What pops said.
It's a necessity in my opinion, to have a thermometer that you trust. Take a digital thermometer and test it like pops said to do, once you know that this thermometer is accurate, it's the bible, that is, it's correct to the point that you will believe it over the digital read out. I always have a thermometer stuck through a potato at grate level (the level the meat is cooking on) so that I know what's going on in there, doesn't have to be fancy either, I bought an 8 dollar thermo from target, tested it, made sure it was good to go, stuck the probe through a raw potato, and then ran it down through the top vent of my MES 30 and set it on the grate next to the meat. Then you know exactly what the temp is and you don't have to worry.
Another thing to consider is that all cuts take a different amount of time, just because something should take 10 hours to cook, doesn't mean it will take 10 hours to cook...which is why it's so important to cook to temp instead of time.