Relative humidity (and pianos)

Del Vento

For the series "better earlier than later", let me start this thread and be as brief as possible (which still will be long, sorry 🤣) rather than exhaustive. Let's start with some background.

Both acoustic and digital pianos have parts made of wood. All acoustic and several upper-level digital / hybrid have critical parts made of wood: action, soundboard, keys. Each instrument is different (e.g. acoustic pianos made by Kawai in the last couple of decades have non-wood-based carbon-fiber action, whereas the lowest tier of digital may not have any any wood at all).

Wood used to be a tree, and a tree absorbed water to survive. Hence it's no surprise that wood contains water. What might be surprising if you have not played with wood in the water is that wood can release and adsorb water.

The air contains water too, and it exchanges it with all the porous surface and materials which are immersed into the air itself (which is everything, unless you put stuff in a vacuum chamber 🤣). Water content in the air is complicated and depends (among other things) on temperature. When the temperature is high(er), the air may contain large amount of water (and I could digress on climate change but I won't for now). When the temperature is low(er), the air may contain smaller amount of water.

If you have to remember just one thing of the previous paragraph, it is that air may contain only up to a maximum amount of water per unit of volume and that value changes with temperature (and pressure, but the variation in pressure on Earth's surface is small, even comparing sea level with the commonly inhabited mountaintops).

The way in which the air exchanges water with other materials is not based only on the water content of the air, but on other factors too, most notably temperature. To "hide" this fact, the "relative humidity" (RH) is utilized. The 0% value correspond to the obvious "absolutely no water whatsoever", which is impractical in areas where humans are, if nothing else because we breath water vapor out…. The 100% value correspond to the "maximum amount" mentioned above. A nice way to think about that is that if you try to add more water into RH 100% air, it does not stay there but "rains down". You may have experience that in a hot sauna, in the bathroom (if you have teenage kids 🤣 like I do), in a rain forest, perhaps in hot springs.

So the 0% value stays the same for any temperature, but all the other numbers change. What do I mean? Say that you have two identical sealed containers of air at a given temperature and it is at RH 45% (ideal piano situation). Now you put one container in the refrigerator and the other in the oven. What happens? Water cannot escape, but temperature changes. Because of "the single thing to remember" above, the heated air may contain MORE water, and the cooled air may contain LESS. So while the amount of water that is in the bottle has NOT changed, the RH has. The hot air has become "too dry" and the cold one "too wet". If you have to remember one thing only about this first message, it's this paragraph. If it's not clear: ask, since it's a moot point to continue without having understood this.

Now, go back in that moist bathroom to clean up after the mess your kids left. Often the walls and other surfaces of the area are colder the the air, even so slightly. As such, the air very close to the surfaces was colder too and (as I said before) could contain less water. So it is possible that water "rains down": not everywhere in the room, just on these surfaces. This is very common on Earth that we gave a name when it happens outside: dew fall. Inside, people in the USA call it "bottle sweating" (since it happens on cold beer bottles so commonly in front of people here). There is nothing about "sweating" or about "falling" in this phenomenon. This is simply water in the air which cannot stay in the gas anymore (because the RH cannot be higher than 100% remember?) and hence turns back to liquid water. It can happen on pianos too and it can have a very bad impact on wood, metal and electronics (most plastics deal fine with it). Ferrous metal may rust, copper-based metals may oxidize into verdigris (which has a anti-lubricating effect and make the action slower), electronics might short… Wood will absorb water and might rot if exposed for long time or… (see below for the short time)

On the other side of the spectrum, air in the house could be too dry, either when heated up in the winter, or when the A/C in the summer is running the dehumidification too aggressively (which is most of the time). In such conditions electronics and metal do fine. Wood instead "breaths out" its water content and become drier and drier. In the process, it shrinks (well, it loses some of the material it is made of, so shrinking must not be so surprising). The point is that drying wood does not shrink uniformly: it does so according to the grain. Hence, it can very easily warp. If the shrinking/warping is severe, it can be irreversible: parts lose their shape (especially problematic in wooden actions which have tight tolerances) and/or crack (especially visible on soundboards). If the change is small, it can be reversible: wood absorb backs its moisture and swells back on its original shape. Or it may swell to a larger ☹️ size, in which case it causes compression problems to itself. Very common in soundboards, and noticeable as growth rings too dark, alternating with normal ones and the occasional crack. Paints somewhat mitigate this problem, but only to a point (and action parts are never painted, as far as I know).

