Over time, and particularly if users store them horizontally instead of vertically, thermometers may develop small air bubbles in their capillary tube. Until the bubbles are removed, such thermometers become useless because their readings are no longer accurate. The two easiest methods for repairing a thermometer are discussed in this article.
Repair by centripetal force
Grasp the thermometer in one hand and toward the end opposite the bulb.
Stand in an area away from obstructions and away from people or objects that might be injured or damaged by a projectile thermometer.
Swing the thermometer with the bulb facing down in a U-shaped pattern similar to the arc of a golf club. Repeat this process six to eight times, then inspect the thermometer. If the bubbles persist, repeat the process. If the bubbles persist after several attempts, proceed to the alternative method, "Repair by heating."
Repair by heating
Prepare a thermometer holder by drilling a hole through the centre of a paint stirrer slightly larger than the diameter of the thermometer. Place the paint stirrer across the top of a small sauce pan, then insert the thermometer through the hole and position it so that the bulb rests about 13 mm (0.5 inches) above the bottom of the pan. Hold the thermometer in place, and wrap electrical tape around the thermometer five or six rotations just above the hole. The idea is to create a "stop" with the electrical tape to prevent the thermometer from slipping through the hole and contacting the bottom of the pan.
Fill the pan with cooking oil just deep enough to cover the bulb of the thermometer, then place the pan on a stove and set to medium heat.
Put on safety glasses and carefully monitor the thermometer. As the temperature exceeds the maximum temperature reading, the alcohol will begin to fill the small overflow reservoir at the top of the thermometer. As soon as the air bubbles reach the reservoir, remove the thermometer from the oil. Do not allow the reservoir to fill more than one-half full. If the air bubbles have not reached the reservoir by the time the reservoir is one-half full, then remove the thermometer from the oil and consider it unrepairable.
The alcohol in alcohol thermometers is flammable. Never heat a thermometer in an open flame. Allowing the expansion chamber to become more than half full of alcohol creates an explosion hazard. The use safety glasses is strongly recommended.