I know this old but where do you place the sensor and how big are your pot? Do you ever calibrate but figuring out pot volume and how much water it can retain? If not and using coco then get a 1 gallon pot .75 US gallon. Fill it with very dry coco. Not complete dry but dry enough so the plant would wilt and die. Measure 3000ml (3 liters). Slowly pour it into the pot and collect the run off. You can dump again just to ensure it’s at 100% field capacity. Measure the run off and minus from the 3000ml. That will tell you how much water the coco is retaining. If you have 1500ml of run off then coco is holding 1500ml and has a 50% field capacity . Also weigh the pot before and after. 1ml weighs 1 gram. So it should also be 1500 grams more. Sensor need to be place about 2 inches high to get a measurement of the wettest part of the pot. Keep adjusting but never use the same place twice. Keep trying until you get a very close reading and calibrate from there.

The 1 gallon is just for figuring out field capacity and you can use any size pot you want to grow in. Every coco brand will have a didferant field capacity. Rockwool is standardize and normally 60% but check for each different size And manufacturer.