"And so I want to know about the RH in the air in my apartment (not outside). I find a hygrometer useful. I take appropriate action based on that--which in my case is simple. Humidipak goes into the body of the guitar, or not. The RH where I live is very benign."
Well, yes and no. If you are fortunate enough to actually live in a constant environment which is neither too wet nor too damp, you're lucky. So are your guitars.
Most of us aren't that lucky. As I noted, this week in Dallas we went from the low teens to the low '70's in temperatures in the course of five days. Temperature drives R.H. (both inside and out) and your HVAC system is only responding after the fact to those changes. The R.H. went from 15% to 89% in that same time. Knowing a bit about that in advance would have helped any guitar player plan to deal with their guitars IMO. Is it so radical to say "be pro-active"? Understanding the woods in your guitar are all expanding and contracting at different rates would help also. Knowing your AC will also dry out your guitar should help.
Paying attention to your guitars is the most important thing you can do because all the rest is too variable in some (most?) climates. Maybe it would help if I said think of your guitar as bread dough. Any bakers out there? The R.H. in the environment affects the amount of liquid you will add to your dough to reach a desired consistency. Yes, it's the inside R.H. that you are working in, but it is the outside R.H. that may drive the moisture content of your dough if you live in a volatile climate.
When it's dry or when it has been dry outdoors, you will feel the dough one way. When it's damp inside or been damp outside for a while, you will feel that too. Your mothers and your grandmothers knew that and they didn't need a meter to tell them. They did so by being knowledgeable and experienced. Be your own grandmother.
The same applies to your guitars. Pay attention to your guitars, they will tell you what you need to know.
Now, I do not advocate just setting the temperature in your oven by feel. No. Meters have their purpose, as long as they are measuring the correct value.
"Certain woods and certain cuts of wood are much more susceptible to cracking than others, it's impossible to predict in any accurate way how the particular woods are going to react to humidity changes."
I agree with that statement and note your guitar is made from several varieties of woods. Pay attention to your guitars, a meter does not know what they feel like or how they sound. Any meter, IMO, is no more than a brief one page outline to Moby Dick.
Remember, folks, the desired R.H. range for your instruments is spread across a 20% range and your guitar is not going to fall apart after a few hours - or even a few days - outside that range. Fortunately, that 20% range exists where most of us are comfortable when our homes are heated to approx. 68-72 degrees. How many more numbers do you need? I just can't help but be amazed at all the hysteria over this subject.
Last edited by soufiej on Mon Jan 22, 2018 3:29 pm, edited 2 times in total.