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:
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  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 
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 🤣)
: 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…
: contact me if you want to start a business with this and we may both become rich