The soil moisture sensor uses the soil conductivity detected in the ground as an analogy of the mount of moisture in the soil. Without calibration, the best that can be said about these readings is that an increase (or decrease) in the reading is indicative of whether the soil moisture is rising (or falling). The unit cannot calibrated in the factory for your soil, since there are a wide range of soil types and each type can carry a different amount of moisture.
If you wish to calibrate your sensor, it is recommended that you follow a recognised calibration process that will take readings when a soil sample has been heated to drive out all moisture and then the readings when the soil is saturated (water just stops running out of the soil). With those readings, you can set the calibration of your sensor in the weather station. A google search on "soil moisture calculation" should set you on the right track.
One of the main things that people ask is "How can I increase the range of this sensor?" The reason why this sensor has a limited range is that it does not have an inadequate antenna which is located in the vertical green section.
When it is realised that the antenna is not adequate for the range that they are requiring, some people have opened the sensor case and attached an antenna wire to the circuit board. There is however a risk that the device is not fully sealed after this and it can be damaged by the ingress of water.
This rather drastic intervention is generally not required to double or triple the sensor's operating distance. The problem can fixed by adding an extended antenna without having to open the moisture sensor.
Looking at the photo of a modified sensor to the right, all that is needed is a piece of wood, a metre or so of hook-up wire and some weatherproof tape.
The business end of the modification can be seen in the close-up of the sensor installation. As can be seen, the yellow wire is wrapped around the vertical part of soil moisture sensor. Not visible because of the tape, is that there are about six (6) turns. Make sure that the tape is weatherproof, otherwise it will ultimately come undone. Self-amalgamating tape will provide a permanent fitting if it is installed correctly. You will need to note the ident numbers on the sensors if you are using multiple units as this will make calibration easier.
To help ensure that the yellow wire sits vertically, a small hole is drilled through the piece of word at the bottom and the top. The yellow wire is fed through the bottom hole, up the back of the wooden stake and then back through the hole at the top and fastened so that the wire will not come loose. It is recommended that tape should be used to ensure that the wire is held in place.
Strictly speaking, the wire between the hole at the bottom to the sensor should have loop in the wire such that it is lower than where the wire exists the hole and where it reaches the sensor body, so that rainwater will drip off there rather than run down onto the sensor.
