I guess this depends on your medium. The one I got had prongs which was rather thick (half pencil thickness) and I had difficulty inserting it deep enough.
I use medium to large bark on larger orchids. Though a little easier on seedling mix.
At the end I stopped using it, the amount of medium/root disturbance didn't seem right.
I am not sure if this is the correct way but I use layer of very large coir chunks in the bottom of the pot and use this as a guide as to if the bark is dry by just touching the bottom of my clay pot drainage hole.
Also I also guess by weight by lifting the pot to see if I can tell how wet it is by weight. After six months I am getting better at it. To check I just feel the drainage hole with my finger to feel how wet the bottom of the pot is
I've never used one, but I don't think they work very well. I have a client who has one in a potted ficus and it is just totally inaccurate.
I believe they actually work by measuring an electrical charge between the salts in the soil, not the water itself. I read that if you stick one in a glass of water, it will say dry!