in 1975.The fundamental basis of the TDR soil moisture measurement speed that to be electromagnetic wave propagate along probe is inversely proportional to the square root of the specific inductive capacity of probe surrounding soil, therefore can come the specific inductive capacity of measured soil according to the speed of electromagnetic wave propagation.Gu the Topp Using such method recorded gas in the soil--DIELECTRIC CONSTANT of liquid mixture, and then utilize the numerical value regression analysis to have found out the water cut of Different Soil and the experimental formula between the specific inductive capacity:
θ＝-5.3×10
-2+2.92×10
-2ε-5.5×10
-4ε
2+4.3×10
-6ε
3 (1)
Wherein θ is the soil volumetric(al) moisture content, and ε is the soil apparent dielectric constant.
Proved the real-time requirement that soil moisture and conductivity measurement based on the TDR method can satisfy quick measurement by a large amount of theories and experimental study, but porous medium object to this complexity of soil, though the variation of water cut θ can cause the variation of DIELECTRIC CONSTANT significantly, but under the condition that the sensor probe geometrical length is restricted, poor Δ T incident-reflection interval that is caused by gas-solid-liquid mixture DIELECTRIC CONSTANT but only is 10-9 order of magnitude second.If will accurately measure to the retardation time of weak point like this, very big from the angle difficulty of radio survey technology, corresponding with conductivity measurement device cost very high based on classic method TDR soil moisture.
The TDR soil moisture tester generally is made up of etc. step signal generator, coaxial transmission line, soil moisture probe and high frequency sampling oscilloscope and is partly formed, as shown in Figure 1, wherein, the 1-RS-232 communication port, the 2-synchronizing generator, 3-pulse producer, 4-sampling oscilloscope, 5-50 Ω concentric cable, the 6-probe; High-frequency impulse generator (pulse generator) sends the high-frequency impulse of 1000MHz, and its coaxial transmission line by 50 Ω (50 Ω coaxial cable) is transferred to probe (probe), because coaxial transmission line and probe impedance do not match, some electromagnetic wave reflects along coaxial transmission line in probe and transmission line junction, remaining electromagnetic wave continuation is transferred to the other end of probe along popping one's head in, because the impedance of probe and soil does not match and causes electromagnetic secondary reflection again.Time between two secondary reflections is the twice of electromagnetic wave along the probe transmission time.Time between two secondary reflections can be measured demonstration by ondograph (sampling oscilloscope).Transmission time can be expressed as:
t＝2Lε
0.5/c (2)
In the formula: t---the time (s) between two secondary reflections; The length of L---probe (m)
The specific inductive capacity of ε---medium (soil); C---electromagnetic wave velocity of propagation (3 * 10 in a vacuum
8M/s)
Can draw the DIELECTRIC CONSTANT of medium thus:
ε＝[ct/(2L)]
2 (3)
(ct/2) is called probe " apparent " length in the formula.Make L
a=ct/2 is then:
ε＝(L
a/L)
2 (4)
Soil conductivity is relevant with signal reflex rate and soil moisture content.
Only minority developed country has grasped technology such as manufacturing and designing the required high-speed sampling oscillograph of TDR soil moisture tester, narrow rising edge step signal generator, high-frequency high-precision time reference at present.And these countries take strict export of techniques restriction to China, make China can't obtain essential most advanced and sophisticated electronic unit and chip, so China can't make traditional TDR soil conductivity tester.The TDR instrument that China produces and R﹠D institution uses is mainly from state's imports such as the U.S. and Canada, about 10,000 dollars of unit price, and the set of equipments price is several ten thousand dollars, owing to China does not still have this product, import price is more high than this ground price of producing country.Because of producer abroad, lack perfect maintenance and fittings supply service, often have to return former producer after breaking down and keep in repair, the continuity of influence monitoring.So, develop the TDR soil moisture, conductivity measurement and the measuring method that have China's independent intellectual property right and have important significance for theories and practical value.
