Mando strings go sharp--why?

[hank]

Willie, I had the same thought take the wood out of the picture with a direct comparison of a Mix and a standard build with the same type/guage strings. Another thought is to use a deflexion compliance tester to measure the rise and fall of the top of the bridge under extreme swings of each of the suspected causes. This same testing method could also throw some light on why some mandolins wake up after use and others that don't. My F5 seems to be more stable now with less change in muted awaking than in years past. Is this my flawed perception or is this reality? My thoughts are based more on intuition than science but it should work if the movement is large enough to be measured with a dial indicator.
Another factor that may be coming into play here is atmospheric pressure. Generally we think of atmospheric pressure affecting a closed container like a ballon or your joints that ache in lower pressure because of less resistance against the joints swelling allowing increased size from inflammation. Again I'll ask the more educated than myself, are the cells of the wood possibly affected by the force of the surrounding air pressure.
Humidity is most likely the leading particular here but generally humidity comes with low pressure as dry air comes with a high pressure.

[Moon]

I live in the Wyoming desert. The average humidity is around 15%, like Las Vegas. I have two bowlbacks, one is a 12-string. They both sound sweeter when it rains, they absorb moisture and fill out. The strings have to be loosened some because of that. Most of the time they stay in tune because the humidity is low even in the summer when the windows are open, just as low as indoor heating in winter makes it.

Because it's always so dry here, I think the wood "drinks" up more water faster than in places where it's only 20-30 percentage different humidity it the rain. We can go from 15-20% to 85% in 24 hours.

I took a bowlback along on a long motorcycle trip to a much more humid place (Wisconsin) and I had read to loosen the strings before traveling because of the possibility of the mandolin swelling and popping strings or warping.

[pops1]

I live in the Wyoming desert. The average humidity is around 15%, like Las Vegas. I have two bowlbacks, one is a 12-string. They both sound sweeter when it rains, they absorb moisture and fill out. The strings have to be loosened some because of that. Most of the time they stay in tune because the humidity is low even in the summer when the windows are open, just as low as indoor heating in winter makes it.

Because it's always so dry here, I think the wood "drinks" up more water faster than in places where it's only 20-30 percentage different humidity it the rain. We can go from 15-20% to 85% in 24 hours.

I took a bowlback along on a long motorcycle trip to a much more humid place (Wisconsin) and I had read to loosen the strings before traveling because of the possibility of the mandolin swelling and popping strings or warping.

Welcome to my world.

[Bill Cameron]

Postscript: I had the 505 refretted a couple of weeks ago. In the course of the job, Brian also tweaked the nut slots for the new fret setting. Plays smooth like silk and I havent broken a string since (in fact I didn't even change the strings for the refret, they were pretty new flats). Not even in 8 hours of St Pats Day gigging. It still goes sharp over a couple of days, but to a less extreme degree.
I'm so happy with it I stopped thinking about new tuners, bridge, and tailpiece. So, good for me, less good for the aftermarket parts industry!

[Willie Poole]

One thing that I have found is that if I have to tuned down when they do go sharp, after a few songs I have to tune up again, what I usually do if I find my mandolin going sharp after sitting for a long period of time I just pull (stretch) the strings a bit and then tune them, they seem to stay in tune longer that way, be careful not to pull too hard though...

The theory about atmospheric pressure is interesting also...