Candy thermometers are an absolute necessity for making many different types of candies, like caramels, toffees, and many fudges. However, it's not uncommon for thermometers to become less accurate over time, and a mis-reading of even 5 degrees can mean a failed batch of candy. Because accuracy is so important, it's a good idea to test your thermometer periodically. Here is a quick and easy method to test the accuracy of your candy thermometer.
1. Insert your candy thermometer into a pot of water, and bring the water to a rolling boil. The bubbles should be constant and vigorous. At sea level, the boiling point for water is 212 degrees Fahrenheit, or 100 degrees Celsius. This will be our baseline.
2. Leave your thermometer in the water for five minutes, to give it time to get an accurate reading. Make sure that the bulb of the thermometer is fully immersed in the water, and that it is not touching the bottom or sides of the pot—this can give a false reading.
1. Now inspect the temperature on your thermometer, making sure that you are eye level with the thermometer and not looking at it from an angle. Is it 212 F/ 100 C? If so, your thermometer is accurate!
2. There's a good chance, though, that your thermometer may be off by a few degrees, ormore. This is very important information! Take this temperature difference into account when doing all future cooking with the thermometer. For instance, maybe your thermometer registers 220 F when inserted in boiling water. You now know that your thermometer reads temperatures 8 degrees hotter than they actually are. So if you have a recipe that calls for a temperature of 240 F, you know you need to add 8 degrees and reach 248 F on your thermometer to get your candy hot enough. Or perhaps you live well above sea level, and your thermometer reads 209 degrees F in boiling water. It may be that your thermometer is entirely accurate, and that is just the boiling point of water at your high altitude. No matter the reason, you will still need to take this discrepancy into account and subtract 3 degrees F from every temperature in a candy recipe. Make a note of the inaccuracy so that you easily remember what your particular "candy thermometer conversion" actually is.
3. Perform this test on a regular basis, to ensure that your conversion is still accurate. It's easy to make this a part of your routine—slip the thermometer into some water that you're boiling for pasta, for instance—and it makes a huge difference in the outcome of your candy-making.
4. If you find that you are regularly getting drastically different results from your calibration, that means your thermometer is no longer reliable and it is time for a new candy thermometer.
5. For high altitude cooking, performing this test is even more important, since the temperature of boiling water decreases at higher altitudes, and if you don't take this change into account, all of your candy recipes will be terribly overcooked. There is also a handy rule of thumb that gives a general guideline to estimating candy conversions at altitude.