When monitoring the temperature in your refrigerator, it is important to understand the difference between air temperature and liquid temperature. Liquid temperature fluctuates a lot slower, and therefore a lot less than air temperature. You may notice that the temperature in your refrigerator goes up and down but you can rest assured that the liquid in the refrigerator is much more stable.
A refrigerator thermostat is usually connected to a sensor inside the unit that monitors the air temperature and turns the compressor on and off accordingly.
When the air temperature rises a certain amount above the set point, the compressor turns on, and when the temperature drops to a certain point, the compressor turns off. The number of degrees above and below the set point is called the differential, usually represented as a +/- number.
The Cooling Cycle
Refrigerators with smaller temperature differentials run more often but for a shorter duration each time. Refrigerators with larger differentials run less often, but each cycle is longer. The liquid temperature usually settles in the middle of the air temperature range.
If you have a differential of +/- 5 degrees and your refrigerator is set to 55, the compressor will turn on at 60 degrees and off at 50 degrees, for an overall fluctuation of 10 degrees. Because liquids do not warm up or cool off as quickly as air, the temperature of the liquid in your refrigerator should only change by less than 1/2 of 1 degree during that fluctuation.
View the chart above to compare how ambient air, internal air and liquid temperatures varied in a wine refrigerator set to 55 degrees over the course of two days. The red line represents the ambient temperature outside the unit, the yellow line represents the air temperature inside the unit and the green line represents the liquid temperature in the bottles. As you can see, the liquid temperature barely changed despite the constant change of the conditions around it.