Development of the thermophysical parameter tester for measuring the soil matrix suction and optimization of the calibration function

•The in-situ thermophysical parameter tester was improved to measure soil matrix suction.•The simulation performance of theoretical model and ML on SWCC was compared.•The calibration function is determined by hybrid algorithm and traditional theory.•The in-situ test performance of the designed instrument and tensiometer was compared.•The systematic error of the designed instrument was analyzed.