When choosing a candy thermometer, look for one that is clearly marked and easy to read. Choose a thermometer with a mercury bulb that's set low enough to measure the heat in the syrup, but won't touch the bottom of the pan. And look for a thermometer with a clip that attaches to the pan. Candy thermometers are available in department and hardware stores, kitchen supply stores, and some large supermarkets.
Before every use, check the accuracy of the thermometer. To do this, place the candy thermometer in a saucepan of boiling water for a few minutes and then read the temperature. If the thermometer registers above or below 212 degrees F, add or subtract the same number of degrees from the recipe temperature, and cook to that temperature.
For example, if the thermometer reads 210 degrees F, cook the candy 2 degrees lower than the recipe temperature. If the thermometer reads 214 degrees F, cook the candy 2 degrees higher than the recipes states. Checking the candy thermometer allows you to make adjustments for any inaccuracy and for weather-related factors.
After removing the candy thermometer from a hot candy mixture, immerse it in hot water. This will protect the thermometer from a sudden temperature change and will make it easier to clean later.