I tested various soil moisture sensors with a DMM, but I cannot find the Volumetric Water Content (VWC) for the SEN0193 model. All I can find is this: Capacitive_Soil_Moisture_Sensor_SKU_SEN0193-DFRobot
(dry < 86.7), (86.7 < wet < 173.4) and (173.4 < very wet).
[86.75/1023] and [173.45/1023]
[0.424 V] and [0.848 V]
The data is only from 1.8 - 2.4 V. So that's another issue.
Aside, this other sensor just gives the equation for the VWC:
http://manuals.decagon.com/Retired%20and%20Discontinued/Manuals/EC-20-EC-10-EC-5-Soil-Moisture-Sensor-Operators-Manual-(discontinued).pdf#page=17