3 ms·

This is cool. wonder if conformal coating over this would be enough to survive the outdoors

by tiagod 4y ago

ortusdux 4y ago
I'm wondering about the longevity of the soil contacting elements. The usual generic soil moisture probes typically have a high failure rate, IIRC due to galvanic corrosion. https://www.digikey.com/en/products/detail/seeed-technology-co.,-ltd/101020008/5482619 https://www.digikey.com/en/products/detail/seeed-technology-...
- _spduchamp 4y ago
  It is my understanding that capacitive moister sensors don't have this problem. Can anyone confirm?
  - simonrobb 4y ago
    That's correct in my experience. Galvanic corrosion was never a problem for the capacitive sensors I developed a few years back (unlike the resistive sensors linked by the parent). However you typically want them more rugged than a bare PCB for industrial settings and since the capacitive field can pass through plastic I had them in a plastic housing filled with resin. Perfectly isolated from the outside environment and hard as a rock. Here's a prototype: https://imgur.com/9yO28CY https://imgur.com/9yO28CY
    - metaphor 4y ago
      I can see how your prototype defends against corrosion, not so much this b-parasite toy. What I don't see anyone talking about is the impact of dielectric moisture absorption[1], which is typically around 0.3-0.5% over 24 hours exposure for cheaper FR-4 on the market, let alone how this product intends to maintain sensor calibration over time. I'd expect to see external corrosion impact at the solder joints first, especially around the battery terminals, but corrosion creep may happen alot sooner than that: vias in the b-parasite prototype are clearly tented[2], but that masking is superficial and won't protect against hygroscopic exposure from the inside; being open source certainly doesn't preclude hobbyists from making even worse material decisions either. [1] https://www.ipc.org/sites/default/files/test_methods_docs/2.6.2.1a.pdf https://www.ipc.org/sites/default/files/test_methods_docs/2.... [2] https://github.com/rbaron/b-parasite/blob/main/img/resized/img1.jpg https://github.com/rbaron/b-parasite/blob/main/img/resized/i...
pjsg 4y ago
I think that this twitter thread describes the horror show in trying to build such a sensor. TL;DR It ain't easy. https://twitter.com/rbaron_/status/1491467468996431873 https://twitter.com/rbaron_/status/1491467468996431873
- metaphor 4y ago
  Wow, I must have been way too slow thinking about how to word this remark[1] that I completely missed your post. Thanks for the design intuition confirmation. [1] https://news.ycombinator.com/item?id=33284221 https://news.ycombinator.com/item?id=33284221

This is cool. wonder if conformal coating over this would be enough to survive the outdoors

by tiagod 4y ago

This is cool. wonder if conformal coating over this would be enough to survive the outdoors

I'm wondering about the longevity of the soil contacting elements. The usual generic soil moisture probes typically have a high failure rate, IIRC due to galvanic corrosion. https://www.digikey.com/en/products/detail/seeed-technology-co.,-ltd/101020008/5482619 https://www.digikey.com/en/products/detail/seeed-technology-...

_spduchamp 4y ago
It is my understanding that capacitive moister sensors don't have this problem. Can anyone confirm?
- simonrobb 4y ago
  That's correct in my experience. Galvanic corrosion was never a problem for the capacitive sensors I developed a few years back (unlike the resistive sensors linked by the parent). However you typically want them more rugged than a bare PCB for industrial settings and since the capacitive field can pass through plastic I had them in a plastic housing filled with resin. Perfectly isolated from the outside environment and hard as a rock. Here's a prototype: https://imgur.com/9yO28CY https://imgur.com/9yO28CY
  - metaphor 4y ago
    I can see how your prototype defends against corrosion, not so much this b-parasite toy. What I don't see anyone talking about is the impact of dielectric moisture absorption[1], which is typically around 0.3-0.5% over 24 hours exposure for cheaper FR-4 on the market, let alone how this product intends to maintain sensor calibration over time. I'd expect to see external corrosion impact at the solder joints first, especially around the battery terminals, but corrosion creep may happen alot sooner than that: vias in the b-parasite prototype are clearly tented[2], but that masking is superficial and won't protect against hygroscopic exposure from the inside; being open source certainly doesn't preclude hobbyists from making even worse material decisions either. [1] https://www.ipc.org/sites/default/files/test_methods_docs/2.6.2.1a.pdf https://www.ipc.org/sites/default/files/test_methods_docs/2.... [2] https://github.com/rbaron/b-parasite/blob/main/img/resized/img1.jpg https://github.com/rbaron/b-parasite/blob/main/img/resized/i...

pjsg 4y ago

I think that this twitter thread describes the horror show in trying to build such a sensor. TL;DR It ain't easy. https://twitter.com/rbaron_/status/1491467468996431873 https://twitter.com/rbaron_/status/1491467468996431873

metaphor 4y ago
Wow, I must have been way too slow thinking about how to word this remark[1] that I completely missed your post. Thanks for the design intuition confirmation. [1] https://news.ycombinator.com/item?id=33284221 https://news.ycombinator.com/item?id=33284221