Initial Hypothesis: Differences in surrounding environmental conditions and the opening of its door cause fluctuations in the interior air temperature of a refrigerator.
Initial Predictions: The interior air temperature will rise by about 2º F through the day in response to rising indoor air temperature. Opening the door will cause an almost instantaneous rise in interior air temperature due to an influx of warm outside air. After opening, the motor and compressor will kick-in and drive down the temperature, powering down when a floor is reached. Interior temperature will then slightly rebound to a steady running temperature.
Reflections After Class Discussion: The biggest gap in my initial understanding was that the compressor will not maintain a steady temperate, but instead will drive down the temperature to a low, after which the temperature will rise due to heat gain until a high is reached and the compressor will again run. Also, the spikes in temperature from opening the door shouldn’t be as long-lived as I predicted, and likely almost unrecordable by the HOBO.
Revised Hypothesis: The cycling of the compressor will cause temperatures to fluctuate between a set high and low throughout the course of the day. The heat of the day will cause more rapid heat gain, and thus a slightly quicker cycling of the compressor.
Revised Predictions: Temperature will fluctuate between 40º F and 45º F throughout the day. A complete cycle will take about 2 hours, but may be as quick as 1 hour in the late afternoon due to faster heat gain. The compressor will drive down temperature more rapidly than heat is gained back, resulting in a zigzag with different slopes for temperature increase and decrease. The only interruption to this cycle will be small irregularities due to the opening of the door.
Methods: A HOBO logger was placed in the center of the refrigerator away from other objects from 11:30PM on August 28th through 10:30AM on August 30th, taking temperature measurements every 5 minutes. My wife and I opened the door three times during the 24 hours: at about 12:30AM on the 28th, at about 9AM on the 29th, and at about 11PM on the 29th.
Observations: Temperature fluctuated pretty steadily between about 36º F and 38º F throughout the 24 hours. Cycles were of consistent length throughout the day, taking about 50 minutes. Heat gain and cooling occurred at about equal rates. Small temperature spikes are visible at about 9AM and 11PM on the 29th. Large temperature rises, to 43º F and 40º F respectively occurred at about 1AM and 2PM on the 29th.
Conclusions: Compressor cycling is the major driver of interior air temperature in a refrigerator. Cycles were faster than I expected, and smaller in amplitude – only 2º F in range. Opening the door did appear to cause small spikes in temperature visible in the graph. Cooling and warming appeared to occur at about the same rate, and the rate of warming was almost constant through the day. The fact that our kitchen is on the east -side of our building, has only a small window, and keeps a steady temperature may explain the lack of variation in heat gain through the day. The fact that our refrigerator is pretty empty may explain why heat gain is as rapid as compressor cooling – I’m curious to see if heat gain is substantially slower in a fully loaded refrigerator. Finally, I’d like to figure out the cause of the larger fluctuations in temperature that occurred at 1AM and 2PM on the 29th.