I am trying tomeasure the resistance of a Watermark moiture sensor, We mainly have to cinsider:
A special circuit is needed to measure the electrical resistance of the Watermark sensor. DC currents must not be allowed to flow through the wet part of the circuit, or else irreversible reactions will occur and spoil the readings. AC excitation avoids these problems, by reversing the polarity of the current many times per second, so that no net reaction takes place at either electrode. The circuit must also isolate the sensor electrodes from galvanic currents in the soil environment. Metal objects such as ground rods or pipes or tanks or other sensors that contact the soil can give rise to underground electrical currents in relation to the electrodes of the Watermark sensor, and those too can spoil the reading and degrade the sensor.
I found serveral way to do it. I am very interrested on this http://forum.arduino.cc/index.php?topic=22250.msg167293#msg167293 but I have pain to understand 1) how work the condensator 2) What should be the value of the condensators 3) How to calculate the of Re, keeping in mind that the resistive value of the Wtermark is from 500 Ohm (Wet) to 28kOhm (dry)
If someone has other work to explain me why we do not neet to inverse the polarity, would be greate!!
Secondely, there is this emply voltage divider: Voltage divider for Watermark If I connect A1, I can get the Output voltage value:
val = analogRead(A1); if(val<1) val=1; Vout = val*Vin/1023;
and then the watermark resistance;
Rwm = (R1*(Vin-Vout)/Vout) ;
But as it states above, the polarity need to be inversed, then, specially if the R1 is not egal to Rwm, Vout will not be the same. That right?
Then, keeping in mind that the Watermark resistance (Rwm) will NOT change wintin 10 second (time if the measure), why Vout is not identical while we inverse the polarity?
I think, R1 should be ega at the maximum value of Rwm. In thar case, Vout will be the same, whatever, the polarity, but one will be posituf, and one negatif, but the same.
I wunder, if some one has a experince with voltage divider, keeping in mind the we need to measure and know the WATERMARK resistance (Rwm).
To calculate the Rwm (watermark resistance), we need t make the measure when D1 High/D2 Low, and make the a new measure when D1 is Low/D2 is High, and make the average. We have now the Rwn. (Better would be to reapet is 10 time and make a new average)
How would suggest me to better us is with an Arduino?
Thank a lot