I check the temperature of my Hamilton Beach roaster oven with a thermometer And there was a difference of 45 degrees. I borrowed a roaster oven of a different brand it had a difference of 40 degrees. I set the dials to 225 degrees on both of them, added 2 and a half inches of water and measured the temperature with 2 different thermometers. Both thermometers read 175 in one oven and 170 in the other. I checked the temperature after 2 and half hours and immediately upon removing the lids. I did notice variance when the element would go on and off, the temperatures mentioned above are a rough estimate of the average temperatures. Can you tell me what I might be doing wrong? Thank you.
Taking a random stab here—and yes, this is a simplification of the physics involved—
If you measured the water temperature, this is expected, assuming you're talking about °F. Water will not get above boiling (212°F, at sea level) no matter how much heat you apply, unless you're using a pressure cooker. Every bit of heat you add once you get it to boiling just goes to boiling it off faster.
Now, as to why it isn't at 212°F, that's more complicated. The water is actually evaporating even below boiling; and this takes energy. (You've no doubt noticed a pot starts to steam before it boils, and that a lid on top will fog up too.) Energy is conserved, so that energy comes from somewhere—it turns out to lower the temperature of the water. How much water is evaporating depends on the humidity in the oven, which depends on the temperature of the air in the oven, how well-sealed it is,—i.e., is complicated. But at lower oven temperatures, it turns out that uncovered water will often be a fair bit below boiling.
The second possibility is that you are indeed measuring the air temperature, and you're seeing this, and there are a few things that could cause this: