Automated soil particle‐size analysis using time of flight distance ranging sensor

Abstract

AbstractParticle‐size distribution (PSD) influences various physical, chemical, and biological properties of soil. The hydrometer test is the most commonly used method for soil particle‐size analysis (PSA) because of its inexpensive instrumental set up and simple use. But manual observations are required to obtain high‐resolution PSDs. X‐ray, γ‐ray attenuation and laser diffraction techniques can measure PSD automatically and faster than the sedimentation methods. However, these techniques have a very high equipment costs. The objective of this study is to develop an automated method for measuring a wide range of soil PSD based on the hydrometer method. Adafruit VL6180X Time of Flight (ToF) distance Ranging Sensor was used to measure the distance between the sensor and the tip of the hydrometer over an 8‐h time interval as particles settled. An initial distance between the sensor and tip of the hydrometer was recorded and used to convert the distance with time to particle size and concentration. The temperature of the suspension was measured at the same time using a waterproof DS18B20 1‐Wire digital temperature sensor. The Arduino Uno R3 was used to program the ToF sensor to record both the distance and the temperature at 5‐s intervals. The recording of the distance with time and solution temperature was converted to particle diameter and concentration based on Stokes’ law. The whole PSD, from 40 μm to 2 μm, was obtained automatically within 8 h. A total of 10 soil samples with different soil texture were used to test the system. The measurement from the automated system was checked against the standard pipette method. Results show that measurement of the percentages finer than 2, 5, 10, 20, and 40 μm obtained by the two methods are highly correlated, with R2 values between 0.857 to 0.896. The hardware to automate the system costs less than $70 (USD) and provides an easy, efficient, and cost‐effective way of measuring soil PSD.