Hi Tom,
To test and measure error in sensor values I've replaced sensors by resistors, and I've tested several values in different ranges (see below). I measure first the resistors with a DMM with 4 digits (so I can see a resistor of 7880 ohms for example, understanding it can be +- 1). Anyway, reading with Arduino I've found that I normally get readings with errors between 3 to 4 %. Now I need to compare errors when using different power supplies to see if there is any significant difference as I suspect. Are these errors of an expected magnitude ?
In my case this error can be even acceptable as I translate this value to water tension in cBar using experimental formulas (provided by sensor manufacturer:
http://www.irrometer.com/200ss.html) depending on the range of the resistor. I'm currently using 2 sensors for soil moisture measurement at different deeps and a temperature sensor to compensate readings.
Resistance < 550 Ohms: CB=0
Resistance < 1000 Ohms: CB=-20.00*((WM1_Resistance/1000.00)*(1.00+0.018*(TempC-24.00))-0.55)
Resistance > 1000 Ohms, but < 8000 Ohms: CB=(-3.213*(WM1_Resistance/1000.00)-4.093)/(1-0.009733*(WM1_Resistance/1000.00)-0.01205*(TempC))
Resistance > 8000 Ohms: CB=-2.246-5.239*(WM1_Resistance/1000.00)*(1+.018*(TempC-24.00))-.06756*(WM1_Resistance/1000.00)*(WM1_Resistance/1000.00)*((1.00+0.018*(TempC-24.00))*(1.00+0.018*(TempC-24.00)))
Idea is to automate the watering decision based on sensor readings... so precision shouldn't be perfect at all.
I'll upload a picture of the circuit prototype as soon I arrive at home. On thing I'm not comfortable is because the high number of wires required for a protoboard that has sometimes weak contacts...
Regards,
Joan