Any thermometer can be good or bad, depending on the thermometer. I have read a lot of people's posts that said to check any thermometer with a standard mercury thermometer, just to check for accuracy. I have a thermometer/hygrometer, that I got at walmart for around $6 that I haven't had any problems with. I think it's called accurite.
I use GQF dial (for wet bulb readings), Taylor digital thermometer/hygrometer and Accurite digital thermometer/hygrometer and all have worked fine for me.
I have found that the ones they send with the incubator are junk and usually off by 6 degrees. These should be calibrated or tossed in the nearest garbage can.
Keep in mind that even if you don't get the perfect thermometer, you can judge the accurate temp by the day your chicks hatch. If they hatch before day 21, your thermometer is reading too low (ex: showing you 100, when it's actually 101). If they hatch late, your thermometer is reading too high (ex: showing you 100, when it's actually 98.5). Once you figure out exactly how it is reading, you can judge your settings and get your hatches to occur consistently every time.
To check thermometer accuracy:
Eggseronious wrote: Get a quart of ice and crush it or use a blender make an ice water mix put the thermometer down in about 2 inches and sit for 4-5 minutes should read 32* degrees make sure its a thick slush of ice water.
Calabrating hydrometer-from Speckledhen.
1. You can calibrate a hydrometer by mixing in a bowl 1/2 cup of salt in 1/4 cup water. Actually a couple TBSP of salt moistened with water is enough.
2.Place bowl inside a zip lock baggie with your hydrometer. After at least 8 hours, your hygrometer should read 75%.
3. If it says, for example 80% you know that it is reading 5% to high and then you can make adjustments in your readings