SF-HP Heat-Pulse Sap Flow Sensors

We currently offer the following different versions:

    SF-HP-N3D2, with three 30 mm long sensor needles. The upper and lower measuring needles are each 6 mm away from the middle heating needle and each have two measuring points at 10 mm and 20 mm from sensor head.
    SF-HP-N3D3, with three 35 mm long sensor needles. The upper and lower measuring needles are each 6 mm away from the middle heating needle and each have three measuring points at equal distances from each other at 5 mm, 17.5 mm and 30 mm from the sensor head.

Advantages of HP-Sap Flow Sensors

• Significantly lower energy consumption compared to juice flow sensors with constant heating (e.g. TDP sensors such as SF-G or SF-L). 
• Equipped with several measuring points in each measurement needle, the radial sap flow depth profile can be recorded.
• The three-wire sensor design enables the direction of flow to be determined, i.e. also the detection of reverse flux as well as the use of the "Dual Method Approach" which combines the advantages of HRM and Tmax measuring methods and thus allows precise measurements in the entire range from slow to fast sap flow velocities.
• The underlying measuring principle itself is less susceptible to natural temperature gradients than compared to that of TDP sensors 
• Analogue output signal stronger (in the range of volts) than with TDP sensors (signals in the range of 1 millivolt), i.e. the relevant requirements for the data logger are significantly lower.
• Sensors are very robust.

Limits of HP-Sap Flow Sensors

• Measurements cannot be taken continuously, i.e. the maximum time resolution is in the range of 10 to 15 minutes
• Depending on the measurement method used (e.g. HRM, Tmax or DMA), the control of the sensor and the data processing of the sensor data generated during the measurement is relatively complex. The requirements on the data logger are therefore high in terms of measuring frequency, flexible programmability and data processing (Campbell Scientific Logger recommended, or our soon available multi-interface for integration via e.g. RS485).  
• Especially in the case of the Tmax and DMA methods, the data logger requires a measurement resolution of 0.01°C and a measurement frequency of at least 120 Hz during the measurement.  
• Depending on the sensor model, 4 (SF-HP-N3-D2) or 6 (SF-HP-N3-D3) measurement channels are required.

Technical Data

Sensor model name   
Heat pulse Sap-Flow Sensor SF-HP-N3D3
Heat pulse Sap-Flow Sensor SF-HP-N3D2

Scope of application    
For measuring the sap flow in woody plants

Suitable for diameter
> 35 mm
> 30 mm (Sensors for smaller diameters on request)

Number and alignment of the sensor needles
3 needles arranged in a line one above the other (1. upper measuring needle - 2. heating needle - 3. lower measuring needle), distances between the heating needle and the measuring needles are 6 mm each

Measurement depths
Measuring points at 3 depths at a distance of 5, 17.5 and 30 mm from the sensor head
Measuring points at 2 depths at a distance of 10 and 20 mm from the sensor head

Sensor dimensions

Sensor head (HxWxD): 45 mm x 20 mm x 16 mm.
Needle Diameter: 1.27 mm;
Needle Length: 35 mm
Sensor head (HxWxD): 45 mm x 20 mm x 16 mm.
Needle Diameter: 1.27 mm;
Needle Length: 30 mm

Output signal
Analog, voltage 0 to supply voltage Vex (recommended: 1 < Vex < 3.3 VDC)

Technical requirements for the data logger used
6x single ended measurement channels, noise-free resolution at least 0.01°C, i.e. 14 bits in the range from 0 to supply voltage Vex.
To use the Tmax, or the Dual Approach (HRM + Tmax) measurement method, the logger requires a measurement frequency of > 10 Hz (i.e. all 6 sensor channels in < 0.6 seconds)
Recommended: ratiometric half bridge measurement, with Vex = Vref of the logger. Data loggers from Campbell Scientific are ideally suited.
4x single ended measurement channels, noise-free resolution at least 0.01°C, i.e. 14 bits in the range from 0 to supply voltage Vex.
To use the Tmax, or the Dual Approach (HRM + Tmax) measurement method, the logger requires a measurement frequency of > 6.7 Hz (i.e. all 4 sensor channels in < 0.6 seconds)
Recommended: ratiometric half bridge measurement, with Vex = Vref of the logger. Data loggers from Campbell Scientific are ideally suited.

Sensor power supply
   
SF-HP sensors require two different power supplies:
1.: Switched excitation voltage (Vex) for the temperature measurement, recommended: 1 < Vex < 3.3 VDC. At best, ratiometric measurement with Vex = Vref of the logger
2.: Precisely timed heating current, 8 seconds approx. 300 mA, at 11 - 13 VDC. Energy consumption per measurement 0.67 mAh at 12V, i.e. 8 mWh. The usable capacity of a 10 Ah 12V battery is sufficient for around 10,000 measurements.

Sensor cable length
5 m, extendable to max. 25 m

Material: Epoxy and Delrin head, stainless steel needles