Collecting some data
In part I the temperature probe was configured to collect time-series data for the internal freezer temperature. The initial goal was to troubleshoot an elusive problem, and alert in case of over-temperature.
Here are some things which were learnt after the first 48h of data have been collected.
Normal operating ranges
The normal range of internal temperatures is colder than I intuitively expected. Of-course I knew that it had to be several degrees below zero degrees Celsius, but if I had to guess I’d say -5 to -10 would ok. Turns out that’s too hot for a freezer, it likes to stay below -15C (5F), with anywhere from -18C down to -25C being normal temperatures for my specific unit.
In the image below one can see the results of playing with different temperature setpoints in the front panel (markers 1, 2 and 3), as well as the quick temperature rise provoked by opening the door for a few instants (markers 4 and 5).
Also visible is the effect of the duty-cycle of the compressor, which stays on for a while (temperature drops) and then turns off for a while (temperature starts to rise). This is normal of models which don’t have an inverter compressor (one which can vary it’s speed to suit the instantaneous demands). On non-inverter models the temperature control mechanism on the unit (often just a simple thermostat) controls the amount of “cooling power” by changing the ration of the on cycle compared to the off cycle.
The head pump in action
A refrigerator is a form of heat pump: since it is not possible to create cold out of the blue (like you can with heat), the mechanism work by moving heat from the inside of the freezer into the environment outside. I had installed two sensors in this experiment, one inside the freezer, and the other outside, just above its metal body. Looking at the image below it is really easy to see the heat pump mechanism in action:
- Every time the compressor turns on, the internal temperature starts to go down
- Immediately, the external temperature starts to climb up
- Once the compressor is turned off, the cycle reverts itself
- Notice the range difference: for an excursion of approx 2C in internal temperature, the external temperature varies by just about 0.7C
- This is easily explained by a combination of the much bigger volume outside, as well as the poor isolation of the room the freezer is in against its adjacent rooms
Duty-cycle in detail
The saw-tooth-like chats above show the effect of the on-and-off working cycle of the compressor. Zooming-in a bit it is possible to measure the time in each phase, as seen below:
The image above shows in detail one particular moment. Changing the settings between “min” and “max”, the “ON” part of the cycle (compressor working, temperature going down) varied from anywhere between 20min to 50min (depends on how high the temperature was to begin with, how full/empty the freezer is, and if there are warm foodstuffs that still need to be frozen)
What did not vary perceptively though is the duration of the “OFF” phase: it was always around 21 minutes. This seems to suggest that this particular model (under normal circumstances) always shuts its compressor down for 21min, varying how long it keeps it ON in order to meet the demands of the moment.