Everyone’s residential oven has hot or cold spots, not all convection ovens operate alike, everyone doesn’t pre-heat the oven for sufficient time to ‘heat saturate’ the surrounding oven walls, and I permanently installed a pizza stone in the bottom of my oven. Oven temperatures fluctuate, greater in gas than electric ovens, the inclusion of a pizza stone helps maintain the temperature. This is also why, I believe, that cast iron cookware is superior to glass in the oven, once the cast iron is up to temperature (typically being started on the stove top) there is much less fluctuation inside the pot’s cooking chamber. My smoker’s cooking chamber temperature varies widely from top to bottom and throughout the smoking cycle and I suspect that barbecue grills have numerous differences between brands and individual operators.
Those are some of the reasons why you need a good thermometer or two or three. Thermometers will help you achieve consistent results.
I am not sure why the FDA overstates many of the temperatures that you need to achieve for safe cooking. If I cooked a lamb roast to 145° F, no one here would eat it. I would be embarrassed to serve a duck breast at 165° F. I am still confused over the recommendation to cook fresh pork to 145° F, while you should cook fresh (raw) ham to 160° F. Raw oysters and steak tartare are obviously not acceptable; ceviche isn’t recognized.
I never cooked with a thermometer until I started making caramel, I read about the manual method for determining soft ball versus hard ball and quickly came to the conclusions that a thermometer was best for me. After ruining a few lamb roasts by seriously over cooking (thinking I could use some rule of thumb minutes-per-pound suggestion) I bought my first in-oven temperature probe. I rarely over cook lamb or roasts now and when I do it is more a matter of medium versus rare, not completely ruined. At least you can make a wicked good curry out of a medium lamb roast.
One thing that I really don’t do often enough is check the accuracy of my thermometers. Thermoworks, great product line, has publish a simple, downloadable guide for making an ice water bath and a cooler guide for determining the boiling point of water at your home, if you are really annal then they have an advanced calculator that lets you factor in your barometric pressure with local altitude to calculate a more precise boiling point.
If you are cooking chicken, turkey, racoon, bear or other local road kill a thermometer will help keep the holidays safe. The USDA’s Food Safety and Inspection Service (FSIS) has a concise summary of common bacteria that consumers need to know about. Frequently it is not the last meal that made you sick, it could be something you ate many days ago.