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Guys just a quick question my workshop is in the loft/attic and this time of year it can get quite cold as I only have the heating on when I'm in it, but my question is when the workshop is say 5°C the humidity is around 60-70% but as soon as its heated to 18°C the humidity drops to 35-45% and I just wondered if relative humidity was measured at a set temperature and if 5°C-70% humidity was equivalent to 18°C 40% humidity? The heat source is an electric oil filled radiator that I switch on normally an hour before I work in there.

This is only build 1 and hobby fun so I won't be spending out on dehumidifiers and heating etc for when I'm not in the shop, and at the moment my solution it to keep things like the plates and box in the house which it relatively stable all year round, as I found out the hard way the temperature swung from 8°C to 28°C and 70%-35% when we had a hot day after a cold damp night and when I went back to it there were 2 large cracks in the back, my own fault for using slab sawn wood which already had a cup before bracing which exaggerated after bracing it pulled out after I'd glued it to the rims and had sanded out before it cracked. Good learning curve though and I'll and up with an ugly duckling once played though.

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Formerly JJH

I learn more from my mistakes than my successes

John, you need to focus on relative humidity at the actual temperature you are at. In essence this tells you the airs tendency to remove or add moisture from wood.

Warm air can hold more moisture than cold air. If you take cold air at 45% RH and warm it without adding moisture the relative humidity of that air will decrease. At that warmer temerature the air holds much less moisture than it is capable of and it has become "thirsty" and will remove moisture from your wood.

I always keep my wood in a small room in the house that I can control cecause it is difficult to control humidity in my shop. I also do my gluing up in that room.

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David White, Toronto

"All my favourite singers can't sing."

John buddy I am not sure if I understand your question as you wrote it but what you do want to strive for is to maintain around 42 - 48% constant RH. Warm air holds humidity better than cold air so that is where the need to also strive to maintain a constant temperature comes in. Besides we humans don't want to be too hot or cold either.

Factories such as Martin will go to great lengths to maintain a near constant 72F and 45ish% RH to minimize loss and damage due to RH fluctuations. I built my shop to maintain the same numbers year around which means that I am humidifying in the winter (now) and dehumidifying in the summer and during some days in between I may be doing both depending on the time of day. In my experience it is far easier to maintain a desired RH level with a constant or near constant temperature.

To absolutely eliminate the possibility of damage and the loss of precision that RH changes can create if one wants to build guitars successfully dealing with the RH and temp issues are a must. Even using work-in-progress storage boxes are not IMHO an adequate solution in as much as a braced top when removed from 45%RH can turn into a potato chip in 20 minutes time.

Good post David!

Also there is a need to have your materials acclimated to the proper RH and temp too which is why David is storing all of his materials in a RH controlled room.

You know as much as we get this question, temps and RH for guitar buildings, I am beginning to think that before one even purchases their first tonewood or tools the RH and temp solution for them in their part of the world really needs to be step one.

Cheers guys my thoughts were that storing woods at 5°C-70% humidity was equivalent to 18°C 40% humidity well not quite equivalent but not too harmful to the wood, and just keep the parts in a relatively stable environment whilst building the guitar, to prevent warping.

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Formerly JJH

I learn more from my mistakes than my successes

Generally you can count on an RH drop of about 40% with every 23 degree F increase. I can't remember how to convert to Celsius but sounds like that is about what you're experiencing. You need to find a place with a more constant temp though. That makes things alot easier. In my shop in the winter I try to keep the temp around 65 F and 40-45% RH. It's difficult for me to go much higher with the RH and if I warm it up to 70 F its even more difficult so I run it a little cool. Good luck with your build.

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"We might not be big but we sure are slow"

I am experiencing the same in the part of my shop that is not climate controlled. I think what happens is as you start heating the humidity rises sharply, and then evaporates with the constant temp. Here in northern New-England we experience sharp changes in temperature, especially during the fall and spring, when it is below freezing at night and can get fairly warm as soon as the sun rises. So what I see in the machine shop in the morning is a huge rise in humidity for a couple of hours, from say 32ºF/30%, to 50ºF/65%, and then the humidity drops down to about 45%. Cold air doesn't hold moisture, but I assume the moisture is still there, trapped in the building and materials.
A dehumidifier doesn't work well, or at all, when it is too cold BTW.

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Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

Using a psychrometric chart, a relative humidity of 65% at 5 degrees C, will give a humidity of about 28% when the air is heated to 18 degrees C. With wide temperature fluctuations, the wood will be rapidly cycling though a range of moisture contents. This is not ideal.

I feel to be successful at building the box at consistant RH is to do it as fast as you can.The sooner the box is completed,the less chance it has to experience RH fluctuation.That's why the big factories get away without many problems with RH.I always try to have everything ready for the build so it can be done in less time to minimise the RH problems that a slower build would create exposing the box to many days fluxuations in RH.Just my 2 cents on building boxes.

I was wondering about this last night. Here's my question, and I think it's related here.

I've been keeping my shop at 70F and 45% humidity. It's winter here and the furnace has been on. At the moment I'm using about 2 gallons of water per day.

If I drop the temp to 60F (and wear a sweatshirt) and adjust my humidifier to keep the humidity at 45% I will use less water and my guitars will not know the difference.

In other words, if I take a guitar from 70F and 45% humidity to 50F and 45% humidity, there won't be any significant change in the amount of moisture content of the wood.

Sound right?

Kirt

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Horton, MI

Kirt bro I think that this is correct. Interestingly the reason why even though warmer air will hold more moisture and in your example you have to add less moisture to cooler air the reason for this is I believe that forced air heating has a drying effect by moving air.

