Building Relative Humidity

Construction and repair of Classical Guitar and related instruments
Jim Frieson
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Building Relative Humidity

Post by Jim Frieson » Sat Jul 01, 2017 10:19 am

The top is braced on the inside , during past winter , at 35% relative humidity .
Now it is 60 % relative humidity .
If I had braced it at 60% and exposed it to 35% , the effect , opposite .
Illustration of the effect of humidity on the broad expanse of tops and backs .
DSCF6498.jpg
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TheEvan
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Re: Building Relative Humidity

Post by TheEvan » Sat Jul 01, 2017 11:26 am

So, if a guitar's destination is high RH, is it better to build it at high RH and vice-versa?

johnd
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Re: Building Relative Humidity

Post by johnd » Sat Jul 01, 2017 11:57 am

I wonder what happens if you seal the bracing so that humidity has little or no effect on the bracing? Would the top develop cracks?

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Michael Lazar
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Re: Building Relative Humidity

Post by Michael Lazar » Sat Jul 01, 2017 1:32 pm

When I build a guitar for a high humidity area I assemble it at a higher RH, sometimes as much as 60%. Most of my guitars are built for the Canadian prairies where the winters can be very cold, hence very dry indoors. Those are assembled at about 30% RH. The humidity at the time of top bracing is not as critical as it is when the top and back are being glued to the sides.

Wood expands & contracts mostly across the grain and very little along the grain. As the braces are not very wide the amount of movement would be very slight so sealing them would make very little difference.

johnd
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Re: Building Relative Humidity

Post by johnd » Sat Jul 01, 2017 3:18 pm

Michael Lazar wrote:
Sat Jul 01, 2017 1:32 pm
Wood expands & contracts mostly across the grain and very little along the grain. As the braces are not very wide the amount of movement would be very slight so sealing them would make very little difference.
Michael,
Don't know if I understand this.
If the braces are sealed and then glued to the top so that the humidity doesn't effect their movement, how would the top move? If the braces don't move, how would the top move?????

Alan Carruth
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Re: Building Relative Humidity

Post by Alan Carruth » Sat Jul 01, 2017 8:39 pm

The top moves because it tries to get wider as the humidity rises, and narrower as it falls. The braces, most of which run at least somewhat across the grain, don't change in length much, so there's a stress set up between the braces and the top. You'd get the same effect if you glued a piece of metal, such as aluminum foil, across the grain of a piece of wood. I used something like that as a humidistat to turn a light bulb on and off through a relay, which controlled the humidity in a box. Eventually the cross grain wood strip broke owing to the stress between it and the metal.

Plywood has layers of wood that cross each other If you look closely at a piece you'll see that the outer two layers have the grain running in the same direction, while the layers underneath those go across, and so on for however many layers there are. Plywood tends to be very stable with humidity changes, but that comes at the cost of lots of stress in the glue lines. Plywood left outdoors tends to come apart over time, even though the glue they use is water proof. The glue doesn't fail, the wood does.

printer2
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Re: Building Relative Humidity

Post by printer2 » Sat Jul 01, 2017 9:20 pm

Think of the wood as a bunch of straws bundled together. Fill up the straws with water and they do not get longer but they can expand and the bundle get wider. The length of the top will not change much but the width of the top would with a change in humidity. Now if you glue braces lined up with the grain of the top, the top will still get bigger width wise with an increase in humidity, narrower with decrease in humidity.

Now if you glue a brace along one side of the top 90 degrees from the direction of the grain you lock that side of the top to the length of the brace. The brace will not change length (in practical terms) with a change in humidity. But the top still will because you have all these tubes that are expanded with the added water. The brace side can't expand but the opposite side of the top where the brace is glued can still expand. So that side of the top expands, the brace side does not, something has to give. And that is the brace. The brace might be real stiff length-ways you can bend it easy enough with some side force. The edge of the brace that is not glued is compressed a little and the glued side slightly made longer.

