SF-G / SF-L Granier Sap Flow Sensors
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.
Advantages
Specific to the SF-G model
- Suitable for tree diameters > 1 cm
Specific to the SF-L model
- Greater measurement accuracy
- Correct determination of the xylem flow zero point when supplemented with dendrometer measurements
- Allows measurement of nocturnal xylem flow
- Additional information about tree growth provided by installing supplementary dendrometers
Limits
Specific to the SF-G model
- The measured values are superimposed by naturally occurring vertical temperature fluctuations in the trunk (up to +/- 2.5 ° C). This leads to measurement errors greater than 50% and may impact the reliability of measurement results.
- Nocturnal sap flow cannot be measured. Because of the temperature overlaps, nocturnal measurement values have to be set to zero.
Specific to the SF-L model
- Not suitable for trees with a diameter < 8 cm
- 3 logger channels are required per sensor
Models
| Model name | Suitable for diameters | Sensor needle // heater element length in mm |
|---|---|---|
SF-G | > 1 cm | 18 // 10 |
> 2 cm | 33 // 20 | |
| 43 // 20 | ||
| 63 // 20 | ||
| SF-L | > 8 cm | 18 // 10 |
| 33 // 20 | ||
| 43 // 20 | ||
| 63 // 20 |
Technical Data
Sensor model name | Sap Flow Sensor | |
Type SF-G | Type SF-G | |
Scope of application | For continuous measurement of the sap flow in wood plants | |
Suitable for diameter | > 1 cm | > 8 cm |
Number of sensor needles | 2 | 4 |
Sensor dimensions | Needle diameter: 1.5 mm; Needle length//length heated in mm, available versions: 18//10; 33//20; 43//20; 63//20 | |
Output signal | Analog, DC voltage -100µV to +800 µV | |
Required logger channels | 1x differential, resolution better than 10 µV | 3x differential, resolution better than 10 µV |
Power supply | Sap flow sensors must be supplied via a constant current source (CCS). A CCS supplies up to 3 SF-G and / or SF-L sensors and continuously needs 85 mA x 12 V. This corresponds to 2.1 amps x hour / day. | |
Sensor cable length | 5 m, extendable to max. 20 m | |