So how to make sure none of this happens to your piano? Well, before we can solve anything we need to see if we have a problem (if it ain't broke…) and if so what the problem is. To do so, we need hygrometers to measure the RH of the air nearby our beloved instruments. Do you think your cheap electronic hygrometer really tells you the RH humidity of your air? Think again!! 😡 Do you think your very expensive electronic hygrometer really tells you the RH humidity of your air? Think again too!!!! 🤣 Next chapter: hygrometers!

David B

I became aware of humidity and its impact on wood instruments when I purchased my first high-end guitar back in 2006. Guitars can get ruined pretty easily by the humidity extremes. Taylor builds their guitars in an environment at 75 degrees Fahrenheit/23 celsius and 47% RH. They feel like this is the optimal temperature/humidity not only for guitars, but for people as well.

It's easy to control the environment of a guitar. Simply keep it in its case and use something like this. I've been using them for years and they work great. Sometimes I like to leave my guitar out of the case when I'm playing it throughout the day. I like to know what temp/RH is in the room so I use something like this.

I can't keep my N1X in a case, but it's down in my basement which is partially underground. Temp/RH doesn't change much. It's stays around 65-75 degrees and 40-50% RH all year long. If I lived in dry or damp climates, I would use a humidifier/dehumidifier depending on the need.

God Bless,
David

dore_m

Luckily the climate here in silicon valley is really moderate, and I mostly worry about the wood expanding and contracting quickly as a result of being near a window or heater/ac. My wife doesn't like it, but I've covered the forced air vent above the piano so the changes happen more with the ambient changes in the room.

Tuning has remained relatively stable.

MHirsch

@Del Vento

Nice post!

I'm sure all those bad things could happen to a piano, if it's exposed to extreme dry air (<30%) or too humid air (>70%) for a long time. The only question is how much time is that long.

Our local indoor values are around 45% RH in the winter and around 60% in the summer, occasionally going down to 35% in the winter (couple of very cold days) or up to 70% in the summer (another couple of days). My local piano tech said it's OK and I don't have to do anything special.

arc

MHirsch My local piano tech said it's OK and I don't have to do anything special.

I would say that Physics would disagree with that statement ;-) Pianos are often placed in environments with a relatively narrow range of temperature variation: say 20-21C during the Winter and up to 25C during Summer. However, the absolute humidity range would vary quite significantly. For example, a change from 60% to 35% RH at 22C reduces the the water content to nearly half . This will have impact on the expansion and contraction of the wood. In locations with moderately cold winters it is not possible to keep RH above 35% inside a heated environment without an humidifier, even if the outside RH is > 80%.

The chart below shows absolute humidity (measured in g/m3) as a function of RH and Temperature.

Doubling or halving the water vapour content for periods lasting several months will certainly expand and contract the wood. The question is if such changes impact tuning and the dozens of mechanisms of the instrument that rely on very small adjustments.

Del Vento

arc The chart below shows absolute humidity (measured in g/m3) as a function of RH and Temperature.

arc Doubling or halving the water vapour content for periods lasting several months will certainly expand and contract the wood. The question is if such changes impact tuning and the dozens of mechanisms of the instrument that rely on very small adjustments.

The absolute humidity is not the important parameter here. RH is, that is the main point: air (and substances it contains) at the same RH level behave the same as much as water is concerned, regardless of absolute humidity level.

However @MHirsch said:

MHirsch occasionally going down to 35% in the winter (couple of very cold days) or up to 70% in the summer (another couple of days)

That is quite a swing and it will definitely have an impact if it were "long term", say 6 months at 35% and 6 months at 70% would certainly destroy the instrument in a few years. But a couple of days? How much of an impact that would have? How can we be sure? For sure the wood will not swell or restrict immediately when exposed to high or low RH, but how long is too long? Paint will certainly mitigate that but by how much?

