The method being used to measure the suction of unsaturated soil is based on the calibration of the suction of filter paper. This method was studied in China and abroad

but for homemade quantitative filter papers the calibration curves can just be used to lower suction. Based on the previous works we have made the calibration of filter paper suction at the higher suction. The calibration curves for the filter papers made in China or abroad are almost the same

which consists of two sections

one with larger slope and the other with smaller slope. We also found that the equation developed by van Genuchten is better to fit the calibration curve.