I am looking to build a device that can measure the dendrometry fluctuations of trees (etc). I essentially want to build this https://publiclab.org/notes/kinasmith/09-09-2016/dendrometers to a t but I don't know where to start. I have admittedly pretty limited knowledge of arduino/microcontrollers and microcontroller kits and sensors.

  • The tension meter from that link does not measure the circumfence. I measures the force from displacing a small part of the string. Do you really want to measure the circumfence? – chrisl Jun 15 '19 at 19:09
  • I don't care about the actual circumference measurements, but I do need to wrap the string around a circular object and measure tension changes. What sort of device would I use to get started or what should I read up on? – Michael Alvarez Jun 15 '19 at 19:13
  • I don't really understand the whole setup, but you can build a tension meter like in the link yourself by using a load cell – chrisl Jun 15 '19 at 19:16
  • I am attempting to measure dendrometry dimensions of trees if that helps and want to do a setup like this but I am a beginner at this blog.adafruit.com/2016/10/25/… – Michael Alvarez Jun 15 '19 at 19:31
  • edited question for clarity – Michael Alvarez Jun 15 '19 at 19:35

The measuring principle, that is describe in that link, is quite cool and simple.

The string is wrapped around the tree. One end is connected to a fixed point (here the casing of the sensor). The other end is connected to a spring, which in turn is again connected to a fixed point. As the tree get's bigger, it will pull at the string. At the first side of the string nothing happens (because it is a fixed point). But the end with the spring will move according to the force of the tree. Force and displacement have a linear relationship for a spring (as long as you don't overload it): F = k*x with the force F, the spring constant k and the displacement x.

The displacement of the springs end can be measured with a linear potentiometer. That is just a simple strip of resistive material with a glider, which can be set to any position at the strip. It will create a voltage divider, giving you an analog voltage depending on the position (most potentiometers are linear, but there are also others). Connect both ends of the potentiometer to Vcc (supply voltage) and ground respectively and the glider to an analog pin of the Arduino. Then you can measure the voltage with analogRead().

As jsotola mentioned in his comment: Be sure to use a string, that will not stretch too much, as this will make your measurement less precise.

  • thanks @chrisl, this is starting to make sense to me. The only part that i'm not understanding is why the spring is necessary and how to actually set up the potentiometer to measure the displacement of the spring. – Michael Alvarez Jun 15 '19 at 23:35
  • get a string and a rubber band (spring) and a couple of small nails and try it out on a tree .... attach the string to the tree and then put your hand under the string to simulate tree trunk expansion ... see how the end of the string moves .... now try it without the rubber band – jsotola Jun 16 '19 at 3:47
  • Would you use a gliding potentiometer or a Softpot? Forgive me if this is the same thing, I may be confused here. – Michael Alvarez Jun 16 '19 at 17:21

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