The thermometers built into the grill which is on the mounted to the top of the grill reads 100°F higher than a thermometer inside of the grill.
One or both of these are true:
- The temperature is actually different at those two locations.
- One or both thermometers are wrong (broken, miscalibrated, etc.).
It’s the first one. I can tell you for sure that there is a huge temperature gradient inside the grill from the distance from the cooking grates to the inside of the lid. The lid thermometer is somewhere between 10-14" away from the grates, in most models.
That’s also why the upper rack is a “warming” rack and not a “cooking” rack (despite some manufacturers - coughweber - trying to include it in the cooking area total).
And the lid thermometer is usually on the lid. Where there’s cold air on the other side.
Most of the built-in thermostats on barbecue grills are garbage. Before speculating on what might be behind the temperature difference, get a thermometer that is accurate with two probes and measure again.
Heat rises, cheapo thermometers aren’t super accurate.
If you can get access to the probe itself, try the two easy extremes… boiling water and ice water. See how far off they are. They will be off, it just depends by how much.
Of course the important metric when it comes to cooking food isn’t the air temp, but the temp inside the food, which is why an internal thermometer is what most people go by.
Thermometers, like most measurement devices, are always accurate until you get two of them. Each device has a specific tolerance (or should, otherwise it’s probably a horrible tolerance), for a grill thermometer this will look like -/+5C/10F. Additionally, everything used to read a measurement needs to be calibrated regularly to ensure proper function, otherwise readings cannot be trusted. For a thermometer, the easily accessible way to calibrate are to use ice water (does it read 0C/32F) and boiling water (does it read 100C/212F). Using these constants will allow you to adjust your thermometer and get a (more) accurate reading.
Could be calibration or hot air rising? Where are you putting the other thermometer in the grill? What happens if you stick them in boiling water?
