The Integral Suspension Pressure (ISP) method is a technique to determine the continuous particle size distribution (PSD) of soil materials by inverse modeling of suspension pressure data in a sedimentation experiment. The objective function in ISP includes externally determined sand fractions and a time series of measured suspension pressures. Practical experience with ISP has shown that the precision of the method for the clay fraction is not satisfying. In this contribution, we present a further developed method called "ISP+" to improve the accuracy. This is achieved by additionally including in the objective function the dry particle mass of a collected subsample of the suspension containing fine material, mainly clay. This regularizes the inverse problem and reduces the uncertainty of the determined PSD in the fine particle range. The application of the methodology is illustrated with five soils tested with an adapted PARIO™ system (METER Group AG, Munich, Germany). The results show a significantly improved precision of the ISP+ results in the range of silt and clay particles and very good agreement with the pipette method, which serves as the reference. The measurement time could be reduced from 8 h (ISP) to about 2 h (ISP+) without affecting the accuracy.

积分悬浮压力 (ISP) 方法是一种通过对沉降实验中的悬浮压力数据进行逆向建模来确定土壤材料的连续粒度分布 (PSD) 的技术。ISP 中的目标函数包括外部确定的砂子比例和测量的悬浮压力的时间序列。ISP 的实际经验表明，该方法对粘土分数的精度并不令人满意。在这个贡献中，我们提出了一种称为“ISP+”的进一步开发的方法来提高准确性。这是通过在目标函数中额外包括收集的悬浮液子样本的干粒子质量来实现的，该子样本包含细粒材料，主要是粘土。这使逆问题正则化并降低了在细颗粒范围内确定的 PSD 的不确定性。该方法的应用通过使用经过调整的 PARIO™ 系统（METER Group AG，慕尼黑，德国）测试的五种土壤进行说明。结果表明 ISP+ 结果在淤泥和粘土颗粒范围内的精度显着提高，并且与作为参考的移液管方法非常吻合。在不影响精度的情况下，测量时间可以从 8 小时（ISP）减少到大约 2 小时（ISP+）。