When I was trying to understand proper guitar building RH requirements I kept getting hung up on the temperature thing until someone, can't remember who, on the OLF told me that the temperature, within reason of course, is more for human comfort than anything else. And.... that maintaining a near constant temp makes it easier to maintain a near constant RH.

My shop is at around 74F because I am always cold, even in the summer......, but besides the costs associated with the fuel that warms my shop there is also the need for me to dump and schlep more water for the humidifier again because the forced air circulation has a drying effect. Also, if you have a big air cleaner run that for 10 minutes and watch what happens on your hygrometers - the shop starts to dry out.

Yes, sounds right.

I've got the exact opposite problem to most of you. It's summer here, and coming into the monsoon season. And while temperature is a fairly constant 32 - 34 Celsius, humidity can go from low 40's to nearly 100%. Trying to air condition a room down to 20 Celsius just isn't an option, but running a dehumidifier is no big deal. I will pull 12 liters of water per day out of a small bedroom during the summer months. Temperature goes up to low 40's Celsius because of the heat generated by the dehumidifier, but RH is 40 -45%. The rest of the house is much, much higher than this.

One interesting thing about my situation is that while it's over 40 C in my work room, it's actually more comfortable than the rest of the house, because the humidity is so much lower. Plus, when using hide glue, I've got heaps of open time.

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Allen R. McFarlen
Barron River Guitars & Ukuleles
Facebook
Cairns, Australia

It's all relative! 70* air will hold a lot more water than 60* air, so 45% RH (Relative Humidity - i.e., relative to the amount air can hold at a particular temperature) at 70* has a lot more water in it than 60* air at 45% RH. That's why, when you have 45% at 60* and raise the temp to 70*, the RH drops to something like 38% or so. If you raise the temp, and do not add moisture to the air, the air will suck the water out of everything around it.

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Waddy

Photobucket Build Album Library

Sound Clips of most of my guitars

Question:
If the humidity evaporates, in a close room presumably, where does the humidity go? Don't the water molecules continue to float around the room?

Physicist I know thinks the lowered humidity that comes from a higher temperature in a room is due to the drop in air density when the air is warmer. Sound reasonable? But it would seem as the air density dropped, would you get higher RH?

I don't know either answer.

If we don't know how varying temperatures affect the wood's moisture content (that's really why we're concerned about this, isn't it?), maybe a moisture meter would be more useful. Or a wood cross-grain contraption that I think Laurent mentioned some time ago. I read where Carleen Hutchins had one hanging in her shop. Two thin pieces of narrow wood that would curl a bit one way or the other, depending on where the moisture content was at that time compared to the moisture content when the two pieces were put together. When it stayed straight for awhile, that was the time that gluing plates could be done.

Pat

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formerly known around here as burbank
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http://www.patfosterguitars.com

Hesh - with all due respect, I believe you are seeing the effects of "wind chill" in the room. Another possibility is that by moving the air you are warming it slightly. Just the energy to run your dust remover can heat up the room. If the room is well sealed and there is no air exchange with the outdoor or other unconditioned space, the total mass of water in the room remains unchanged. When you run a forced air system within an enclosed space all the electrical energy used is converted, ultimately, to heat. The motor warms and exchanges heat with the air, the air rubs against itself... etc..

I used to date a girl whose roomates thought they gained water weight at night while they were sleeping! They argued quite vehemently about it.

I guess they were hygroscopic girls.

Pat Foster wrote:
Question:
If the humidity evaporates, in a close room presumably, where does the humidity go? Don't the water molecules continue to float around the room?

Yes, the mass of water has not changed.

In HVAC circles there are two humidities: relative and absolute humidity. Relative is expressed as a percentage of the total mass of water the air can hold at that temperature. In other words if a volume of air at 70*F can hold 4 pounds of water and there is only 2 lbs in the volume you are at 50% RH.

Absolute humidity is in units of lbm (pound mass) per foot cubed. RH is calculated by the following formula: RH = [(AH @ T1) / AH @ T1sat)] X 100. Where AH @ T1sat is the mass of water/volume of air at the given temperature at saturation, i.e. it cannot hold anymore water. The volume and temperature units drop out and you have a dimensionless number expressed as a percentage.

Watching the weather reports the RH is related to the Dew Point, which is the temperature that the air has to reach to be at 100% RH. At the dew point the air is considered saturated and cannot hold any more water. Typical dew points for the northern tier of states at this time of year are in the 10*F to 15*F range. The air temperature cannot fall below the dew point without percipitation.

Most all household building materials, espescially wood, are considered HYGROSCOPIC or water absorbing. So, lowering the temperature in a room will raise the relative humidity until the air/shop system is at equilibrium. Wood is seeking a moisture content equilibrium with its surroundings. It is based on the RH in the space, air temp and the absolute humidity as well.

Sorry for the long post, but since I am not able to contribute much in the way of guitar building and I do know something about this I wanted to share. If anyone has questions, I would be willing to try to answer them. If I have only muddied the waters, let me know!

Thanks,

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Steve Walden
Aspiring Builder,
Bartlesville, OK

Steve I stand corrected and I thank you kindly my friend for doing so.

There is also the possibility at least in my shop of methane saturation.........

As for hygroscopic girls I would remind you that this phenomenon is likely not exclusive to women....... And you have just coined a new term as well psychohygroscopics........

I bet they were beautiful.

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David White, Toronto

"All my favourite singers can't sing."

Thanks for the edification, Steve!

Dave, I too would bet they were.

Pat

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formerly known around here as burbank
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http://www.patfosterguitars.com