Very slow reaction time?


#1

Story: Even in a cold winter it’s recommended to ventilate rooms for a few minutes daily. Sometimes we open a window and forget about it, and then it gets very cold in a room.

One of my plans with buying a Ruuvi tag was to detect this and send an alert that the room is getting too cold. But it’s not working because of a seemingly very slow reaction time.

Example 1: Room temp 23°C, I put the Ruuvi tag in a fridge (this is safe, right?), the fridge’s own display shows 5°C. After about 15 minutes, the Ruuvi is still only down to 14°C. I think this is very very slow.

Example 2: Room temp 21°C, we have a digital thermometer in the room, and a Ruuvi tag stuck on the wall. We open the window. In 5-10 minutes, I can feel on my body that the air is very cold. The digital thermometer shows 13°C. The Ruuvi tag didn’t move at all, still showing 21°C. I wait 5-10 more minutes, still nothing, so I close the window. Then it takes well over 30 minutes to heat the room back to the original temperature, and all the time Ruuvi keeps showing 21°C. Given that the digital thermometer reacts almost instantly, the Ruuvi’s zero response is actually very concerning.

Is this behavior expected?


#2

Sample log when putting it into the fridge: https://pastebin.com/Gt3AC9Rz . So it’s about 0.9°C/minute.

As for the wall, probably the wall stayed warm and the Ruuvi was really measuring the temperature of the wall and not the ambient air. I still find it weird that the reading didn’t move at all. I tried placing Ruuvi away from the wall and will keep experimenting.


#3

This is a test where devices are put in a freezer and taken out:

The reaction times you’re experiencing are because the temperature sensor is located inside waterproof enclosure.

You can also explore a live demo here:

graphs.2kgwf.fi/dashboard/db/ruuvi


#4

This graph very closely matches my own calculation as well, -30 degrees in about 30 minutes and then it slows down even more. Is this because of the specific type of plastic used in the enclosure maybe?

BTW, I’m getting better results by not putting the Ruuvi tag on a wall.


#5

It’s partially due to the plastic having low thermal conductivity, but the bigger issue is the air inside the enclosure, which has a low thermal conductivity as well and since it’s a closed enclosure, the air inside changes really slowly and most of the temperature change is due to the heat transferring and not the air itself going around.

If you use the tags without the enclosure or the enclosure open, the change is near-instant (at this scale), considering that the air in the room does not change instantly, and some heat is being radiated from other parts of the tag and the surroundings. These are probably the reasons why you get better results by not purring the tags on a wall.


#6

I thought air can permeate the covering on the small hole. I guess opening this hole is safe when used inside. Do you know of materials that have better conductivity? Like 3d printed stuff?


#7

The air can, and will, but it’s slow :slight_smile: most 3D-printed stuff are plastic or similar materials, with pretty much the same thermal conductivity. To minimize the reaction time, you should use the tags without the enclosure and close to the window, ideally closer to the floor in a position where the air can change as freely as possible as the cold air will first fill the lower parts of the room