Well, the answer is that we cannot be sure, however there is one proxy that we can use to assess it, and that is (surprise): tuning. Changing the length or tension of a string by even a small amount will change its pitch in a very audible way. The swelling or contracting wood in the soundboard does change the pressure in which it presses the strings (yes, the soundboard presses the strings, otherwise it would not act as a soundboard). That change in pressure will cause a change in pitch. Because of the characteristics of the instrument, it's almost impossible that the whole instrument will go up (or down) in pitch perfectly in a way that would be very hard to hear. What happens is that some strings go up (or down) more than other and you'll hear an out of tune instrument. So if this happens, it means the RH has changed the size of the soundboard and that is more bad the more the pitch has changed.

arc The fundamental question is what is actual contribution of humidity to the tuning

Hopefully the description above answer this part of the question. The regulation part, is more complicated to asses.

arc Are seasonal or longer-term humidity changes the major factor impacting the instrument? What happens to an instrument placed in an environment where temperature and humidity are fully controlled? We do know that the materials of a piano react to humidity. But we also know that these materials are subject to very significant forces. Is humidity really the driving factor?

I have not done lab research in this myself, but I know there are some people that have, and I have read some of their publications [1]. The answers to all these questions is YES.

In fact, it is often thought that the tuning of a piano changes because the tuning pins in the pinblock slip. That is not the case besides instruments in disrepair. A good pinblock is able to hold the tuning pins forever. The piano goes out of tune exactly by the process I described above: up when RH goes up and down when RH goes down. The catch is that when the RH goes back to where it was (assuming there is an annual cycle), the instrument does not go exactly where it was in pitch because the wood is not like an uniform piece of metal, so there are tiny variations which will cause a "detuned" status.

arc What happens to an instrument placed in an environment where temperature and humidity are fully controlled?

I don't recall where [1], but I read that if it is not played it stays perfectly in tune forever. If it is played, especially with intense repertoire, the hammering of the strings will cause detuning.

[1] I'll see if I can find the references for all these claims I am making, but I am sure I am remembering them correctly.

arc

The fundamental question is what is actual contribution of humidity to the tuning and regulation of a piano. Are seasonal or longer-term humidity changes the major factor impacting the instrument? What happens to an instrument placed in an environment where temperature and humidity are fully controlled? We do know that the materials of a piano react to humidity. But we also know that these materials are subject to very significant forces. Is humidity really the driving factor?

EB5AGV

arc I can tell you that a perfectly working piano (I played it on its previous location) which was kept on an recording studio with pretty constant temperature and RH (lower than at my home), when placed at my home, in just a couple months, needed some regulation work (escapement was uniformly unadjusted). I learned how to do it and, after that adjustment, it has been working fine for months, passing the end of the Winter, Spring, Summer and now beginning Autumn.

So, yes, RH has been a big factor on my piano. But it seems to be stabilized at its new conditions. I will be sure when a full year has passed, though.

Del Vento

arc Wood doesn’t care about RH but about how much water vapour is in the environment.

That's quite the opposite. No substance cares about the absolute value of water vapor (sorry, US spelling here 😆). Not even water itself. On the other hand, all other substances, and especially water itself cares about RH. The RH greatly affects the evaporation and condensation rates of water, as well as the hygroscopy (and the similar property deliquescence). Not a lot of explanation, but at least an authoritative source here

The wood exchange water with the air mostly with evaporation and hygroscopy, so (like pretty much everything else) it is greatly affected by RH and not at all by absolute humidity which is an useless concept for this discussion.

arc

Del Vento Yes, evaporation and hygroscopy depend on RH. But that is half of the story. Wood and other hygroscopic materials reach an equilibrium point with the environment through water evaporation and absorption (this is the so called “moisture content equilibrium”). But the equilibrium point is not linearly related with RH (it is also a function of AH). As temperatures decrease, the equilibrium is reached with higher RH values. In reality, at low temps the equilibrium is only reachable in humidity controlled environments since RH needs to be near saturation level. This basically means that materials can continue losing water content even when RH is high.

Del Vento

arc Yes, evaporation and hygroscopy depend on RH. But that is half of the story. Wood and other hygroscopic materials reach an equilibrium point with the environment through water evaporation and absorption (this is the so called “moisture content equilibrium”). But the equilibrium point is not linearly related with RH (it is also a function of AH)

You are absolutely correct, I wanted to keep things simple, but it looks like that's impossible on the internet 😆 -- In my opinion, if we take into account what you are talking about we will complicate this discussion so much that we would lose all our audience.

In fact, In the normal range of temperature and humidity found in homes, the equilibrium moisture content (EMC) for wood is linear in RH with good approximation, and does not depend much on temperature either. From wikipedia:

from https://en.wikipedia.org/wiki/Equilibrium_moisture_content [1]

Sure, if you go to the very dry or very wet, or outside of the room temperature range things change, but that is not what we are talking about here.

