Having an accurate, reliable thermometer is crucial to having a good hatch. I recommend not using the bulb/red liquid thermometers that may be included with your incubator. The best place for those is the garbage. You can go to Wal-Mart and buy one for $10. Although this may be great for givng you an idea of the indoor temperature of your home, it is not designed to work in the 99.5 degree range for incubation.
If you ask me the IncuTherm Plus is the thermometer to use. This thermometer reads both temperature and humidity. The
temperature is rated to +/- 1 degree Fahrenheit in accuracy. A very helpful feature with the IncuTherm Plus is that it has a remote sensor, which allows you to place the senor where you need it in the incuabtor with the display/controls on the outside. When I used this thermometer I was using a HovaBator 1602N, the feature I found most useful was the min/max memory function. Before I went to bed I pushed the min/max button. When I woke up in the morning I was able to see how high the temperature got as well as how low it got. This gave me a good idea of what the average temperature was over several hours and I was able to adjust the manual thermostat accordingly.
Another great use for the IncuTherm Plus is for building your own cabinet incubator. By using multiple IncuTherm Plus Hatch Monitors you can see in real time what the temperature is in different parts of the container. This helps you to find those hotspots and configure your incuabtor accordingly.
You can't go wrong with the IncuTherm Plus. Learn More About the IncuTherm Plus.