Banana Plant Trial Result (Traditional Design)

To test the probe described in my previous blog post, I deposited the probes in a banana plant I had handy.

How I built the probes by inserting heater wire and type T thermocouples into hypodermic needles. More resources can be found in my last post and in the following paper by Davis et al. Post here. Paper here. 

The graph result of the test is below:

As you can see, the top probe was not reading the correct temperature. This was probably an issue with the accuracy because it remained surprisingly precise meaning that the results shouldn't be completely discarded. Daily trends are evidence that the sensor could be working somewhat. This is nowhere near the accuracy I intend to get with my final design.

However, there definitely is something interesting going on here. The data was recorded over an interval of three days. Examining the Differential plot, there seem to be roughly three major up-and-downs that PROBABLY are related to the day-night cycles of photosynthesis. 

 

Grainer Sap Flow (the Status Quo)

Before I got started designing the SMT sap flow, I built and tested the probe sap flow sensor popularized by Grainer. This design popular with researchers probably because of its inexpensive BOM ( ~$10/sensor ) and promisingly low labor ( 1 hour/sensor ) which puts the total price at ( ~$60/sensor ).

When I replicated several of these probes I found significant difficulty in making them. The 36 gauge wires are especially difficult to work with and the nichrome wire heater probe was almost impossible to wind to a high enough resistance. For the 7 I built, only 2 were usable enough to record meaningful data. This is not a process I would recommend to anyone.

Here is the BOM used by Camden Lowrance in his University of Georga thesis in 2014 and the simplified one I created for this project. Camden's includes a base station and mine does not.  

The biggest difference is that I used a thermocouple amplifier chip instead of building my own. This greatly simplified the design and made fabrication a lot easier. I chose an Adafruit Universal Thermocouple Amplifier MAX31856 Breakout board. I made sure to set it to the Type-T setting. Adafruit has an excellent guide on how to use one of these on their website here. 

 

Posted by Brett Stoddard

Intro to the Project

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.

 Here is an image of the rough schematic for the build.

Here is an image of the rough schematic for the build.

 

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].