So, I repeat: while the following statement is not accurate enough for a lab making scientific or technical measurements of high precision, for the purpose of a the "health" of a piano in a inhabited home (i.e. at decent comfort levels for the humans) the water exchange of wood with air (i.e. the thing that makes the wood swell and shrink and possibly ruin the instrument) depends only on RH.

Please hold on tight for the hygrometers 😀

[1]: @CyberGene it looks like the insert-image-from-link (the one on the right) does not work, I had to download it and reupload it (with the one on the left): can you please check that?

Del Vento

Del Vento Please hold on tight for the hygrometers

Time has come. So before we could do anything to make our beloved pianos be safer, we need to know what is going on, i.e. measure what the RH of the room is (and if the location is not inhabited, measure the temperature too: by the way, the common "shiny black", aka EP, aka Ebony Polish, technically polyester coating of the pianos irremediably cracks at freezing temperatures… which arguably could be the most expensive damage of the situation, even if it's just cosmetic)

Anyway, there are three ways of measuring the RH of the air:

Thermodynamics
Mechanics
Electronics

The first method is the most accurate, but also the one that requires lots of work: wet-bulb thermometers (with the constant refill maintenance), dry-bulb thermometers, psychrometric charts… So in my opinion is not feasible for the average piano owner.

The second method is the most inaccurate and it bases itself on the change of mechanical properties of a material (such as wood to swell and shrink). One common kind I studied as a kid, and it was the inappropriately 😆 called in my native language "female hair hygrometer" (wait what? man can't grow hair, especially not long ones? 👨‍🦲) -- the actual name is hair tension hygrometer. There are many source of inaccuracy, but the one I want to highlight here is that the core of the sensor is a material which changes with humidity, such as paper or hair. Those changes can be irreversible, making the instrument report different values for same RH level, depending on what happened in earlier measurement cycles. Not good (and actually the reason why RH changes can be damaging to pianos too)

The third method is the one commonly used in modern hygrometers, which are often electronic. Without entering in the details of what happens to the sensor itself, let me mention from a "bird view" how they operate: the presence of water in the air changes "something" about the electrical properties of the sensor (remember your mother's advice: don't touch a power switch or a power outlet with your hands wet) and the device measures that change. This has two problems: first that when water content is very low, the sensor does not sense. In other words, these devices struggle to measure anything real RH below 20-30% (they might fictitiously report values lower, but it's unlikely to be accurate). Second, the very nature of the sensor is to be exposed as much as possible to the water in the atmosphere. Again, without entering into the details: electricity, water, metals… it may remember you of chemical reactions such as oxidation, rusting and other related things (electrolysis). Just hand waving here, but we are not designing a hygrometer, so I think this is sufficient to convince you that the electronic devices too have some kind of "irreversibility" like the mechanical ones, in that they could show two different RH values on two different days when the actual value is the same -- depending on what happened in between. And obviously the vice versa: show same RH value on two different days when the actual value is different -- depending on what happened in between. So how to deal with this? First with the experimental observation that such drifts are relatively slow, i.e. these changes take several months, even years to happen. So if we are somewhat sure of what we measured today, we can be equally sure with what we will measure tomorrow (the exception is if we dropped the hygrometer in the toilet, but that seems to be the fate of smartphones more than hygrometers 🤣). However we cannot be equally sure that what will be measuring next year is correct. What can we do? Calibration!

Nature has provided us with a nice process called deliquescence (not related to the criminal-delinquent AFAIK 😅) which consists on the following. A slush mixture of (certain) salts and water in an open container placed inside a larger, closed container [1] creates an environment of known and constant RH! Let me stress that again: if you place a (large enough) slush mixture of salt and water in a bowl, then place the bowl in a sealed room, the room eventually reaches and maintain a constant RH. There are three caveats:

the process is somewhat slow
the slush needs to have enough water to fill the room (if/when it is too dry)
the salts in the slush need to not be fully saturated, because they may need to "suck out" water from the room (if/when it is too humid)

Besides that, this is the perfect humidity control system. Why don't we use it for musical instruments? Well, actually we do, that's exactly the product @David B mentioned in the second post of this thread: Boveda for Music (it originally was developed for cancer-causing cigars, ban me 🤣). He uses that for his guitar, people use it for bowed strings, and I won't enter into the business for the clarinets which is more complicated. Now, there's a little more to that product, because I don't know about David, but my daughter would not be happy to have a slushy mixture into an open container inside the case of her beloved violin, especially not when riding her bike to school 🚴‍♀️

