SensorTile.box #FromTheCustomer: Fridge opening detection (Mode2 + Cloud)

[Eleon BORLINI]

Do you know how to monitor your fridge and detect whether a family member is crawling into the kitchen in the middle of the night and indulging in a bit of snacking? Follow this Share you activity post to get an insight on a practical use of the SensorTile.box!

[Eleon BORLINI]

ST.box in the fridge

[Andrea VITALI]

What are you monitoring? PS do not forget the rubber cap on the USB plug or the moisture may leak inside with bad consequences for the electronics inside

[Eleon BORLINI]

Hi Andrea, along with temperature and humidity for the fridge status monitoring, Kamilla measured also the variation of the magnetic field when the fridge door opens and closes. When someone opens it for about 3 minutes, and then closes again, the magnetic field in Z-direction changes from approximately 200mG to 400mG. Meanwhile, also humidity and temperature readings changed correspondingly. She explains it (correctly, I believe) by the fact that the cool air inside the fridge rushes out towards the door when you open the door. The correspondent graphs are shown in her (interesting) article. Regards

[Andrea VITALI]

​How can humidity change if the box is rated IP67? The only hole is connecting the external environment with the microphone not with the environmental sensors... there is no dedicated gasket for the humidity sensor (design guidelines for HTS221 not only highlight the need to have good contact with the environment but also that air flow must be guaranteed, so one would need two holes not one for the humidity alone)

[Eleon BORLINI]

Right, but also supposing that the ST.box is fullyIP67, assume that the the tool is at a certain temperature, let's say 25°C, when outside the fridge, and the air inside the box at the same temperature. When placed inside the fridge, the ST.box cools (let's say down to 4°C), and the air inside it cools too. From physics, the humidity content of cold air is under reasonable assumption less than the content in the same amount of hot air. This is shown in the graph posted by Kamilla (1st decreasing ramp of the graph). When the fridge opens, the humidity increases because the environmental air temperature increases, and the water vapor quantity that can melt in the air is higher. This is the second ramp in the below graph. When the fridge door closes back, the humidity still drops and it keeps on dropping if the fridge remains closed (3rd ramp). Regards

[Andrea VITALI]

​thanks - please note that if the humidity inside the box drops, then there is condensation on the electronics which can have bad consequences (as said above) - measuring an unwanted side-effect does not look very clever...

[Eleon BORLINI]

You are right, it's important to close the of the ST.box inside its case in a dry environment, to avoid an eventual condensation. For a standard env humidity (50%) and a standard temperature drop (from 25°C to 5°C for a fridge) the condition should be safe, but of course the user must be careful especially in case he uses the ST.box in mode 3 and then he works far from the ambient temperature.

[Andrea VITALI]

​You should look at a psychrometric chart: if you want 100% RH at 5 degC then this is 30% RH at 25 degC (not 50% as you suggest). One should then stay below that, let's say 20%, just in case the temperature drops below 5 degC, to avoid condensation in the target temperature range.

[Eleon BORLINI]

I know, but the condensed water amounts to a small fraction of the water in vapor phase, so from a statistical pov it is difficult to damage the HW of the ST.box. It is of course a slightly unsafe condition that can be prevented by the user placing the ST.box inside the case in a dry environment (not so dry, the suggested rH level for an ideal environment in winter is 40% at 25°C). Regards