navindra why is it the relative humidity that's important as opposed to the absolute humidity?

Ooh good questionā€¦ I do think absolute humidity is important, as in, itā€™s not completely irrelevant.

But my understanding is that moisture moves around (for lack of a better term), and itā€™s RH that determines how it movesā€¦ if the RH is high, itā€™s easier for moisture to ā€œmove intoā€ the piano ā€¦ i.e., be absorbed by the wood and cause the wood in the piano to swell (and we all know why thatā€™s bad).

If the RH is low, itā€™s easier for moisture to ā€œmove out ofā€ the piano, i.e., for the wood to dry out, which causes the wood to shrink and in worse cases, crack.

When the RH is around 40-50%, thatā€™s when moisture moves to or from the wood the least.

Itā€™s different for other materials (say, fabric for example). So I think this is why itā€™s RH that matters, not just absolute humidity. Because as the temps go up and down, and the RH moves accordingly, it changes the ability of the moisture (thatā€™s already there, the absolute humidity) to move into and out of materials in the room.

Hereā€™s another interesting (and I believe relevant) tidbitā€¦ in the paperwork for my Venta humidifier (note, not a de-humidifier), it explains that when you first turn on the unit and start putting more moisture into the air, the first thing that happens is that soft materials in the room soak up that moisture ā€” curtains, plush furniture, carpeting. During this time, hygrometers may show little or no increase in RH. But once these items have soaked up enough moisture, then the additional moisture coming from the humidifier starts to stay in the air more and thatā€™s when youā€™ll start to see the RH go up on your hygrometer.

So thereā€™s probably a better, more technically accurate way to explain why itā€™s RH that matters, but hopefully this helps a little bitā€¦

      Oh, one more thing to add (sorry for posting so much, thereā€™s nothing I love more than a leisurely Saturday morning breakfast and posting on Piano Tellā€¦ šŸ˜…

      So, back to the question of why an AC causes a drop in RH.

      Normally ā€” say, outside in the open air ā€” if the temps drop, the RH increases and if the temps rise, the RH decreases. This is because cooler air can ā€˜holdā€™ less moisture, and hotter air can hold more moisture. Thatā€™s why many climates are more humid in the summer and less in the winter. (But not deserts, I donā€™t know anything about deserts and someone else can chime in on that matter).

      So you would think that AC, i.e., cooling the air, should cause the RH to rise.

      But the important point is that with an HVAC, the cooling is artificial ā€” first, warm air is blown across a cooling coil (with refrigerant) and then the air is moved around the house by artificial means ā€” the air handler, which is like a massive fan.

      Any kind of movement of air can contribute to a drying effect, hence the AC causing a (usually beneficial) drop in humidity even though the temp dropping would otherwise cause a rise in humidity.

      In a desert climate, this is different because itā€™s so much drier outside and I donā€™t know how the parameters are different. (Itā€™s also why I am super curious about whether @rogerch is in a desert or desert-like climate)

      With regard to heating, itā€™s already less humid, and on top of the air being moved around by the air handler (the fan) the air is also being heated. So even though an increase in temps outside lead to an increase in overall RH, indoors with the heat on, it leads to an overall decrease in RH because the air is being in both artificially warmed, and artificially moved around ā€” both of which result in a drop in RH.

      So basically, HVAC systems, whether in heating or cooling mode, usually result in a drop in humidity. Itā€™s just that in the summer, this is usually beneficial for pianos because the starting point is a higher RH. But in the winter, the starting point is already a lower RH, so usually the drop in RH caused by heating is bad for pianos.

      So, in conclusion (you all thought Iā€™d never finish!) absolute humidity is important and relevant, but itā€™s relative humidity that has the more direct impact in the piano.

        ShiroKuro I live near Sacramento, California.

        After reading your post I did some additional research and I now understand that air conditioning units are also de-humidifiers! Thanks for pointing that out! It makes sense because our air conditioning system produces water.