Summary of the invention
(1) technical matters that will solve
The technical problem to be solved in the present invention is to overcome that electromagnetic wave is propagated in the existing TDR soil moisture measurement technology time is difficult to accurately measure and the research and development difficulty that causes is big and defective such as measuring equipment cost costliness on sensor probe.
(2) technical scheme
For solving the problems of the technologies described above, technical scheme of the present invention provides a kind of soil moisture based on phase-detection, conductance instrument, comprising:
Signal generator is exported sinusoidal detection signal vt of the high frequency with same frequency, amplitude and initial phase and reference signal vr to circulator and chronotron respectively;
Circulator, the sensor probe in soil to be measured is exported described detection signal vt by concentric cable and impedance transformer, isolates the reflected signal vt that returns through the sensor probe end reflection and exports to phase detectors and wave detector respectively;
Chronotron makes the transmission time generation of the reference signal vr of described signal generator output export to phase detectors after delaying time;
Phase detectors, the phase differential between the reflected signal vt that the reference signal vr and the described circulator of the output of described chronotron are exported is converted into proportional with it d. c. voltage signal, exports to analog to digital converter;
Wave detector carries out rectification to the reference signal vr of described chronotron output and the reflected signal vt of described circulator output, obtains its voltage magnitude Vr and Vt respectively and exports to analog to digital converter;
Analog to digital converter carries out analog-to-digital conversion and exports to the travel-time counter respectively and reflection coefficient calculates device the signal of described phase detectors and wave detector output;
The travel-time counter is according to the travel-time t of phase difference value calculating detection signal vt on described sensor probe of analog to digital converter output;
Reflection coefficient calculates device, according to the voltage magnitude Vr and the Vt signal calculated reflection coefficient ρ=Vt/Vr of analog to digital converter output;
The moisture counter, according to described travel-time t and the water cut calibration formula of demarcating in advance calculate soil moisture content θ to be measured;
The Conductivity Calculation device, according to described water cut θ, described reflection coefficient ρ and the conductivity calibration formula of demarcating in advance calculate soil conductivity σ to be measured.
Wherein, described chronotron is the time-delay cable.
Wherein, described impedance transformer is an impedance inverter circuit.
Technical scheme of the present invention also provides a kind of method of using this measuring instrument measured soil moisture, it is characterized in that, may further comprise the steps:
Generation has the sinusoidal detection signal vt of high frequency and the reference signal vr of same frequency, amplitude and initial phase;
Circulator sends described detection signal vt in the soil to be measured sensor probe by concentric cable and impedance transformer, isolates the reflected signal vt and the output of returning through the sensor probe end reflection;
Make the propagation time delay of described reference signal vr, equate with the transmission time of detection signal vt on circulator, concentric cable and impedance transformer time delay;
The reference signal vr and the phase differential between the reflected signal vt of described delay output are converted into proportional with it d. c. voltage signal, are converted into digital signal subsequently;
Calculate the travel-time t of detection signal vt on described sensor probe according to the phase difference value that comprises in the described digital signal, and calculate soil moisture content θ to be measured according to the calibration formula of funtcional relationship between described travel-time t of sign that demarcates in advance and the soil moisture content θ.
Further, this method also comprises, demarcates the funtcional relationship between described travel-time t and the soil moisture content θ in advance.