So can we use that product for pianos? After all we don't bring pianos around so the open container business is less of a concern. Yes we can, but as I said the process is somewhat slow and unless you want to place the piano inside a ziplock bag (see below why), it requires large quantities of salt/water. Which we could make in the form of a nice vase or something to be placed under the piano. So this is possible, but I have not explored it yet fully [2]

What can be easily done is placing a small cup full of the salty slush, take the electronic hygrometer and put both of them inside the smaller ziplock bag which can hold both. Seal the bag and wait a day or two. After that you can be sure that the air has reached the known RH level for that particular salt. And you know what your hygrometer reads instead, which can be close or far. Repeat with at least 2 different salts (I recommend 3 as a Goldilocks not-too-much-work, and not-too-inaccurate balance), and you're done. With cheap hygrometers you have to do mental (or paper 😄) interpolation. With expensive ones you have to find the instruction manuals which you have tossed in the trash which picked it up with Pianoteq on Monday (quoting @MacMacMac). Every year or two, repeat the process (you can use the same salty slush, which I save inside small jars -- closed when stored, open when doing the calibration). Done, now you know with reasonably accuracy your actual RH levels even if you have a cheap hygrometer.

Wait which salts to use you ask? Fair enough, that's what I use:

Pure sodium chloride (NaCl) aka kitchen salt, not the iodized kind, but probably not much different - to calibrate at high RH
Lithium chloride (LiCl) - to calibrate at low RH
Potassium carbonate (K2CO3) - to calibrate at optimal RH

Say that your hygrometer measures 40% when it's with LiCl and 60% when it's with K2CO3 and 80% when it's with NaCl. Well, you know the ideal environment is at the measured 60% (which will be close to actual 45% RH).

You can buy all these three products on amazon (and the first at the grocery store too 🤣)

[1]: don't you wonder how scientists decide to make such experiments? even Volta making a stack of alternating disks of two different metals with acid-soaked stuff in between them? by I digress…

[2]: contact me if you want to start a business with this and we may both become rich

MacMacMac

If I read your chart correctly, for any value of RH, the EMC varies very little over the range of temperatures in which a piano (and a pianist) would operate.

Del Vento

MacMacMac If I read your chart correctly, for any value of RH, the EMC varies very little over the range of temperatures in which a piano (and a pianist) would operate.

Well, it depends what "very little" means. In pure number, it could be a change of a few percents of EMC (say from 7% to 14%), however that is a doubling of the water content and it will have a dramatically change as far as the swelling/shrinking of the wood is concerned (which is our main issue)

MacMacMac

Very little ...
Look at the gap between the blue line (hot summer day, 86 F / 30 C) and the black line (water freezes 32 F / 0 C).
Nowhere between those curves does the EMC differ by more than a fraction of one percent.

And, given that we play the piano over a much narrower range than 0 C - 30 C, the point becomes moot.

Del Vento

MacMacMac And, given that we play the piano over a much narrower range than 0 C - 30 C, the point becomes moot.

Ahaha! Sorry I misunderstood your earlier post. That's is totally right, for a fixed RH level. In fact you noticed that I did not mention temperature as a concern!

On the other hand, RH does not stay constant on its own. Depending on geographic location and thermal systems of the home, the RH may change of 20% (or more!) between summer and winter, and that will cause water content of the wood to change which is the problem that this whole thread is about. Are we on the same page now?

arc

Del Vento the RH may change of 20% (or more!)

I am on definitely in the "more" region 😀 During the last year, the lowest indoor humidity recorded by the hygrometer was 31% (February) and highest was 77% (July). But the RH during the warmer months varies wildly. Indoor temperature is pretty stable at ~21C (~70F) in that room, getting to 24-26C (75-79F) during daytime for a few weeks during Summer. I have a humidifier in the piano room, so I manage to keep the minimum RH at ~45% for most of the time during the dry period.

MacMacMac

I was with you all along.

Then this EMC thing came into the picture, which was new to me.

But the chart showed that EMC doesn't matter in practical situations. It's all about RH.

