How Can You Tell If a Kitchen Thermometer Is Inaccurate?

A kitchen thermometer may be inaccurate if meat cooked under the same conditions turns out inconsistently, or if the thermometer fails to show the expected reading at a known reference point, such as boiling water.

How do you do the boiling-water test? At sea level, boiling water is 100°C / 212°F. Immerse the thermometer in boiling water and wait a few seconds. If the reading differs from this value by more than 2–3°C / about 4–5°F, the thermometer is out of calibration. Water boils at a slightly lower temperature at higher elevations, so a small temperature difference may be normal at higher elevations. The ice-water test works in a similar way: add plenty of ice to a container of water and wait a few minutes. The mixture should be around 0°C / 32°F. A significant difference from this reading may also indicate a calibration problem.

What other signs should you look for? On a digital thermometer, a display that responds slowly, stays fixed, or keeps jumping between readings may indicate a weak battery or a faulty sensor. If replacing the battery does not solve the problem, the thermometer itself may be defective. On an analog dial thermometer, the needle not resting exactly at zero or sticking under a cracked glass face is a common sign of failure. A mismatch between the thermometer reading and the cooking time can also be a clue. If meat cooked for the recipe's specified time and temperature has an internal temperature much lower or higher than expected, the thermometer is suspect and should be checked using the boiling-water test. Meat cooked with an inaccurate thermometer may appear to reach a “safe” temperature on the display while remaining undercooked inside. Do not rely on a questionable thermometer for critical measurements, especially for poultry and ground meat, until it has been checked with the boiling-water test or replaced. Color and texture do not show that meat is safely cooked and cannot replace a thermometer.

How do you check an infrared, noncontact thermometer? The boiling-water test cannot be applied directly to these devices because they measure surface temperature, not the temperature of a liquid. Check them using reference surfaces specified by the manufacturer or solid objects at a known temperature, such as ice cubes. Some probe thermometers have a small adjustment nut under the dial. If the boiling-water test shows a deviation, turn the nut with a wrench to reset the needle. Most digital thermometers do not have a physical adjustment; if they are inaccurate, replace the unit.

Checking a frequently used thermometer with the boiling-water test every few months can improve safety, particularly for critical uses such as cooking meat. Check it again after it has been dropped, struck, or left in the sun for a long time. Accuracy can vary between thermometer models: inexpensive models powered by a single battery may become inaccurate more often, while calibratable models used in professional kitchens may stay accurate longer. Testing a new thermometer in boiling water as soon as you take it out of the box can reveal a manufacturing defect or damage from shipping before you rely on it.