        In my house I observe higher relative humidity inside than outside in the summer, which I always attributed to the cooling of the air by the AC. Maybe cooling the air from our high summer temperatures down to our indoor temperature more than compensates from the dehumidifying action of the air conditioner. Or something else is going on. Whatever is going on it's great for the piano!

          @rogerch ahh, I have never even visited California, let alone lived there, but I'm guessing the summers are maybe much drier there? Interesting!!

            Well that makes sense. And so of course some of what i wrote then might not apply to your region.

            I love this topic.
            My easy version: if it's too dry the wood will crack. If it's too wet the strings will rust. šŸ˜†

              navindra One very basic thing I didn't understand ā€” why is it the relative humidity that's important as opposed to the absolute humidity?

              I'm far from an expert on this, but I think @ShiroKuro hit the nail on the head.

              ShiroKuro If the RH is low, itā€™s easier for moisture to ā€œmove out ofā€ the piano, i.e., for the wood to dry out, which causes the wood to shrink and in worse cases, crack.

              This right here. Very cold air holds more moisture, so if you have 15 "units" of moisture in the air, in cold air it would be only 10% RH because the air can absorb more, but in hot air it would be 80% RH, because relatively, the air is almost at saturation.

              This is important because the water moves in and out of the air, and in and out of things in contact with the air, depending on the moisture level between them - they are always seeking equilibrium.

              So seasoned wood for pianos is typically around 40-50% RH. If the ambient air is lower RH, the air can hold more, and thus wicks moisture out of the wood, drying it. If the ambient air is higher RH, the lower concentration of moisture is now in the wood, and the wood draws the excess moisture from the air into itself. This is all irrespective of the actual amount of moisture in the air, it's all about concentrations seeking equilibrium.

              That's why you can use a PLS with a heat bar to change the RH under the soundboard - if the RH in the air is low, the heater turns on, warms up the air, and that air can no longer hold as much water, so the relative humidity increases, and now the air is less likely to need to pull water out of the wood in the piano.

              At least, thinking about it that way makes it easier for me to understand.

                    Rubens My easy version: if it's too dry the wood will crack. If it's too wet the strings will rust.

                    And even before the strings rust, my understanding is because the wood shrinks when it's dry, and swells when it's wet, over time this will cause the wood to warp, it will cause joints to loosen, and it will cause glue to fail and lamination to separate. And yes, enough cycles and the wood will crack when dry. This shrink/swell "breathing" is the slow death that is the fate of every wooden acoustic piano.

                    Controlling the humidity minimizes this movement, and prolongs the steady-state life of the instrument!

                      Would the first sign of bad humidity management be the acoustic piano having a hard time maintain tune?

                        Rubens if it's too dry the wood will crack. If it's too wet the strings will rust.

                        Yep! this is the most important part, and why we all make ourselves crazy trying to get our indoor climates stable and piano-friendly!

                          I live in Montreal, Canada, where the indoors are dry in the winter and wet in the summers. In addition to humidifiers (during the winters) and dehumidifiers (during the summer), another thing that helps is to keep the indoor temperature on the cold side (less heating) in the winter and on the hot side (less AC) in the summer! I think this is the easy/stupid version of what @ShiroKuro said earlier, but it works for me. My grand piano has amazing tuning stability.

                            ShiroKuro But the important point is that with an HVAC, the cooling is artificial ā€” first, warm air is blown across a cooling coil (with refrigerant) and then the air is moved around the house by artificial means ā€” the air handler, which is like a massive fan.

