Differences between air and liquid temperature readings

There are a variety of factors that impact the ability of an appliance to cool and/or maintain a specific desired temperature setting. In general, an internal air temperature variance of 4-6 degrees can occur depending on how recently the compressor completed a cooling cycle, how frequently the door has been opened/closed and the ambient temperature of the room or location where the appliance is installed.  High levels of humidity in the room or location where the unit is operating will negatively impact the cooling performance of your appliance. If possible, try limiting the number of times the door is open and closed to minimize the amount of warm, humid air that can enter into the interior refrigeration cabinet.  

Proper Temperature Measurement

To properly measure liquid temperature, a temperature probe or other style of liquid thermometer is needed.  This is a far more accurate method of measuring performance than simply measuring air temperature.  A liquid thermometer placed in a cup of water will work if a liquid probe device is not available.  It is important to note that temperatures may vary from location to location within the interior cabinet, so taking several readings may provide the most accurate result.  Air temperature can increase quickly while liquid temperature will be much less impacted by short periods of air temperature fluctuation.
Test Data

To illustrate the temperature fluctuation that occurs within a refrigeration appliance please refer to the below test results from a unit set to 55 degrees over the course of 24 hours.  

** Note that the  3°F spike that occurs at approximately 4:30 PM was the result of the the unit's door being opened temporarily.
More Information
Refrigeration appliances should be installed in locations where the ambient temperatures are between 50°F and 95°F. Operation outside these temperatures may result in lowered efficiency or cause other adverse effects such as difficulty maintaining temperature or the inability to reach a desired temperature.