I always used a pinless meter that appeared to be very accurate, it may have been off a percent or so but any stave it said was dry, was dry when I started working it down. I left a new battery in it, these newer batteries leak and this one ate up the battery terminals and put my meter out of action, I am not working much osage now so I haven't had it fixed.
The way to never burn out a bulb in your drying box is to insulate it well, put more lights in it than you need, use 100W bulbs and put them on a dimmer switch, I have a string of 3 100W bulbs in my drying box. You can get 100 degrees in your box with the lights barely glowing, this way they never burn out. I changed the bulbs in my box from 60W to 100W about 15 years ago and never had a bulb burn out. I cranked up my dimmer switch one time and my box got up to 145 degrees if I remember right.