Plain and simply I don't trust digital or outdoor thermometers. Relying on a thermometer with a 3 or 4 degree difference in calibration can cause us to set out incubator temperatures to a level that is too high or too low. We can live with a cool incubation but a hot one will kill our developing eggs. Although I do have a very accurate scientific thermometer they do tend to be expensive with some being as much as the cost of an incubator itself. Here is a simple process that I've come up with to check the accuracy of both built in digital thermometers as well as aftermarket digital ones and even the cheapo mercury/alcohol ones that come in so many incubators and it is much cheaper to do. This can even be done in an incubator full of eggs (if there is an empty spot) although the egg turner should be disengaged. Fill a small dixie cup with warm water and put it in the incubator. Warm water will make it so that it will come up to incubator temperature more quickly. The cup should be at egg level and will have a similar capacity as an egg although the capacity really doesn't matter. With a larger incubator such as a Top hatch you can use several cups to determine whether there are warm and cool zones in the incubator. After a while, the water will stabilize to the incubators temperature. As we are aware forced air incubators tend to have less of a temperature differential from top to bottom. You then use a regular medical thermometer to get a reading of the temperature of the water. Medical thermometers are designed to be accurate through the ranges that we are most interested in holding. If you have a thermometer (digital or glass) next to the cup of water and the reading is quite different than the reading of the water then there is a calibration problem. Remember a good glass medical thermometer is a great resource to have on hand. Also remember that companies buy as cheap of components as possible and that could very well be the thermometers. Hope this can help someone.