The easiest way to keep track of a refrigerator's internal temperature is through use of a refrigerator thermometer. These devices are designed to remain inside a freezer or refrigerator over time and provide readings of the ambient air temperature. The temperature of food or liquids can also be measured.
A refrigerator thermometer measures the temperature of the air, and that temperature can change rapidly when the refrigerator door opens. For this reason, the thermometer should be situated in the back of the refrigerator, preferably not on the top shelf, and should be checked only after the refrigerator has remained closed for at least an hour.
To measure temperatures with a contact thermometer, place a container of water as close to the center of the refrigerator as possible, and leave it there for at least 24 hours. When checked, its temperature should be somewhere between 34 and 42 degrees Fahrenheit. The freezer temperature can be checked by placing the thermometer between two packages of frozen food that have been inside for at least 24 hours. The reading should be somewhere between -5 and 8 degrees Fahrenheit.
If the freezer is at optimum temperature but the refrigerator is too warm, the vents that carry cold air between the two units may be blocked. Move packages of food to provide clear air flow, and the refrigerator's temperature should return to safe levels.