Calculating the RH for different Temperatures

[ChrisE]

I just use a cheap digital hygrometer.

I think the number it's showing calculates the RH based on the current temperature, meaning I shouldn't need a calculator to know if my guitars are in danger of drying out. Am I right?

[fazool]

this post might help, I hope

[justonwo]

Originally Posted by ChrisE

I just use a cheap digital hygrometer.

I think the number it's showing calculates the RH based on the current temperature, meaning I shouldn't need a calculator to know if my guitars are in danger of drying out. Am I right?

Yes, relative humidity is the number displayed. There is no calculation required. And RH is the only number anyone needs to concern themselves with.

[DCCougar]

Originally Posted by Haasome

The warmer air is, the more water it is capable of holding.

This is undoubtedly true, as I've heard it said before. But from my rudimentary physics understanding, it doesn't make sense. Warmer air means the air molecules are moving around faster. I don't get how this would allow more moisture in the air. It would seem to be the other way around.

[charles Tauber]

Originally Posted by Silly Moustache

Hi folks, just revisiting this at the change of seasons.

I used Rod's calculator : http://rods-music.caoulet.com/humidity.htm

This told me the followng based on Collings advice that they build their guitars at 75 degrees f @ 49% RH.

Using his calculator tells me something rather different : i.e.

60f = 82% 65f – 69% 70f = 58%
80f = 42% 85f = 35%

Is this correct do you think?

Say this to yourself over and over again: "49% relative humidity is 49% relative humidity regardless of temperature."

If you want to maintain your guitar at "factory humidity", you want 49% relative humidity, regardless of temperature.

Heating or cooling THE SAME AIR without adding or removing moisture to/from that air will give you the result you've calculated from Ron's webpage, the same result as simply moving horizontally across (constant absolute humidity) the physchrometric chart.

You've repeatedly stated that you neither heat nor cool your home's air. Thus, the "conversion" you are calculating is irrelevant and does not apply. It ONLY applies if you are heating or cooling air to determine what its relative humidity is AFTER heating or cooling that air, without adding or removing moisture while heating or cooling.

In principle, it isn't that complicated. Simply maintain the relative humidity at your desired level.

[mawmow]

What a well stuffed topic !

In the book referred to, table 4-2 shows that if a guitar was allowed to get from some 60 f 40% RH to 90 f 70 % RH, when the wood content would reach new equilibrium with room air, the water wood content would have increased from 7,6 % to 12,6 %.

Figure 4-3 represents a diagram of a log section where different cuts show how humidity content exerts different degrees of alteration along longitudinal and radial direction of wood fibers : wood grain thickness is more affected than its lenght.

To me, it explains how our guitars are significantly affected by extreme (as well of more subtle ?) room air changes.

[charles Tauber]

Originally Posted by mawmow

Figure 4-3 represents a diagram of a log section where different cuts show how humidity content exerts different degrees of alteration along longitudinal and radial direction of wood fibers : wood grain thickness is more affected than its lenght.

In most species of wood, the change in length is, practically, negligible. What the Figure 4-3 shows is how the orientation of the end grain in a board - how the board is cut relative to the tree - influences how that board changes geometry as the humidity content changes.

The real take-away is that most wood species change by different amounts between radial and tangential directions. How wood is cut - it's end grain orientation - determines how that piece of wood will move in response to humidity changes.

To me, it explains how our guitars are significantly affected by extreme (as well of more subtle ?) room air changes.

Let's use Sitka spruce as an example. Let's assume a constant indoor temperature of 70 degrees F and a RH that changes from 65% in summer to 40% in winter. That represents a change in moisture content in the wood of about 6%.

Published figures for Sitka spruce state that a change of .0015 for the radial direction and .0026 for the tangential direction can be expected for each per cent of change in moisture content. That is, 6x.0015=.009 for a 6% moisture content change for radial, and 6x.0026=.0156 for tangential.

For a 16" wide piece of Sitka spruce, over the change from 65% to 40% RH, we would expect about 16"x.009=.144" radial direction change and about 16"x.0156=.250" in the tangential direction. That's about 5/32" in the radial direction and about 1/4" in the tangential direction.

A grain that is oriented with the radial direction across the width of the board is quarter sawn; grain that is oriented with the tangential direction across the width of the board is slab cut or flat sawn. That is, the quarter sawn piece will shrink by about 5/32" while the slab cut piece will shrink by about 1/4" for the same 25% change in relative humidity. Since a quarter sawn guitar top is constrained about its edges and can't change width, the top flattens when attempting to shrink, and domes/bulges when attempting to expand. That takes the bridge and strings with it, changing action seasonally, if not moderated.

The values for rosewood are similar. The arch on a rosewood back will flatten to accommodate that shrinkage, until it can't flatten further and it cracks to relieve the internal stresses due to further shrinkage. The amount of expansion/contraction for slab cut rosewood is almost twice what it is for quarter sawn rosewood for the same change in humidity.

[Mr. Jelly]

Try this.



https://www.hand2mind.com/pdf/versat...vestG6p6-7.pdf