Technical scheme of the present invention also provides a kind of method of using this measuring instrument measured soil conductivity, may further comprise the steps:
Generation has the sinusoidal detection signal vt of high frequency and the reference signal vr of same frequency, amplitude and initial phase;
Circulator sends described detection signal vt in the soil to be measured sensor probe by concentric cable and impedance transformer, isolates the reflected signal vt and the output of returning through the sensor probe end reflection;
Make the propagation time delay of described reference signal vr, equate with the transmission time of detection signal vt on circulator, concentric cable and impedance transformer time delay;
The reference signal vr and the phase differential between the reflected signal vt of described delay output are converted into proportional with it d. c. voltage signal, are converted into digital signal subsequently;
Calculate the travel-time t of detection signal vt on described sensor probe according to the phase difference value in the described digital signal, and calculate soil moisture content θ to be measured according to the calibration formula of funtcional relationship between described travel-time t of sign that demarcates in advance and the soil moisture content θ;
Reference signal vr and described reflected signal vt to described delay output carry out rectification, obtain its voltage magnitude Vr and Vt respectively, and are converted into digital signal;
Calculate reflection coefficient ρ=Vt/Vr according to voltage magnitude Vr in the described digital signal and Vt, and calculate soil conductivity σ to be measured according to the calibration formula of funtcional relationship between the characterization signal reflection coefficient ρ, the soil moisture content θ that demarcate in advance and the soil conductivity σ.
Further, this method also comprises: pre-determine funtcional relationship between reflection coefficient ρ, soil moisture content θ and the soil conductivity σ by rating test.
Wherein, the calibration formula of funtcional relationship is between described sign reflection coefficient ρ, water cut θ and the soil conductivity σ: σ=(1468 θ
2-531.76 θ+75.474) (ρ-1)
2
(3) beneficial effect
Soil moisture based on phase-detection of the present invention, conductivity measurement, its high-frequency circuit uses the sinusoidal signal of single-frequency to replace step signal, use phase detectors that the phase differential of incoming signal and reflected signal is converted to the d. c. voltage signal that is directly proportional with it, can obtain the travel-time of electromagnetic wave on probe by measuring d. c. voltage signal, wave detector carries out rectification with reference signal and reflected signal, obtain its voltage magnitude respectively and export to analog to digital converter, can further obtain soil moisture content and conductivity by demarcating.Owing to do not adopt the quick step signal generator and the high frequency sampling oscilloscope technology of traditional TDR technology, greatly reduce cost and technical difficulty.Its technical feature is with suitable with quasi-instrument abroad, but price only is 50% of an external product.
Description of drawings
Fig. 1 is the schematic diagram of the TDR soil moisture tester of prior art;
Fig. 2 is the structural drawing according to the soil moisture based on phase-detection of the present invention, conductance instrument (P-TDR);
Fig. 3 is the graph of a relation that is used for demarcating soil conductivity and signal reflex coefficient under the embodiment designated volume water cut of rating test of conductivity formula in the method according to measuring instrument measured soil conductivity shown in use Fig. 2 of the present invention;
Fig. 4 is used for demarcating the embodiment quadratic polynomial model coefficient of rating test of conductivity formula and the graph of a relation of soil volumetric(al) moisture content in the method according to measuring instrument measured soil conductivity shown in use Fig. 2 of the present invention.
Embodiment
The soil moisture based on phase-detection, conductance instrument that the present invention proposes are described as follows in conjunction with the accompanying drawings and embodiments.
