I recommend a basal body thermometer if you are serious about fertility charting. Why? Because a basal thermometer will give you temperature readings to the one-tenth of a degree. This is very important when charting because every tenth of temperature has significant meaning.
Why not a regular or fever thermometer? Because most regular or fever thermometer will not give temperatures to the one-tenth of a degree. Most give readings in two-tenths of a degree. This would cause you to misinterpret your basal body temperature and may cause you to miscalculate when ovulation occurs.
Digital Basal Body Thermometers
These thermometers are very good. They run off of a small watch battery and are very accurate. They usually tell you your temperature within a minute or two. Most beep at several second intervals to let you know its working. The newer digital basal thermometers even have a little light to allow you to read the temperature if you take it before the sun rises.
The drawbacks of a digital basal thermometer are the battery. All batteries will go dead eventually and unfortunately you have no way of knowing just when. With the newer models, the battery will probably go dead faster with a light to drain them. When the battery starts to go dead you may not receive an accurate reading. Usually they will last for quite a while; I used mine for over a year before needing to replace the battery. One way of assuring accuracy and good battery life would be to replace the battery before it dies. You can replace it at several months to one year at avoid the battery dying during a morning reading.
Glass Basal Body Thermometers
Glass basal thermometers are always accurate. You never have to worry about the battery going dead. You can take your temperature first thing in the morning and lay the thermometer to the side to read later and be assured that your reading will still be there. Some say that glass thermometers are more accurate than digital thermometers. I think this is because of the chances of the battery going dead in the digital ones, although in my research both are equally accurate barring that the digital thermometer has a good battery. Glass thermometers are usually less expensive than digital thermometers and if not broken can last for years and years.
Glass basal thermometers take longer to receive an accurate reading than a digital thermometer. You have to be sure to leave the thermometer under your tongue, or armpit, or wherever you take your temperature for the amount of minutes the manufacturer states. Glass basal thermometers can be broken easily because they are glass. You will want to be sure to keep these up and out of the way of small children.