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The Cheap Vegetable Gardner created this soil moisture sensor using nothing more than picture wall hanging hooks, a soda straw, hot glue, and some Plaster of Paris.
After a little research there was the classic science class method of creating a homemade sensor by simply putting two galvanized nails in the soil and measuring the resistance between them. A major problem with this solution is the soil composition can significantly vary the resistance and keeping the spacing between the nails could be troublesome.
After a little more research I came across the method that has been working well for over 50 years. This method includes taking two metal probes and inserting them into a gypsum block and measure the resistance between them. The gypsum absorbs the water and provides a decent range of resistance and moisture measurement.Unfortunately I was fresh out of gypsum, so I looked around the garage and found a good substitute, Plaster of Paris...
I beleive plaster of paris is gypsum but simply heated up first?
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Yes, it is. Heated and powdered. To use it, you add water, which makes it harden into "standard" gypsum. Somehow I doubt the original idea involved drilling holes in a gypsum block instead of using plaster and water. Someone didn't make their homework before submitting the story.
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Yes, you both are correct. That was my assumption was that as well but looking at the box and only seeing calcium sulfate hemihydrate on the box I was thoroughly confused. After a quick check on Wikipedia (should have done this before) calcium sulfate hemihydrate + Water + drying = Gypsum. Thanks for bringing this to my attention and I have updated the post to avoid confusion to others.
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