Be illustrated in figure 1 as according to of the present invention based on the soil moisture of phase-detection, the structural drawing of conductance instrument, as can be seen from Figure, this measuring instrument comprises:
Signal generator, it links to each other with chronotron with circulator, and respectively to circulator and chronotron output detection signal vt and reference signal vr, wherein, vt and vr are the high frequency sinusoidal signal with same frequency, amplitude and initial phase after the generation high frequency sinusoidal signal; Thus, use the sinusoidal signal of single-frequency to replace traditional step signal;
Circulator, it links to each other with signal generator, impedance transformer, phase detectors and wave detector, detection signal vt enters the sensor probe in the soil to be measured after by concentric cable, impedance transformer, and in probe end generation reflection, after this, circulator is separated the reflected signal vt and the incoming signal of probe end, and reflected signal vt is exported to phase detectors and wave detector; Wherein, impedance transformer can be impedance inverter circuit;
Chronotron, it links to each other with signal generator, phase detectors and wave detector, make the propagation time delay of the reference signal vr of automatic signal generator, just in time be the transmission time of detection signal vt on circulator, concentric cable and impedance transformer the time delay of chronotron; Wherein, chronotron can be the time-delay cable;
Phase detectors, it links to each other with circulator, chronotron and microprocessor, phase differential between the reflected signal vt that the reference signal vr of chronotron output and circulator are exported is converted into proportional with it d. c. voltage signal, and the analog to digital converter of exporting in the microprocessor carries out analog to digital conversion; Thus, use method for detecting phases to replace traditional high-speed sampling oscillograph method;
Wave detector, it links to each other with chronotron, circulator and microprocessor, the reference signal vr of chronotron output and the reflected signal vt of circulator output are carried out rectification, obtain its voltage magnitude Vr and Vt respectively and the analog to digital converter exported in the microprocessor carries out analog to digital conversion;
Microprocessor: link to each other with wave detector with phase detectors, the signal of its output is handled, calculate soil moisture content to be measured and/or conductivity as required; A progressive ground, it comprises:
Analog to digital converter carries out analog-to-digital conversion and exports to the travel-time counter respectively and reflection coefficient calculates device the signal of described phase detectors and wave detector output;
The travel-time counter is according to the travel-time t of phase difference value calculating detection signal vt on described sensor probe of analog to digital converter output;
Reflection coefficient calculates device, according to the voltage magnitude Vr and the Vt signal calculated reflection coefficient ρ=Vt/Vr of analog to digital converter output;
The moisture counter, according to described travel-time t and the water cut calibration formula of demarcating in advance calculate soil moisture content θ to be measured;
The Conductivity Calculation device, according to described water cut θ, described reflection coefficient ρ and the conductivity calibration formula of demarcating in advance calculate soil conductivity σ to be measured.
Because circulator, concentric cable and impedance transformer are very little to the decay of signal, can ignore, then reflection coefficient ρ has reflected that signal is in the signal attenuation that is caused by the dielectric loss and the conduction current of soil when sensor probe propagates, and the specific inductive capacity of soil media is relevant with the soil volumetric(al) moisture content, therefore reflection coefficient ρ and soil moisture content θ, soil conductivity σ satisfy certain functional relation, and this funtcional relationship can be decided in advance by rating test; The Conductivity Calculation device can calculate soil conductivity according to reflection coefficient ρ and this calibration formula; Therefore, also comprise in the microprocessor:
Storer stores in advance the calibration formula of funtcional relationship between the calibration formula of funtcional relationship between the sign travel-time t that obtains by rating test and the soil moisture content θ and sign reflection coefficient ρ and soil conductivity σ and the water cut θ.
Wherein, preferably, described sensor probe is the parallel three bar types probe of stainless steel, by impedance transformer or other settings its characteristic impedance value with concentric cable is complementary, thereby reduces the reflection of signal at the probe head end, and then the raising measuring accuracy.
Wherein, above-mentioned signal generator, circulator, chronotron, phase detectors, wave detector all can pass through hardware realizations such as related circuit, chip, cable, and it is respectively techniques well known, is not described further at this.
Wherein, the program that is embedded in of above-mentioned microcontroller is for carrying out the software of corresponding operating according to the workflow of measuring instrument of the present invention.
The present invention also provides the method for using above-mentioned soil moisture based on phase-detection, conductance instrument difference measured soil moisture and soil conductivity, comprising: high frequency sinusoidal signal generator output frequency, amplitude and the on all four test signal of initial phase (vt) and reference signal (vr); Be the travel path of test signal vt as shown in phantom in Figure 2, dotted line is depicted as the travel path of reference signal vr; Particularly, vt enters sensor probe and produces reflection at the probe end through circulator, concentric cable and impedance inverter circuit, and circulator is separated reflected signal and sent into phase detectors and wave detector; Simultaneously, vr also input phase detecting device and wave detector after chronotron postpones, should just in time equal the transmission time of vt on circulator, concentric cable and impedance inverter circuit its time delay; Phase detectors are sent into microprocessor after the phase differential between reflected signal vt and the reference signal vr is converted to proportional with it d. c. voltage signal; Microprocessor obtains the travel-time t of electromagnetic wave on probe according to phase difference calculating, calculates soil moisture content according to the water cut calibration formula that is provided with in advance again; Wave detector carries out rectification to reference signal vr and reflected signal vt, obtains voltage magnitude Vr and the Vt of vr and vt, these two voltage magnitudes after the A/D conversion, send into the reflection coefficient ρ that is used for signal calculated in the microprocessor (=Vt/Vr); Last conductivity calibration formula according to setting in advance can calculate soil conductivity.Wherein, demarcate in advance by test before described water cut calibration formula and conductivity calibration formula are and implement measure and obtain.
