This is the gold standard, and it’s almost too easy. Fill a glass with crushed ice, then add just enough cold water to cover it. Let it sit for two minutes until the mixture is slushy and stable—no floating ice cubes allowed.
Insert your thermometer probe into the center of the ice-water mix, making sure it doesn’t touch the glass. It should read exactly 32°F (0°C). If it doesn’t, you’re about to become a DIY calibration guru.
Fun fact: Pure water freezes at 32°F, but tap water impurities can shift it by a tenth of a degree. Use distilled water if you’re feeling particularly scientific—or just use tap and name your thermometer “Approximately Correct.”
The Boiling Point Test (For the Adventurous)
This one is trickier because altitude messes with your results. At sea level, water boils at 212°F (100°C). For every 500 feet above sea level, subtract 1°F from that number.
Bring a pot of distilled water to a rolling boil. Don’t use a shallow pan, or the probe might touch the bottom and give you a false reading. Insert the thermometer into the water—not the steam—and wait 30 seconds.
If you live in Denver (5,280 feet), your boiling point is about 202°F. Don’t panic if it’s off; just do the math. This method is perfect for impressing friends at dinner parties who think you’re a mad scientist.
