Probe spacing of sap flow sensors affects the quantification of sap flow in plants. The assumption of nocturnal zero flow conditions, required by most non-destructive methods for probe spacing calibration, is often invalid in natural biological systems and extremely hard to validate with in situ measurements. Our study aims to present a novel method for accurate in situ determination of probe spacing. The method uses nonlinear curve fitting with two mathematical models applied during low flow conditions. Numerical simulations were conducted with three types of probe misalignment and demonstrated that errors in heat pulse velocity (V_h) (< 2.0%) is less than the error associated with commonly-used probe spacing correction methods. Field experiment on European beech (Fagus sylvatica L.) comparisons with corrected probe spacing applied to the heat ratio (HR) and Sapflow+ method produced improved sap flux densities, with both methods agreeing very well (root mean square deviation < 1 cm³ cm⁻² h⁻¹). The proposed method allows real time probe spacing measurements at low flow rates, alleviating some uncertainties associated with zero flow assumption. The new method can hence successfully be used to estimate in situ probe spacing for operational measurements of sap flux density.

液流传感器的探针间距影响植物中液流的量化。大多数用于探针间距校准的非破坏性方法所需的夜间零流量条件的假设在自然生物系统中通常是无效的，并且极难通过原位测量进行验证。我们的研究旨在提出一种准确原位确定探针间距的新方法。该方法使用非线性曲线拟合，在低流量条件下应用两个数学模型。对三种类型的探头未对准进行了数值模拟，并证明了热脉冲速度 ( V _h) (< 2.0%) 小于与常用探头间距校正方法相关的误差。欧洲山毛榉 ( Fagus sylvatica L.) 的现场实验与应用于热比 (HR) 的校正探针间距和 Sapflow+ 方法的比较产生了改进的树液通量密度，两种方法都非常吻合（均方根偏差 < 1 cm ³ cm ^{- 2}小时^-1 )。所提出的方法允许在低流速下进行实时探头间距测量，从而减轻与零流量假设相关的一些不确定性。因此，新方法可以成功地用于估计原位探针间距，以进行液流密度的操作测量。