In one embodiment of the invention, funtcional relationship is between soil moisture content θ and the signal propagation time t: θ=-3.1363t
3+ 24.016t
2-60.442t+50.15, wherein, θ is volume moisture content m
3/ m
3, t is signal propagation time ns.
The scaling method of present embodiment with the Tu Shui ratio be 1: 5 soil extraction conductivity (σ) as the soil conductivity standard, reflection coefficient and the conductivity relation of P-TDR are demarcated.With analytically pure KCl and the soil sample of deionized water with 8 kinds of different conductivity of interval preparation of 0.3dS/m, the design load of soil sample conductivity (dS/m) is: 0,0.3,0.6,0.9,1.2,1.5,1.8,2.1, because soil itself has faint conductivity, so the actual conductivity and the design load of soil sample have minute differences behind the adding KCl, therefore, the actual value of 1: 5 soil extraction conductivity of each soil sample as the soil sample conductivity measured with PET2000 type electric conductivity instrument in the test back.To above-mentioned 8 soil samples with 0.05m
3/ m
3The soil moisture content increment add deionized water successively until saturated, measure the reflection coefficient of soil under each water content levels with P-TDR.By analyzing the Changing Pattern of reflection coefficient, study the validity of P-TDR measured soil conductivity, and measurement result is demarcated.Environment temperature is controlled at about 20 ℃ in the experimentation.Adopt parallel three bar types probe in the experiment, the stainless steel probe diameter is that 4mm, the long 30mm of being, interrod spacing are from being 20mm.
Particularly, test preliminary work comprises: will test with folk song and do the sieve of back with maximum diameter of hole 2mm, and be put in the baking oven temperature with 105 ℃ then and dried 24 hours, it is standby to be cooled to room temperature under the environment of drying.The PVC bucket internal diameter that is used to load soil sample is 15cm, being that 25cm, this test set needs the soil volume V that loads deeply
s=800mL, and mark is done at the corresponding height place in bucket is so that remain unchanged the soil sample volume when loading at every turn.According to pre-determined soil unit dry weight ρ
b=1.7g/mL calculates the quality m of required dry ground
s=ρ
bV
s=1360g takes by weighing 8 parts of the dry ground of this quality with TC10KB type electronic balance.The maximum capacity of TC10KB type electronic balance is that 10kg, sensitivity are 0.1g.Soil conductivity σ (dS/m) according to experimental design calculates the value that each soil sample need be mixed KCl.Computing method are: if will make soil sample Tu Shui ratio is that the conductivity of 1: 5 soil extraction is σ, concentration known is that the KCl electrical conductivity of solution of 1mol/L is 128.8dS/m in the time of 25 ℃, the KCl molecular weight is 74.5g/mol, and the density of water is 1000g/L, and the soil quality of soil sample is m
s, then required KCl quality m
KClFor:
m
KCl＝σ/128.8×74.5×5m
s/1000 (5)
Test procedure comprises:
Dose volume water cut θ=0.05m at first
3/ m
3Soil sample.Get 0.05V
sDeionized water is with LA164 type electronic analytical balance (range: 160g, minimum division value: 0.1mg) take by weighing the required KCl value of soil conductivity of test design, it is dissolved in this deionized water.Pour dry ground and KCl solution into metal basin and fully mix, pack in the PVC bucket with layering filling method.After filling 8 soil samples by above-mentioned steps, sealing and standing 24 hours measures and writes down the signal reflex coefficient ρ of 8 soil samples then respectively with P-TDR;
Take by weighing 0.05V once more
sDeionized water left standstill 24 hours in the PVC bucket of packing into after the soil of itself and each soil sample fully mixed again, obtains 8 solutes content is constant but volumetric(al) moisture content is increased to 0.10m
3/ m
3Soil sample, measure and write down the signal reflex coefficient ρ of 8 soil samples once more;
