The Weather Fire Dwarfs or: Some thoughts on sensors – Part 2

Picture by Alexas_Fotos via pixabay CC0

A common misconception is that sensors know the room temperature and relative humidity. They don’t. There is no weather fire dwarf who walks to the artifacts and says “Well, here we got 51% relative humidity”. A sensor measures what the temperature and humidity is right where it is. If it sits 2 meters above the ground, it may measure a beautiful 51% and 21 °C (70 °F) while your artifacts are overheating at 28 °C (82 °F) and dry out at 34% behind the glass in your display case with a badly done light installation. You might panic glancing at the 32 °C (90 °F) in your gallery while in fact it’s just that direct sunlight falling on your sensor. Or, for that matter, it’s placed over one of those heaters and gets roasted.

One of the difficulties you face when you decide on where to put a datalogger is that only in very rare cases there is something like a “room climate”. In most rooms, there are several climates. There is that outside wall that is always a little colder than the rest of the room. There is that wall that has bright sunlight from 11 a.m. to 2 p.m. in winter and 9 a.m. to 4 p.m. in summer. There is the space at the entrance which always gets a flush of cold and damp air as soon as someone opens the door. Then, there are your display cases.

All things try to reach equilibrium, which means that they strive to reach a common room climate, but while doing so, they create different and sometimes problematic room climates. The warmer air will inevitably go to the colder areas, but when reaching there, the once nice 51 % at 21 °C (70 °F) might become 68% in that 16 °C (61 °F) corner of two outside walls. 1

To get a good idea of what happens where in your room or gallery, you might wish to place several sensors. Life and the museum sector being what it is, you probably don’t have money to do that. What you can do, however, is measure at different areas, moving your datalogger to get an idea what happens where. Then, for the permanent location of your datalogger, choose the place that captures the climate your artifacts experience best.

Next up, we want to take a look at the difference between cheap and pricey sensors, which is – besides other things like WiFi functionality – what determines the price of a datalogger.

Stay tuned for part 3 of this article.

  1. Fun fact: This is why you often end up with a damper cellar when you try to dry it out by opening the windows in summer, but have a good chance to succeed in winter.
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