The SF-L sensor is an improved Granier sensor (SF-G). The improvement enables the correction of sap flow calculation by separate measurements of natural vertical background temperature gradients within the sapwood. Without correction (original method after Granier), natural temperature gradients within the sapwood can occasionally lead to extensive errors of up to 50% in sap flow estimates (cf. also Do, F., & Rocheteau, A., 2002). Using the SF-L sensor, temporally variable background temperature gradients can be subtracted from the temperature difference between the two needles of the sap flow sensor. Resulting sap flow measurements are significantly more accurate and the important reference point of zero sap flow (ΔTmax) remains stable over long periods.
A very helpful option for the exact determination of the point of zero flow is additional continuous information about the water saturation status of the tree body as provided by electronic dendrometers. This additional information allows a new definition of zero-flow conditions: sap flow = 0, when the relative humidity in the crown is 100%, and the tree body is saturated with water (indicated by the dendrometer curve). The precise dendrometer based determination of the zero point makes the usually performed every night zeroing (after Granier) unnecessary. It hence enables the detection of nightly sap flow.