With 0.05m
3/ m
3The soil moisture content increment repeat above-mentioned steps to soil volumetric(al) moisture content and reach 0.25m
3/ m
3The time, leave standstill that the soil sample surface has small amount of moisture to ooze out after 24 hours, it is saturated to illustrate that soil has reached, measure the signal reflex coefficient of each soil sample after, enter next step;
The soil that from each soil sample, takes a morsel, be 1: 5 ratio calculating and take by weighing deionized water in native quality ratio, the amount of added deionized water in should considering in the computation process to test is evenly mixed to leave standstill after 24 hours and is got clarified solution, uses PET2000 type electric conductivity instrument to measure the conductivity of clarified solution.This value is the conductivity standard value of each soil sample.
The reflection coefficient of measuring-signal is distinguished in the above-mentioned test of present embodiment to the soil sample (sandy loam) of 8 different conductivity under 5 kinds of soil moisture content situations, obtain 5 groups of 40 reflection coefficient ρ altogether, and measurement result is shown in Table 1; Figure 3 shows that soil conductivity and signal reflex coefficient graph of a relation under the designated volume water cut.
Table 1
Adopt linear model and quadratic polynomial model that measurement data is carried out the match test respectively.Consider when reflection coefficient ρ=1 and represent there is not signal attenuation, corresponding conductivity should be 0, and conductivity answers dullness to reduce with the reflection coefficient increase, so the model tormulation formula adopts the form suc as formula (6), formula (7).
Linear model: σ=A (ρ-1) (6)
Quadratic polynomial model: σ=A (ρ-1)
2(7)
The fitting result of model sees Table 2.The coefficient of determination (R in the his-and-hers watches 2
2) analyze discovery, linear model fitting effect under the low water content situation is better, and the quadratic polynomial model is then just in time opposite, considers practical situations, selects the conductivity measurement model of quadratic polynomial as P-TDR.At this moment, R under each water cut situation
2Value is all greater than 0.812, and root-mean-square error (RMSE) is then less than 0.294dS/m.
Table 2
The coefficient A of calibration formula is as shown in Figure 4 under each water content levels in the quadratic polynomial model.As seen, calibration equation coefficient A is the function of soil moisture content, and the employing quadratic polynomial is carried out match to the A (θ) that concerns of A and θ, the result has drawn matched curve as the formula (8) among Fig. 4, coefficient of determination R
2=0.99.
A(θ)＝1468θ
2-531.76θ+75.474 (8)
With formula (8) substitution formula (7), get the calibration formula of P-TDR measured soil conductivity:
σ＝(1468θ
2-531.76θ+75.474)(ρ-1)
2 (9)
The R of further 2 two kinds of models of analytical table
2Value finds that for linear model, when soil moisture content increased, fitting precision continued to descend, and for secondary model, (θ=0.20m when soil moisture content is big
3/ m
3) fitting precision is the highest.If therefore wish under all water cut situations, all to keep higher measuring accuracy, can consider soil moisture content is divided into high and low two intervals, adopt linear and secondary model modeling respectively.
Above embodiment only is used to illustrate the present invention; and be not limitation of the present invention; the those of ordinary skill in relevant technologies field; under the situation that does not break away from the spirit and scope of the present invention; can also make various variations and modification; therefore all technical schemes that are equal to also belong to category of the present invention, and scope of patent protection of the present invention should be defined by the claims.
Claims (9)
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