I have one that is able to be calibrated..but..I also have the walmart kind..I just use the walmart kind.  I am not bragging by no means, but it's just that I have hatched for so many years, that I can pretty much go by touching.  I know "how" I like my eggs to feel.  When I go to turn, I am thinking, this is just right.  Sometimes, oh, too warm, or oh, needs to be up a bit, it's too cool.  I know, some are going to say..what?   I sound like Goldilocks in the three bears.  
I do put the thermometers in the bator, kind of go with that once I feel how they are, and keep the temp where it is reading at the time..works pretty good.  Humidity.  I don't go real high with that anyway, so if it is reading 60 to 62, I know I'm close enough.     Just want to give all of those out there that are worrying, don't worry too much, you are probably doing better than you think.  Keep the thermostat as close as to what your bator suggests.  Have fun!
I'm just one that hasn't worried to much about a couple of degrees difference in the bator throughout the day.  I've had some great hatches, I've had some bad, the bad ones were usually shipped ones that took too long to get here.  I find that past three days, I could be in trouble having having a decent hatch.  Also, shipping in the cold..worse than shipping in the warm.  Cold temps on those eggs, then they sit in a P.O. ...  warm up, sweat/weep..not a good thing.   Could also be bad in real warm temps.  Hot outside, then sit in air condition.  Shipped eggs are a gamble for sure.  And it isn't just because of handling.  
  But oh yeah, that counts too.