Hi,
I've no direct experience of these sensors, but have previously looked at some of the "theory".
A fundamental requirement for any long term reliability (accuracy or even functionality) is for there to be no net (i.e. dc) current flow between the electrodes (to avoid any electrolytic action or corrosion). That might be done by occasionally reversing the current direction (using an "H-bridge" output circuit) or by driving the electrodes from a (capacitor-coupled) ac (sinewave) generator. I believe some high quality commercial moisture sensors use a Very High Frequency oscillator and measure the "dielectric constant" (i.e. capacitance) of the water (which itself is highly complicated). I found only one reference which quoted an operational frequency, of around 70 MHz.
One of the ebay photos does show "symmetrical" R-C components beside the "electrodes" connector, so maybe it makes a "legitimate" (ac) measurement, but since the PCB appears to have hardly any other passive components, I do wonder how it is making the measurement.
Cheers, Alan.
I've no direct experience of these sensors, but have previously looked at some of the "theory".
A fundamental requirement for any long term reliability (accuracy or even functionality) is for there to be no net (i.e. dc) current flow between the electrodes (to avoid any electrolytic action or corrosion). That might be done by occasionally reversing the current direction (using an "H-bridge" output circuit) or by driving the electrodes from a (capacitor-coupled) ac (sinewave) generator. I believe some high quality commercial moisture sensors use a Very High Frequency oscillator and measure the "dielectric constant" (i.e. capacitance) of the water (which itself is highly complicated). I found only one reference which quoted an operational frequency, of around 70 MHz.
One of the ebay photos does show "symmetrical" R-C components beside the "electrodes" connector, so maybe it makes a "legitimate" (ac) measurement, but since the PCB appears to have hardly any other passive components, I do wonder how it is making the measurement.
Cheers, Alan.