There's no such thing as an accurate stud finder for plaster walls.
Long version:
The backside (inside face) of a plaster wall has big lumps of plaster, oozing through the lathe; that's how the plaster stays stuck to the lathe. So there's a lot of variation in how thick the plaster is, and that confuses the heck out of a studfinder.
What are you trying to hang?