A word of warning about humidity...

pakhan

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Dear friends,

I've been living in Singapore which is up to 30 Celsius, 90% relative humidity and even with a dry room, we've been running into issues with guitars until recently....

When I looked into relative humidity (RH) & absolute humidity (AH). It changed how I set up my dry room. Actually, I had looked into this when I was in London but had simply ignored it until now.

This info would be useful for anyone looking to care for their instruments, as the calculations can be worked either way for overly or under humid environments. I am using as a reference a conversion table issued by a German industrial site.

The main problem we all face is RH, but I believe that we should be looking at AH instead. Absolute humidity, is the actual amount of moisture in the air. IE, the amount of water vapor, usually discussed per unit volume.

Relative humidity on the otherhand is a %, ratio of the partial pressure of water vapor to the saturated vapor pressure of water. It is affected by pressure and temperature.

So even if we consider that the pressure is the same (it may not be), temperature alone is enough to throw things if we look at RH alone.

Say my uke is made in the USA. In a climate controlled environment of say 20 Celsius, 60% RH. This translates to an AH of 10.4 g/m3. (black circle)

It comes to me, at 30 Celsius, 90% RH. This translates to an AH of 27.3 g/m3. (black circle) This is almost 3x the original humidity. Even if I manage to get the RH down, to say 60%, the AH is still nearly double at 18.2 (yellow circle). This was the previous setup, and I was finding the guitars were still being affected.

In fact, to achieve an AH approximating 10.4, I should be looking at an RH of between 30-40% if I am at 30 Celsius. (green circle). The good news is that my dry room temperature is closer to 25 Celcius, so after switching to a target of 50%, I find things normalising.

The situation may be reversed in ukes, if you stay in a dry climate and the ukes were manufactured in a hot humid environment, say, vintage ukes in Hawaii. You would be looking at humidifying to close to 100% at 20 Celsius to even have a hope of matching 30 Celsius, 60% RH. Luckily old school luthiers were quite cunning and tried to perform critical tasks during periods of lower humidity.

Hope that helps and sheds a little light on humidity.

Incidentally, the pressure will also affect humidity, so the closer to sea level, the more humidity the air can hold and vice versa.


rg-diag.jpg
 
Lol, I can understand- basically even at the same % relative humidity, if you're at a higher temperature, the absolute humidity is higher.
 
I understand it and thanks for the info. I have several ukuleles in Thailand and some in the desert and some in Colorado.

This graph will help me explain to the caretakers what I need from them.

THanks
 
So what is an ideal humidity level then? I'm assuming it must be somewhere midway between tropical Africa and the Deserts of Siberia, is it? ;)

My studio, where most of my instruments are out of their cases and on display on stands, averages around 50% humidity (heating from a single radiator) and the winter temperature varies between about 16 at night and 20 when the heating is working (early morning and from sundown to 11pm). We are 360 metres above sea level and the general climate is Mediterranean. My most valuable instruments were either made in Mexico (Martin DX 1 AE) or wherever Kala and Ohana make their mid-range instruments (I'm guessing SE Asia).

So, given all that should I be happy or worried?
 
So, let me see if I have this straight...

Oh, fiddlesticks. I don't.
 
Well, overall its rather simple. What he is saying is that the humidity is also affected by the temperature you have. So 50% humidity at 10°C is not the same as 50% humidity at 20°C. Thus it might be that if the temperature where your instrument has been built in is very different from the temperature you have, it doesn't work that you only try to get the percentage humidity right, you have to adjust according to this chart, possibly even needing a higher percentage of humidity than you thought.
 
What is the rationale behind using AH instead of RH? Doesn't the air's ability to hold moisture have more impact on how humid the wood gets?
100% RH in certain conditions can cause mould...
 
Well most of us live in climate control, why not just keep the temp and RH constant?

Bingo. Builders try to cover the bases by building to 50%RH and most likely a comfortable room temperature, let's say 72 degrees. That way you have the Sahara vs. Rain Forest range fairly well covered.
 
What is the rationale behind using AH instead of RH? Doesn't the air's ability to hold moisture have more impact on how humid the wood gets?
100% RH in certain conditions can cause mould...

The AH is a more accurate measure of how much humidity there is in the air as opposed to RH. As most folks don't take temperature and pressure into account when reading RH. SO simply using RH without temperature (and pressure) is like saying, this uke will cost 20% of a person's salary without stating the salary. As opposed to saying in AH, this uke costs $1000 which doesn't require further information.
 
So its -5F here in Colorado and about 10% RH, I should be ok....

I like to use the natural weight of the snow to glue my tops on ;)

Unless someone decides to store your ukes at 30 celcius, 90% RH........ with their own weight! :p
 
I am not able to see how the weight of water vapor in a cubic meter of air translates to the availability of water vapor to escape or penetrate the wood of an instrument. RH tells me the availability of water to desiccate (10%) or saturate (90%) wood. Is H2O at 8grams/m3 at 10% RH going to affect wood the same way 8grams/m3 at 90% RH does?
 
I am not able to see how the weight of water vapor in a cubic meter of air translates to the availability of water vapor to escape or penetrate the wood of an instrument. RH tells me the availability of water to desiccate (10%) or saturate (90%) wood. Is H2O at 8grams/m3 at 10% RH going to affect wood the same way 8grams/m3 at 90% RH does?

That OXK formica is looking better and better all the time. :confused:
 
I am not able to see how the weight of water vapor in a cubic meter of air translates to the availability of water vapor to escape or penetrate the wood of an instrument. RH tells me the availability of water to desiccate (10%) or saturate (90%) wood. Is H2O at 8grams/m3 at 10% RH going to affect wood the same way 8grams/m3 at 90% RH does?

This was exactly where my brain went. It's not a matter of how much water is in the air, but how available that water is to affect the integrity of my uke, isn't it? A low relative humidity would indicate to me that the dryness of the air, for lack of a better term, would be tending to pull more moisture out of the objects around it, and with a wooden instrument that's not a good thing.
 
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