Comparing two DIY gypsum sensor with a commercial Watermark SS200 sensor. With the Vinduino-R board, connected to the Thingspeak web site, I collected measurement data -every 4 minutes- for 3 weeks. I used soil taken from my vineyard (decomposed granite).

For the Watermark sensor, there is a lot of calibration data available to convert measured resistance to soil water potential.
Below is the comparison result. The Watermark shows more sensitivity in wet soil range. Both sensors show a temperature dependency. For watermark sensors, temperature compensation equation are well documented.
In summary: both sensors types look very usable for irrigation control.

In the vineyard we saw the first grape berries changing color, "Veraison" in grape grower terms.
The vineyard is looking well, even under the current reduced irrigation conditions.