Over time, your thermometers will lose their accuracy. Even you have the best candy thermometer, the big change that it will be out by a few degrees and make up the difference between great candy and ruined candy.
Since accuracy is so important, you will have to test your thermometer periodically to prevents undercooking, burning or scorching. This is not the complicate, difficult task to find out how accurate it really is.
#1 How to check candy thermometer’s accuracy?
In order to check the accuracy of the thermometer, simply bring a pot of water to the boil. Put your candy thermometer into this pot of water. Ensure that it is boiling during the process and bubbles should be constant and vigorous. The boiling point for water is 212 F or 100 C and it will be the baseline for the accuracy of your thermometer.
Leave the thermometer in the boiling water for a few minutes to give it time to read the accurate temperature, make sure that the bulb of the thermometer is fully immersed in the water. Preventing the bulb from accidentally touching the bottom of the pot, which will give false readings.
After a few minutes, take the thermometer out and check the temperature, making sure that you are eye level with it and not looking from an angle. If the temperature showing is 212 F, your thermometer is accurate.
#2 Tips for using candy thermometer
If your thermometer may be off by a few degrees or more, you will need to add or subtract any difference when determining the end-point of the boil of your sugar slurry. For example, if it registers 210 F and you want to cook your syrup to the 235 F, then you have to subtract 2 F when cooking and cook until it reaches 233 F.
Perform the test regularly to ensure your thermometer is still accurate.
If you notice that your thermometer regularly showing the drastically different results after the test, it is better to purchase on the new one.
Make sure the quantity in the pot great enough to get an accurate reading off the thermometer, the bulb of the thermometer should be fully immersed. Insufficient quantities usually result in over-cooked syrups.