Trees Can Talk
Measuring sap flow to determine the health of an orchard
Executive Summary
The sap flow project was a continuation of a multi-year project run by the OPEnS Lab and College of Agriculture at OSU, sponsored by the Washington State Tree Fruit Association. Previous years had left the project in an unknown state. Critical pieces of the project were missing and undocumented. We were tasked with the challenge of creating an accurate, affordable sap flow meter within the year.
Towards that goal we made significant developments. We increased the measurement resolution of the RTD amplifiers by 30x, resulting in very precise temperature measurements with minimal quantization error. We adapted the firmware to use protothreads, which both enforced the desired sample rate and allowed up to five probes to connect to one datalogger. We developed an Arduino-compatible low-memory DSP library that supports arbitrary order FIR and IIR filters. We developed state-of-the-art Arduino debugging tools, using a watchdog timer to tell you where and when the program hung. We used a Raspberry Pi as a low-cost LoRa hub, while leveraging standard web software on the Pi to provide data visualization for the farmer without internet. We made a compact and feature-rich PCB that stacks with Adafruit Featherwings for extensible projects.
Although we did not reach our accuracy goal, we believe the project is a success. We overcame many obstacles and achieved quality solutions, prioritizing simplicity, effectiveness, and cost.
What it does
A lot of work has gone into determining best-practices for keeping a healthy orchard. Extension services provide advice on everything from soil amendments and fertilization to which pesticides to use and when to spray. However, less is known about water usage. How much water do trees use? Is the orchard getting irrigated evenly? Does it have adequate drainage?
Measuring the sap flow gives you access to that information, since sap is primarily water. If you measure the sap flow on every tree, you can correlate water usage with fruit size and quality to determine the optimal watering practices, and potentially predict which trees are healthier and which may be diseased.
How it works
Our probe works by creating a heat pulse in the sapwood, and sensing which direction the pulse travels. It uses three small probes inserted into the trunk, following the direction of the grain. The middle probe has a heater which is turned on for several seconds. The upper and lower probes measure the temperature of the sapwood during and after the heat pulse. The relative amplitudes of the observed pulses indicates the direction and velocity of sap flow. You can find more about the Heat Ratio Method in "An improved heat pulse method to measure low and reverse rates of sap flow in woody plants" by Burgess et al (2001). The picture to the left is from a simulation of a thermal model of the tree, as observed by the upper and lower probes.
Who we are
Vladimir Vesely focuses on analog design, but also finds fulfillment working with embedded systems and computer graphics. Vlad brings extensive experience working with diverse teams through his years as a staff member in OSU's engineering residence hall.
Marshal Horn specializes in measurement, testing, and control. He has experience with embedded systems and low-latency processing. Currently he is developing skills in modeling dynamic systems and design validation. In his spare time he goes backpacking with his wife and plays disc golf.
Josh Barksdale enjoys developing low-power embedded systems, particularly with wireless communication. He has experience with embedded C, Linux, analogue circuit analysis, and testing/verification. He led code development on an Espressif ESP32 microcontroller for a low-power distributed sensor project last year.