How to calibrate Polder instant read thermometer

With thin cuts of meat (like chicken breast or a steak), you want to make sure you put the thermometer in to the thin side of the meat so that the whole thermometer goes in to the meat, rather than putting it in through the "top" so that only a little of the thermometer is in the meat. If you're putting it in the top, you can get wildly inaccurate temperatures. Also make sure that the thermometer isn't touching a pan, exposed to air, or touching a bone. Any of those can have a negative effect.

Also, you only need to cook chicken to 74C / 165F. That may be part of the problem in your case.

You may also want to check that your thermometer is accurate. You can stick it in boiling water to check that it reads 100C / 212F (assuming of course it goes up that high).

I use the same hole if what I am checking is large, like a loaf of bread, and I don't want to poke it full of holes...although with my instant read thermometer, the hole is not particularly large.

If the hole is a large proportion to the item (big hole in the side of a cookie...I know, absurd, but you get the idea) where you think heat can run down that tunnel you've made, then be concerned, but I can't imagine any situations where it is likely that the thermometer hole is going to let enough heat in to alter the cooking.

I CAN imagine a situation where a thermometer in place could help transmit heat to the center and make it, potentially, cook quicker. We used to put a large nail in the center of a potato that we were baking in the coals of a fire so that the steel would help transmit heat to the center of the potato to make sure it got done evenly, but I don't know how much of a difference even THAT made.