Thermal sap flow sensors were initially proposed in the paper [German paper] in [date of german paper] by [german paper author]. Since then, several key improvements have created a sensor that semi-popular amongst researchers. However, their current high price point has reduced adoption.
A good case study for the modern state of commercial thermal sap flow sensors is Dynagague. Their probe style sap flow sensor which costs around $300-500 for a single sensor and an additional cost of ~$400 for the datalogger.
By using a few tricks in the design, we at the OPEnS Lab in contribution with [Friend of Trees] hope to design a sap flow sensor that costs less an $100 to produce including labor (priced at $50/hour) that meets the same performance at Dynagague. I plan to do this by redesigning almost everything to incorporate modern SMT manufacturing techniques open sourced communication protocols.
This is the BOM for the first draft design. The raw eagle design documents can be found at [https://github.com/stoddabr/sapflow/tree/master].
To open them you will have to download my parts library from [https://github.com/stoddabr/EaglePCB].