Del Vento

Ok, so that was a lot about hygrometers. I hope I have not lost all of you in the process. I tried to be enternaining or funny to make it a easier read (my wife says I always fail at that, but since most of the readers of this site are in a different demographic…)

In any case, I left out a couple of things that I am going to include now:

List of many salts useful for this calibration and their properties
The above critically lacks Potassium Carbonate which is the most useful for our purposes

If you want to read someone else providing instructions on hygrometer calibration, here is a good one and here is another

Note that I provided Internet-Archive links to the original sites, since they might go away before our own site and some future reader will appreciate that. If you want to go to the original site, just edit the URL in the obvious way.

Del Vento

Last step is humidifiers and dehumidifiers. This is a tricky one, but I'll try to keep it as simple as possible.

Dehumidifiers are easy. You have two options: units that actually remove water from the air and units that just warm up the air (remember, the same absolute water content correspond to different relative humidity level depending on temperature). If your high RH happens in summer you need the former, if it happens in winter the latter could be a better choice (but the former category works too). The good news is that an A/C already works as the former, and all normal heating systems already work as the latter, so you might not need to do anything. The units that remove water from the air require more maintenance, so keep that in mind.

Humidifiers are difficult. You have water and you want to send it out in the air. A recipe for spilling, mold and algae growth. Some technologies (like ultrasonic) sound very nice and good, but they are crap: water is not evaporated, just "spit out" in the air and will quickly attach everywhere, bring at least scale in the form of a fine white dust everywhere (if it goes well -- if it goes bad it will bring also rot, mold and algae). Ask me how I know it…. (for non-US readers: because I suffered such problems).

So the best humidifiers are the ones which actually evaporate the water, with heat. The high heat are great (bringing water to a boil, hence avoiding mold and algae which die at boiling temperatures), but expensive to operate because of the high extent and latent heat of the water (it takes lots of energy to heat and bring water to boil). Scale is a problem at the heating element, and the constant high temperature ruin the inexpensive ones pretty quickly (ask me how I know it….) -- The low heat ones are much less expensive to operate [1] but have similar problems.

The most critical component of the humidifiers is water. Tap water has too much scale for long term operations, unless you like to constantly clean all the parts of the units, which is difficult and time consuming. Distilled water is expensive and lacks chlorine which makes the life of algae and mold much much easier. However the chlorine in the tap water is too low, because it needs to be safe for drinking, so it is not completely effective. The best solution I have found is to install a reverse osmosis filter for tap water. That is good to avoid Flint-like water issues (Google them if you are not aware) for your drinking water, so you can budget it in non-piano-related expenses 🤣. The reverse osmosis is so efficient that it produces basically distilled water, very cheaply. For drinking there is usually a last stage with adds some calcium, magnesium and other stuff, since drinking distilled water long term is no good for humans. I don't want the calcium and magnesium scale in my humidifiers, so I installed also a second tap before that last stage, so I can draw the distilled water when needed. I then add chemicals to the water to prevent mold and algae.

My humidifier and dehumidifier is the Dampp Chaser Piano life saver system, and the chemicals I use are the ones from the same brand. I think it is working much better than anything else I tried before. Before I was trying to manage the whole room rather than just the area next to the piano. It is essential to make sure its humidostat is calibrated, which I do as a two step: calibrate and hygrometer in the ziplock as described above and then place the calibrated hygrometer next to humidostat to see if it switches at the right point. You might place the piano in a ziplock large enough, if you so prefer 🤣

Another options seems to be http://musicsorbonline.com/humidity-control/piano-humidifier/ but I am not sure how effective and convenient it is.

[1]: technically the energy needed to evaporate the water is the same, however the low heat ones evaporate the water at lower temperature, so they save the energy needed for that temperature increase. Moreover, they do not provide all the needed energy for evaporation, but "suck" some of the energy from the environment

arc

Del Vento My humidifier and dehumidifier is the Dampp Chaser Piano life saver system, and the chemicals I use are the ones from the same brand. I think it is working much better than anything else I tried before. Before I was trying to manage the whole room rather than just the area next to the piano.

Not sure if the Dampp Chaser is humidifying the piano or just the underside of the soundboard. Most of the piano, including the action, is likely to be at room humidity. I remember someone (in PW?) that tested a Dampp Chaser system in a grand piano with hygrometers placed inside the piano and in the beams under the soundboard. The hygrometers inside the piano stayed at room humidity, and only the ones under the piano read a higher RH.

Not sure how accurate this test was. But the fact is that the Dampp Chaser holds ~2 litres of water and the manual says to refill it weekly. In contrast, I need to fill my humidifier with 10 litres of water every 2 days during the colder season to keep the whole room at 45-50% RH…