Exploring Basement Humidity With A Raspberry Pi

Sometimes a hack isn’t about building something cool. Sometimes it’s more tactical, where the right stuff is cobbled together to gather the information needed to make decisions, or just to document some interesting phenomenon.

Take this impromptu but thorough exploration of basement humidity undertaken by [Matthias Wandel]. Like most people with finished basements in their homes, [Matthias] finds the humidity objectionable enough to warrant removal. But he’s not one to just throw a dehumidifier down there and forget about it. Seeking data on how well the appliance works, [Matthias] wired a DHT22 temperature/humidity sensor to a spare Raspberry Pi to monitor room conditions, and plugged the dehumidifier into a Kill-A-Watt with a Pi camera trained on the display to capture data on electrical usage.

His results were interesting. The appliance does drop the room’s humidity while raising its temperature, a not unexpected result given the way dehumidifiers work. But there was a curious cyclical spike in humidity, corresponding to the appliance’s regular defrost cycle driving moisture back into the room. And when the dehumidifier was turned off, room humidity gradually increased, suggesting an unknown source of water. The likely culprit: moisture seeping up through the concrete slab, or at least it appeared so after a few more experiments. [Matthias] also compared three different dehumidifiers to find the best one. The video below has all the details.

We always appreciate [Matthias]’ meticulous approach to problems like these, and his field expedient instrumentation. He seems to like his creature comforts, too – remember the target-tracking space heater from a few months back?

16 thoughts on “Exploring Basement Humidity With A Raspberry Pi

  1. A lot of people do not realize what water does in the presence of air. Water is considered a contaminant in any industrial process. Knowing the dew-point of a gas is very important. From natural gas delivered to your home. Or nitrogen used in paint mfg process. It’s all important.
    What a large part of the masses don’t understand is moisture does not respond to pressure. It goes against intuition and does the opposite. Let’s say a pipe under 500PSI has a leak. Not out of the question. The dew point inside the pipe is lower than the point outside the pipe. 99% would think not a problem the pressure inside the pipe would keep the water vapor out. The 1% know the answer. It goes into the 500PSI stream.
    Water vapor is very tiny in size. Remember H-2-0 molecules in size. So to answer his problem of water getting back into the basement. It can’t be stopped. It will get in. You can only manage how damp it will get. It will never get dry.

    1. I second this! I remember running a gas chromatograph and mass spec years ago, we had an increasing problem with water contamination after a carrier gas cylinder change. Turns out that water vapour was entering a 2000psi gas cylinder via a slightly leaky safety burst disc. Never would have believed had I not seen it…

    2. You mean to say a pipe with a dry gaseous medium at 3.5 MPa overpressure springing a leak will cause the gas inside the line to get wet? how so? would the water vapour from outside diffuse faster than the gas is escaping (~speed of sound)?

      1. Please don’t be confused. Water vapor pressure and atmospheric pressure are two different things. Both want to equalize. High pressure wants to go to lower. High water content wants to go to lower content.
        Best home experiment to see is. Take a hot shower all the water vapor rushes out of the shower when the door opens. Even when no air rushes in.
        Still confused, “Not many people understand water vapor pressure.” Google it. There might be a good wiki.

          1. Imagine opening a sliding door. Given that the air pressure is the same on either side, why would there be a flow of air?

            Of course, the problem with this experiment is that usually the air in the shower is heated, and so you can imagine that the flow is due to convection.

  2. Have run compresor based dehumidifiers and wheel based ones. The latter certainly put out a great deal more heat, which is helpful by product (the wheel is heated to regenerate it) for using then in temperatures below circa 18C where the compressor ones really dont work very well at all.

    Which is beign used in the article?

    1. all the dehumidifiers Matthias used look like compressor based units. most domestic portable dehumidifiers in north america are compressor based afaik…

  3. Good approach on tracking your home and understanding what is happening. But he is missing a calculation – if both temperature and relative humidity change, then the conclusions could be wrong

    Suggestion: update the graph to show absolute water content in the air… Not the relative numbers

  4. Never mind that…. I can see plenty of uses for that idea in tracking how humidity in a city apartment can make a mess of things while the people who live there are away and do not have their AC running during a typical summer time period when away on their rarely used vacation. In fact…. I might even do that.

  5. Unless you are renting and desperate for more livable/usable space in an unsealed basement, don’t waste your time with dehumidifiers. It’ll likely fail. Otherwise, seal all cracks and entry/exit-ways below the foundation line except for one seal-able low-point emergency drain (there should already be one), then apply a coat of waterproof masonry cement to the inside surfaces of the basement. Also if it isn’t done already, make sure the flow of your outside downspouts is directed away from the foundation and dispersed in a simple gravel bed. If you are going to heat the basement in winter and there are windows you want to keep, make sure they are insulated double-pane so you don’t get condensation on the inside. If that’s too expensive and the windows aren’t important, board them up with some added insulation.

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