# Output Units of Soil Moisture Sensor

I have an Octopus Soil Moisture Sensor connected to a Rasberry Pi through an MCP3008 ADC chip. The sensor is working correctly, but I have no idea what the reading values mean.

I say its working correctly because when I place the sensor in known dry soil and take a reading, I get a 'dry' result. When placed in known wet soil, I get a 'wet' result. These delineations I was able to find in this datasheet.

However, that is the only datasheet I can find, and in the Specification section we are given this table:

My ultimate goal is to turn on either a valve or pump when the moisture content in the soil drops below a certain percentage of either weight or volume (SI units of either grams / gram or inches / inch, which are the units mostly used in the industry). I have found scholarly articles which contain easy to use equations for converting between different units of measurement, including going from a dielectric constant measurement to % Moisture Content.

I believe the output value is either somehow related to the drop in voltage observed or an indirect measurement of resistance, but I cannot be sure. Without some kind of units I cannot perform any calculations to obtain the result in the units I desire. According to this, correct values of the dielectric constant should not be above ~80 or so for water.

I tried emailing the company I purchased the sensor from but without a response after 2 weeks.

The datasheet I linked above does state this:

This sensor uses the two probes to pass current through the soil, and then it reads that resistance to get the moisture level

Can I take that as meaning that the output units the sensor is giving me is in Ohm meters? If so, I find it odd that the same document doesn't also list that as the units in the specifications. Does anyone here have any experience with this sensor, or know where I can find a datasheet that provides the units it is measuring in?

• I would suggest to bypass the intermediate units, to calibrate the sensor, to make calibration curves: output [A/D counts] vs. moisture content [g/g] or [other industry standard unit]. I can also join Majenko in criticizing the makers of the sensor for lack of metrology information. Here's another post on conductivity measurement which could provide additional outlook. – Nick Alexeev Dec 29 '14 at 22:57