Having an accurate thermometer is a key component to successful incubation. One of the challenges with thermometers is that it is difficult and most of the time expensive to find an extremely accurate thermometer. This is why when you are considering which thermometer to use or purchase it is important to pay attention to the stated accuracy range.
The accuracy range is how close to the actual temperature the thermometer can measure. For example the IncubatorWarehouse.com IncuTherm™ series thermometers stated accuracy is plus or minus 1 degree F; this means if the thermometer is reading 99.5 degrees Fahrenheit the actual temperature could be up to 1 degree F higher or lower than what the thermometer is reading.
Many people try to compensate for the accuracy range by using multiple thermometers in the same incubator. This actually causes more confusion and doubt for most people on what the actual temperature is. This is because it is difficult to know which thermometer is reading closest to the real temperature. It is possible and common that they are operating within the stated accuracy range but in opposite directions. For example, one could be off by 1 degree F high and the other 1 degree F low which would cause them to always have different readings and make it impossible to know which one is correct.
So this raises the question, how do I calibrate my thermometer? The best answer to this question is experience. Basically nature doesn’t lie, meaning the way to determine how accurate your thermometer is, is by the results of your hatch. If your eggs hatch on time the thermometer is reading the true temperature. If they hatch up to a day early then you know the actual temperature was up to one half a degree higher than what the thermometer was reading. If they hatch up to a day late the actual temperature was up to one half a degree lower than what the thermometer is reading.
Most thermometers that are used in incubation do not provide a way to actually calibrate the reading of the thermometer. The best way to do this is to either make a mental note of the difference or to write on the thermometer how far it is off and which direction. Once you know how much and in which direction you thermometer is off you can adjust the temperature in your incubator to compensate. For example, if you had determined that the actual temperature was a half a degree higher than what your thermometer was reading you would adjust the temperature so that the thermometer is reading 99.0 degrees. This would mean that the actual temperature in the incubator is 99.5 degrees.
This might sound like a lot of trouble to go through but if you desire to have better hatch results it is worth it. Just make sure once you get a good thermometer and have it calibrated to not lend it to friends or relatives because you might not ever get it back.