I just bought a water meter for my indoor cactis and need to know where root level is and where I should keep it watered.
Cacti Potted Plants
Certified GKH Gardening Expert
I know this isn't the answer that you want to hear, but you will throw that meter out in frustration. It will not read accurately unless the soil is moist. Usually too moist for cacti to remain for very long. Do you know the type of cactus? Or can you upload a photo? This can help me to determine the water requirements for the specific cactus.
Usually, cacti like to be on the dry side. They like a well draining cactus mix, and only like to be watered when almost fully dry.
My rule of thumb is that if you can stick your finger down and feel moisture anywhere in the container as far as you can feel, then it does not need water. This will give you the most accurate knowledge of how much moisture there is in the container. If you can feel any at all, then it is fine as it is for another day or so.