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The SF-8 Sensor enables to monitor diurnal variations of sap flow rate in plant stems and trunks. The operation of SF-8 sensor is based on a constant heat method with symmetric disposition of thermo-sensitive elements. A small heater warms a part of a stem to 2-3 degrees above the ambient temperature. Junctions of the thermopile are located at an identical distance above and below the heater. If there is no flux in the stem, the distribution of temperature is symmetric; therefore the output signal of the thermopile is close to zero. As a result of xylem flux, the temperature increases in the direction of the flow and decreases in the opposite direction. The thermopile output follows the gradient of temperature. SF-type sensors provide only relative measurement of sap flow rate. 
Two components sap flow rate in trunks may be distinguished: 1) amount of water passing through the plant for transpiration and 2) amount of water used for replenishing diurnal loss of water content in plants. The last component may be manifested in continual sap flow during nighttime.
Analysis of diurnal behavior of sap flow rate versus vapor pressure deficit or potential evapotranspiration (if available) is a fruitful method for investigating limiting factors of transpiration (See Fig. 1 below).
It has been found from research carried out for heat-pulse invasive sap flow probes that the tissue growth around the probe wound does not introduce significant error. 
Thus, the SF-8 sensor allows investigating intimate effects of environmental factors on water balance and transpiration of plants.


Fig.1. Exemplary records made on bell pepper plants. On April 4, sap flow followed VPD that is normal. Next day, the higher VPD level caused stomatal response and opposite trend of sap flow (this period is marked by red rectangle). The following conclusions can be made: 1) the level of VPD=2 kPa was critical for stomatal response and 2) stomata were partly closed during more than 3 hours on April 5. 



The SF-8 Sap Flow Relative Rate Sensor includes a sensor with 4 probes and a dc powered signal conditioner with optional power adapter for unipolar power supply. 


Length of probes: 10 mm
Diameter of probes: 1 mm
Dimensions of sensor: 70L x 10W mm
Diameter Range of Suitable Stems: above 15 mm 
Output: 0-2 V or 4-20 mA (optional)
Supply Voltage: 5 Vdc 15%
Power adapter operates on 10 to 30 Vdc
Dimensions: 120 x 70 x 37 mm (signal conditioner