                            This is such a good point. ACs, heat pumps, refrigerators and most of the refrigerant-coil dehumidifiers are all ultimately the same device, what differs is which direction it runs, and where the "waste heat" is placed. The AC runs ambient air over cold coils to cool the air down, and that immediately causes condensation on the surface of the coils (like filling a glass with ice and leaving it in a warm room). For an AC, that is byproduct waste water and there's usually a drip line to get rid of it. A dehumidifier works the same way, but the entire point is to draw water out of the air, and the "waste" product in this case is heat, because it's a heat pump and the heat has to go somewhere. In an AC, that waste heat is also produced, but it's re-directed outside using a big fan. For a dehumidifier, they just pump the warm, now dry air, out into the room.

                              rogerch

                              Thank you @rogerch! I might be off on this point, and different ACs may work in different ways. I think ShiroKuro explained it well--*although an AC, or air conditioner (yes, I'm thinking of the ones that cool down your house) cools down the air, meaning it can now hold less water and raising the RH, (this causes the RH to rise given the same amount of moisture in the room), it must furst it pass air over a cold coil, which causes condensation out of the air onto the coil (thus reducing the moisture in the air). So it could net out differently depending on efficiency, how long the AC runs, etc.

                              But if you have an AC that draws in outside air, or one that is oversized and cools down the room very quickly, you could end up in a situation where higher RH air ends up replacing the warm air in your house. I'm sure a number of other things are possible too, depending on the variety of AC/heat pump setups that exist.

                              In my house, whenever the AC is on, the RH drops precipitously, and my dehumidifer never turns on.

                              *edited to correct vapor capacity

                                iternabe

                                I think so. I don't have any direct experience with this, but it's been reported often that humidity swings cause tuning instability (and is one of the selling points of the Dampp-Chaser Piano Life Saver, which is a dedicated humidity control system). Presumably you get all kinds of flex/swell/shrinkage in the soundboard, bridge, pinblock and case/rim that cause this to happen.

                                  Gombessa This right here. Very cold air holds more moisture, so if you have 15 "units" of moisture in the air, in cold air it would be only 10% RH because the air can absorb more, but in hot air it would be 80% RH, because relatively, the air is almost at saturation.

                                  Hey Gombessa I think this is backwards. Warm air can hold more water than cool air. As temperature goes up, RH goes down if absolute humidity stays the same. At least thatā€™s my understanding and itā€™s consistent with my observations where I live: as the outside temperature changes throughout the day the outside RH changes in the opposite direction.

                                  Since you have studied humidity way more than I have I may have either misunderstood you or else Iā€™m just wrong. What I wrote seems consistent with https://themachine.science/relative-humidity-and-temperature/

                                      rogerch Hey Gombessa I think this is backwards. Warm air can hold more water than cool air. As temperature goes up, RH goes down if absolute humidity stays the same.

                                      Ugh, yes you're right, I'm all in the middle of confusing myself. Warm air can hold MORE moisture than cold air, so RH goes down because it has now more capacity relative to what it could hold at a cooler temp. I have to go back and edit a few posts/responses to correct the record here šŸ™‚

                                        Rubens I live in Montreal, Canada

                                        [OT]
                                        Oh nice! Montreal is a beautiful city. I lived there for a decade. It's amazing. Pierre Marchand is not too far away and Esmonde White as well... would love a report on Oliver's pianos with the Fandrich action one of these days.
                                        [/OT]

                                        However, after a decade in Montreal, I could easily survive "winters" elsewhere in a t-shirt and shorts... I don't miss those -40 deg winters at all and it's not like the summers don't get hot either. I would have thought it would be hard to do optimal piano climate control under those Montreal conditions, but it sounds like you're doing just fine based on the other thread!

                                        --
                                        Beach L168, Wish, baCh, WOW Petrof, Fake Steinway, VS
                                        TIME

                                            navindra
                                            Thanks! Yes, the only problem is that the missus is not too happy about me turning down the heat in winter and the AC in the summer. Very counterintuitive for normal people!

                                              Iā€™m glad Iā€™m not the only one who is willing to suffer with a cold house in the winter to protect the piano! šŸ˜„

                                              @iternabe another sign of high humidity might be sticking keys.