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USDA says that you need to get a piece of meat above to 140 degrees with in 4 hours or you are at risk of breeding bacteria. between 40-140 is the danger zone.
My little smoker doesn't have a temp gauge on it but I learned something that works great (idea came from someone on here of course).
I used to just hang the probe from my digital thermometer through one of the grates but that was a horrible idea. Then I noticed someone had their probe stuck through some dark oblong shape, come to find out it was a potato and you can insert your thermometer through a potato (have seen oranges and wood used too) and just make sure the tip with the sensor in it sticks out far enough. Then your probe will stay stationary and if you need to move it you can just grab your tongs.
Oh, and I am talking about one of the digital oven thermometers with the cord you can just leave in the smoker and run the cord out the side.
Here is a picture where I circled what you would need to stick out of the end for a proper reading. Need a new one though, it no longer seems to take proper readings.