So in cases where the humidity is greater than when the pieces were joined the top side is bowed out. In lower humidity cases the brace is roughly the same, the diameter of the straws is less, the top is shrunk down width wise. Again the top exerts a side force on the brace, the top is sunken in. Hope this helps.
Fred

Jim Frieson
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Re: Building Relative Humidity

Post by Jim Frieson » Sun Jul 02, 2017 6:35 am

TheEvan wrote:
Sat Jul 01, 2017 11:26 am
So, if a guitar's destination is high RH, is it better to build it at high RH and vice-versa?
Winter day in Toronto . Bitter cold , very dry .
A player from Miami up to do a gig was referred to me , called me at night .
His guitar is cracking right before his eyes he says .
Guitar made in Almeria at who knows what RH . A really good Gerundino Fernandez .
He had it for many years in Miami . No problems perfect condition .
One day in Toronto and it has multiple cracks top and back .

So in answer to your question , I think it wise to build a guitar taking into account the lowest RH it is likely to run into .
Even in humid places like Tokyo , sudden drops in humidity can and do occur .

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souldier
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Re: Building Relative Humidity

Post by souldier » Sun Jul 02, 2017 2:31 pm

I understand that it is detrimental if a guitar is built in high humidity and goes to a place with low humidity. How risky would it be if a guitar is built in low humidity such as 25-30% then goes to a place with high humidity like 60%+?
"Success grants its rewards to a few, but is the dream of the multitudes.
Excellence is available to all, but is accepted only by a few." - Christopher Parkening

SteveL123
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Re: Building Relative Humidity

Post by SteveL123 » Sun Jul 02, 2017 3:09 pm

Alan Carruth wrote:
Sat Jul 01, 2017 8:39 pm
The top moves because it tries to get wider as the humidity rises, and narrower as it falls. The braces, most of which run at least somewhat across the grain, don't change in length much, so there's a stress set up between the braces and the top. You'd get the same effect if you glued a piece of metal, such as aluminum foil, across the grain of a piece of wood. I used something like that as a humidistat to turn a light bulb on and off through a relay, which controlled the humidity in a box. Eventually the cross grain wood strip broke owing to the stress between it and the metal.
.................
Can you provide details of your humidistat? Thx!

printer2
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Re: Building Relative Humidity

Post by printer2 » Sun Jul 02, 2017 4:02 pm

souldier wrote:
Sun Jul 02, 2017 2:31 pm
I understand that it is detrimental if a guitar is built in high humidity and goes to a place with low humidity. How risky would it be if a guitar is built in low humidity such as 25-30% then goes to a place with high humidity like 60%+?
The danger in going dry is the wood gets pulled apart in tension and cracks occur. Going to high humidity puts the wood of the wood into compression, wood can take a compression load better than tension, takes more force before the fibers get crushed. Eventually something has to give and you get the glue let go between the braces and the panels. How risky depends on how the guitar is made and what wood. I have put together two bodies (one SS and one nylon) at 25-30% RH. Need to finish them and see how they manage.
Fred

Alan Carruth
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Re: Building Relative Humidity

Post by Alan Carruth » Sun Jul 02, 2017 8:12 pm

Not many details on the humididstat. I used a cross grain cutoff from a spruce top, so it was about 8" long and maybe 1/2" wide, by 1/8" thick or so. I used Super glue to stick a strip of tinned brass stock, say 1/8" wide to the center of one surface running the length of the strip. One end was glued to a base block. At the other end I set up a contact with an adjusting screw where it would touch the metal strip as the humidity rose and the strip bent. This was hooked up to a battery that operated an old relay. When the relay was triggered by the contact it turned on a light that heated the air up in the box where everything was housed. As the air warmed up the humidity went down, when it got low enough the light went out I could put wood or parts that I was working on in the box to dry them out. A the time I was working in a basement shop that was impossible to dehumidify in the summer. So long as stuff lived in the box most of the time it stayed dry enough.

