I've got 8 ds18b20 (knock-off I assume) temperature probes monitoring the temperature in a freezer. I'm outputting the value of those probes each minute to an RRDTool graph using a Raspberry Pi.
However, occasionally I get some very strange readings and was wondering if anyone here would be able to help me figure out why.
Here's a graph for one of the eight probes (they're all pretty much the same):
AS you can see there are 8, somewhat evenly spaced spikes in the graph.
The interesting thing is that I have 2 other probes also connected to the same 1-wire network that are just outside the freezer. They do not have any spikes at all:
The probes are hanging in free-air inside the freezer, not touching any surface.
Initially I thought that maybe there was some sort of electrical interference, maybe caused by the freezer's compressor. And this may still be the case, I'm not sure, but the fact that the 2 ambient temperature probes don't show any affect indicates to me that it's probably not interference. The external probes are pretty close to the freezer as well.
Also there's the fact that immediately after the spike, the temperature doesn't return to the value that it had before the spike, but seems to be cooling down and slowly leveling out as it reaches the freezer's set temperature.
I'd like to rule out electrical interference before continuing my investigation. If anyone knows whether this is possible, I'd really like to hear from you.
Some more information that may help:
It's a chest-freezer, It's not in optimal state as there's a small "leak"(?) there's water around it's base, I use old cardboard to absorb the water and it usually evaporates from the cardboard quite quickly.
So it looks like it's a daily thing. Here's some more graphs for the same sensor:
I'm guessing that this is just a 'feature' of the freezer, and even though it looks highly inefficient, I'll just have to live with it.