Sap flow sensors are used to continuously measure water uptake in woody plants. The SF-G model corresponds to the classic, two-needle model developed in 1985 by André Granier.
Our four-needle SF-L sensor is an updated Granier sensor. The patented improvements correct the naturally occurring and time-varying vertical temperature gradients in the sapwood. Without correction (i.e. the original Granier sensor, SF-G) and despite isolation, these natural temperature gradients can lead to a temporary over- or underestimation of the measured sap flow values of up to 50% in extreme cases (cf. Do, F., & Rocheteau, A., 2002). Additional temperature measurements with the SF-L sensor enable a correction of the measured sap flow values, which prevents errors caused by the natural vertical stem temperature gradients. This results in a significantly higher accuracy and a stabler zero-point (ΔTmax) of the xylem (sap) flow.
A very helpful option to exactly determine this zero-point would be to use a dendrometer in order to continuously measure the water saturation level of the tree body. This additional measurement allows for a precise determination of zero sap flow conditions: sap flow = 0 if the relative humidity in the crown at 100% and the tree body is 100% saturated with water (no change in diameter according to the dendrometer data). The improved SF-L sensor combined with dendrometer-aided precise determination of zero flow conditions, makes a running nocturnal zeroing of the sap flow (according to Granier) unnecessary. This process enables the recording of a nocturnal xylem flow.