When we look at the soil moisture at a specific site over 12 months we see a range from completely dry to saturated but the volumetric water content readings dont range from 0% to 100%. This is because the amount of water a soil can hold depends on the soil type. 

soil temp graph

There are some excellent websites, such as the AgVic soil monitoring project that take the raw reading and then convert it to a percentage of soil water capacity at that specific location, ending up with a range of 0- 100 %.

Harrow soil moisture

For a rough guide of what values to expect from your soil moisture sensor, we can look at the chart below provided by AgVic.

Crop lower limit (%) Drained upper limit (%)
Coarse sand 2 6
Fine sand 4 10
Light sandy clay loam 10 23
Loam 13 27
Sandy clay loam 13 28
Clay loam 14 32
Clay 25 40
Self mulching clay 25 45

The time chart at the top of the page is from Vasey, Victoria with sandy clay loam. Apart from some outliers after heavy rain events, we can see the range is from 13% when dry to high 20's when very wet. This lines up well with the expected results from the AgVic chart.

In this case we can think of 28% volumetric reading as the "100% saturated" value for that specific location.

Hopefully this information can help in the understanding and interpretation of the data, which is the whole reason for measuring and monitoring any of the variables on your farm.