Research On Detector For Sapwood Moisture Content Of Living Trees Based On Capacitance Method

The traditional moisture content measuring instrument can only measure the moisture content below 30%of wood materials.The moisture content measurement of high-moisture content media such as standing wood has been a hot issue at home and abroad.Aiming at the issue,a theory of water content detection by capacitance method was proposed.This theory refers to a parallel-plate capacitor,and a plug-in measuring probe is designed to convert the change in the dielectric constant of the trunk of a standing tree into the actual capacitance value of the circuit element C1 of the moisture content sensor.The moisture content sensor outputs a periodic signal.The depth and the mass water content of the medium change,but the waveform is always square wave,which has good measurability.However,the output square wave at this time is an analog signal,which needs to be converted into a digital signal by a lower computer,and then substituted into a preset formula,and the specific value of the moisture content of the live sapwood is read on the display screen.In this paper,the overall scheme of the sapwood quality sapwood moisture detector is designed.The specific design and manufacturing process of the sensor,the hardware of the lower computer,the software of the lower computer,the power supply module and the shell are introduced.Then the whole machine is debugged and given about dimensions,technical parameters,and usage methods of the sapwood moisture content detector for living tree;simulation of the probe capacitance value,actual measurement of the probe capacitance value and the output square wave period;The calibration test was performed by the detector,and the relationship between the moisture content of the living tree sapwood and the output signal period was obtained.The optimal calibration equations for conifer and broadleaf trees were selected,the detector zero calibration was performed,and the recommended insertion depth was given.The DMC-1 detector measures the water content of trunk sapwood indoor and outdoor.The conclusions are drawn as follows.(1)The probe part(copper rod)is equivalent to a parallel plate capacitor,and the approximate calculation of the probe capacitance value is between 0.07 and 5.67pF;the COMSOL software is used to 1:1 simulate the probe capacitance to obtain the capacitance value between 1 and 8 pF;the capacitance value measured by the VC6013 digital capacitance meter continuously changes between 0.1 and 8 pF;the output waveform of the moisture sensor is unchanged with the DS1052E oscilloscope,and the signal period varies with the inserted medium.The results are consistent,so the capacitance value of the probe or the period of the water content sensor output waveform changes regularly with the dielectric constant(water content)and the insertion depth.It has a one-to-one correspondence and can be used as a test.An effective method for the quality moisture content of the sapwood of living standing trunks.(2)There is a quantitative relationship between the moisture content(y,%)of the trunk sapwood and the square wave period(x,μs)output by the detector.Under certain tree species and insertion depth,it can be fitted as two or three times polynomial,where 1cm insertion depth is better.(3)DMC-1 moisture content detector can be used to detect the water content of living tree sapwood,and can detect the moisture content range of 0～190%,and the measurement accuracy is±2%.