Many of them are not very accurate at all. They might have been way back when. But now they are cheap pieces of plastic and sometimes the dials won't even move.
That being said, most newbies will stick them on the sides of the tank or at the very top of the tank. Which makes them even less accurate. There really is no good place to put those things at the snakes level unless you want them to be laying right on the substrate. Then they are kinda hard to read.