a health monitor for trees and fruit

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The OpenDendrometer solution provides insights on critical plant health information. It is a valuable tool for managing and studying plant growth.

OpenDendrometer can help answer questions such as:
Is a tree experiencing water stress?
Has fruit or vegetable growth rate slowed?
What are the long term growth behaviors of trees in a specific region, of a specific species, ...?

A dendrometer is an instrument used to measure diameter changes of trees and fruit. Small micrometer changes in diameter occur in trees and fruit throughout the day as part of healthy water transport processes. Comparisons of these daily diameter changes can help predict the onset and occurrence of water stress. Diameter measurements collected over weeks and months show the rate and magnitude of plant growth. In particular, weekly diameter growth data can be analyzed to understand if fruit growth rate is tracking well for harvest. 

The OpenDendrometer is a low-cost digital dendrometer with wireless data transfer capabilities. In simplest terms, it is device that measures, logs, and wirelessly reports changes in linear displacement. It uses commonly available electronics components and fabrication techniques to ensure that others can easily build their own OpenDendrometer. All design files and information will be made freely available after completion of the first proof-of-concept prototype.

  • Cost Estimate

    John Opsahl06/11/2022 at 15:33 0 comments

    Initial cost estimate for a single OpenDendrometer is $70. The 3D printed parts are the largest contributor to total cost. So if you have access to a 3D printer, you could build an OpenDendrometer for around $45.

    I have planned to develop a custom PCB to make assembly very easy, but it's likely possible to make all the connections with jumper wires. Using jumper wires would shave an additional $8-10 off the total cost. 

    All costs for purchased items in the table below are based off of list price on I would guess that sourcing all of the purchased components from Aliexpress could reduce total cost by 20-30%. The downside is that lead time for items purchased from Aliexpress is usually somewhere between 3 and 8 weeks.

    Qty Description Cost Per Unit Total Cost
    1 digital tire tread depth gauge $8.99 $8.99
    1 ESP32 microcontroller $6.66 $6.66
    1 lithium battery, 1800mAh $7.18 $7.18
    1 micro SD card module $1.00 $1.00
    1 micro SD card $4.99 $4.99
    1 DS3231 RTC module $3.40 $3.40
    1 logic level shifter, 4 channel $1.60 $1.60
    2 2N7000 mosfet $0.08 $0.16
    1 custom PCB $10.00 $10.00
    1 custom 3D printed parts $25.00 $25.00
    1 screw, 200mm $1.00 $1.00
    Total $69.98

  • 3D Printed Weatherproof Enclosure

    John Opsahl06/08/2022 at 04:38 0 comments

    I have always thought of FDM 3D printed parts as somewhat porous. As though defect holes in the wall of the part are inherit to the hobby level 3D printing process. I assumed that some kind of sealant would need to be applied to achieve any level of water ingress protection. Recent testing of the first OpenDendrometer enclosure prototype has shown that these assumptions are not true. 

    Now, I am only trying to develop a weatherproof 3D printed enclosure (some might call it water resistant); not a waterproof one. So the requirements are less challenging, but I still expected to encounter issues. Incidentally, I think I achieved a weatherproof 3D printed enclosure on the first iteration.

    Really all I did was to copy the foam cord water seal strategy used on almost all small ingress protection rated electronic enclosures. It involves a foam cord gasket between the case and the cover. With all fasteners between case and cover on the outside of the gasket seal and features in each to either capture or compress the foam cord.

    To test the seal, I put a piece of tissue paper inside and placed it in a bowl of water. After five minutes in the bowl of water -> voila, the tissue paper was still dry. 

    Even without applying any sealants to the 3D printed parts, it achieved a 5 minute waterproof seal. Which should be more than enough for just a weatherproof enclosure that will only see occasional precipitation. 

    I suspect that UV exposure may damage the plastic enough overtime to cause water ingress, but all testing is going to be fairly short at less than a month. I will worry about how to combat long term UV exposure once the design passes system level testing. 

  • Preliminary Competitive Analysis

    John Opsahl05/15/2022 at 20:24 0 comments

    The following table details existing commercial, academic, and DIY dendrometer solutions: 

    Source Model Link
    Channel Technology Circumference DC2 & DC3
    Channel Technology Circumference Dendrometer 1 (DC1)
    Channel Technology Fruit and Vegetable Dendrometer (DF)
    Channel Technology Root and Aquatic Plant Dendrometer (DRO)
    Environmental Measurement Japan MIJ-02 Rotary Type3
    Wafer Sensor, Inc. Automated Dendrometer D6
    ICT International DEN1 Dendrometer ZN12-O-2WP Point Dendrometer
    Environmental Measurement Japan Absolute Linear Dendrometer
    kinasmith Dendrometers!
    Phytech Fruit Dendrometer
    OregonState Dendrometer

  • Initial Component Selection

    John Opsahl05/15/2022 at 20:07 0 comments

    Displacement Measurement. A digital tire tread depth gauge is the lowest cost linear displacement measurement solution that I have found so far. The most important consideration is that it has the required resolution of 10um and minimum range of 12mm. Fortunately, others have already determined how to use an Arduino to log length measurements from these digital gauges -> The only real challenge being that these devices use 1.5V logic so a logic level shifter is required. 

    Microcontroller. The ESP32 is the obvious choice to meet the low-cost and wireless data transfer requirements of the OpenDendrometer.

    Real Time Clock. I have used DS3231 RTC modules in the past for data logging projects with great success. The primary reason for using the DS3231 module is that the ESP32 internal clock is not accurate enough for the month or greater operational period. The second reason is that I will likely be configuring the DS3231 to wake up the ESP32 from long periods in sleep mode in an effort to minimize ESP32 power consumption.

  • Preliminary Requirements

    John Opsahl05/15/2022 at 19:33 0 comments

    The following is a list of preliminary requirements for the OpenDendrometer: 

    Type Requirement
    shall cost less than $60 in materials
    shall accommodate tree diameters >=100mm
    shall be able to be configured as a point dendrometer
    shall <= 10um linear displacement measurement accuracy, point dendrometer
    shall range of up to 12mm, point dendrometer
    should be able to be configured as a circumferential dendrometer
    should <= 0.025mm diameter change measurement accuracy, circumferential dendrometer
    should be able to be configured as a fruit dendrometer
    should <= 0.1mm diameter change measurement accuracy, fruit dendrometer
    should be human tamper proof
    should be wildlife tamper proof
    shall log length measurement twice an hour
    shall be capable of measuring both expansion and contraction
    shall alert the user when battery charge is low
    shall transfer data via Bluetooth
    shall be contained in a weatherproof enclosure
    shall operate within temperature range 0 to 55°C
    should operate within temperature range -20 to 55°C

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