That setup was based loosely on the humidity gauge that my violin making teacher had. She took a strip about 2" wide off one edge of a sheet of plywood (so it was 8' long), and milled off one of the surface plies. The uneven construction caused it to bend with changes in humidity. She mounted it on one wall of the garage she used as a shop, so that one end would move up and down with changes in humidity. Whenever the local radio station announced the R.H., she'd check it, and mark the number on the wall at the end of the pointer if had not already been done. Eventually she had a pretty accurate gauge. Trevor Gore has talked about making gauges on this principle by gluing strips of wood together.

One issue with this is the one I ran into with my relay trigger: when the humidity goes much lower than what it was when the strip was glued on the shrinkage sets up a lot of stress in the cross grain strip. Eventually it cracks, and the thing doesn't work any more. Recently one of my students gave me something that might solve that problem. It's called an 'Old Woodsman's Weather Gauge'.

It's a twig, about 18" long, that has been cut from the trunk of a small tree, with a short section of the trunk wood. The bark has been peeled off. For reasons that will become apparent I know this one is some sort of hard wood. In life the twig grew out perpendicular to the trunk; parallel to the ground. That's important, too. In use, you nail the mounting section from the trunk to the wall, and the free end of the twig rises and falls depending on the R.H. When the humidity is low the twig points up, and when it's high it points down.

This works because there's 'reaction wood' in the twig. 'Cold creep' would make a twig that's is parallel to the ground droop, as you see in 'weeping' willow or cherry. To prevent this the tree builds up 'reaction wood' that has fibers that run at an angle to the length of the twig. This fights that tendency to droop, but also causes the reaction wood to change in length much more than normal wood; about 10-15 times as much. In the living tree this is not an issue, since the wood is saturated with moisture anyway. It can cause problems for us when we dry the wood out, though. At any rate, softwoods grow reaction wood on the lower surface of a branch, while hardwoods grow it on the top. That's why I knew this was a hardwood twig; it bends up as it dries out because the upper surface shrinks more due to the reaction wood. Although a band of very heavy reaction wood can cause a split in a piece this is abnormal: the cell structure is pretty well tied in, and, in this case, the whole twig can move to relieve the stress.

In theory, then, all you need to do is find a nice twig, set it up with the contacts you need, and calibrate it, and you could have a pretty accurate humidity gauge. In practice; who knows?

Jim Frieson
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Re: Building Relative Humidity

Post by Jim Frieson » Mon Jul 03, 2017 11:09 am

Varnish the braces and the top won't move . But what if you get different braces next time , or different varnish .

It reminds me , Joe and Howard go fishing . They don't catch anything .
They get to a place in the middle of the lake and Holy mackerel !! they are just reeling them in . Incredible .
What are we gonna do boy ? How can we remember this here spot . ? Says Howard .
Joe thinks a spell and he say , I got it . We'll just make good and sure we get the same boat .
Howard thinks on it and says .... But how do we knows we gets the same boat , eh ?
Ah , you're daft , says Joe . We'll put an X on the side of the boat .

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James Lister
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Re: Building Relative Humidity

Post by James Lister » Mon Jul 03, 2017 12:17 pm

SteveL123 wrote:
Sun Jul 02, 2017 3:09 pm
Alan Carruth wrote:
Sat Jul 01, 2017 8:39 pm
The top moves because it tries to get wider as the humidity rises, and narrower as it falls. The braces, most of which run at least somewhat across the grain, don't change in length much, so there's a stress set up between the braces and the top. You'd get the same effect if you glued a piece of metal, such as aluminum foil, across the grain of a piece of wood. I used something like that as a humidistat to turn a light bulb on and off through a relay, which controlled the humidity in a box. Eventually the cross grain wood strip broke owing to the stress between it and the metal.
.................
Can you provide details of your humidistat? Thx!
There's a few pics and info about the one I made here: viewtopic.php?f=11&t=107791&hilit=hygro ... 5#p1145948

James
James Lister, luthier, Sheffield UK

SteveL123
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Re: Building Relative Humidity

Post by SteveL123 » Mon Jul 03, 2017 3:16 pm

Thanks Alan and James for the info! It's nice to have a big Hygrometer made out of strips of wood that you can instantly see the RH from across the room! I will try to make one.

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