Relative humidity and Actual moisture content

woodrat · Post by **woodrat** » Wed Sep 02, 2009 6:39 pm

Hi everyone, I am a bit puzzled by something. According to all the literature and received wisdom from other sources guitars need to be assembled between 40 and 45% relative humidity (in Jim Williams book he says below 60% probably written in the days before portable dehumidifiers) I understand that and have heeded this advice and so have not had any humidity related (or lack thereof) disasters thankfully.
I would like to know if there is a huge (and therefore something to to take into account) difference between 45 % RH at 10 deg C and 45% RH at 35 deg C as this is the range that I find I mostly have to deal with temperature wise where I live. As I understand it there is a difference in the amount of water in the air at those different temperatures at the same RH but is it substantial or slight enough not to be that important.
Thanks in advance for any replies to my post.

John

kiwigeo · Post by **kiwigeo** » Wed Sep 02, 2009 7:38 pm

Heres a fairly good website that explains it all:

http://www.bom.gov.au/lam/humiditycalc.shtml

I try and keep the temp in my workshop fairly steady. During the winter I have a heater running 24/7 on a low setting to keep temp at around 20degC.

Allen · Post by **Allen** » Wed Sep 02, 2009 8:01 pm

As far as I understand it doesn't matter what the temperature is, as long as your RH is in the range you are wanting to build at. Working at 25 degrees is a lot more comfortable than 35 -40 that I have to put up with here in the summer, but it sure does make using hide glue a lot easier.

I haven't worked out a way to convice the accountant that I need to put in air conditioning into my workshop before she gets it in the lounge room.

Hesh1956 · Post by **Hesh1956** » Wed Sep 02, 2009 9:48 pm

Allen wrote:As far as I understand it doesn't matter what the temperature is, as long as your RH is in the range you are wanting to build at. Working at 25 degrees is a lot more comfortable than 35 -40 that I have to put up with here in the summer, but it sure does make using hide glue a lot easier.

I haven't worked out a way to convice the accountant that I need to put in air conditioning into my workshop before she gets it in the lounge room.

This is my understanding too.

When I started building I thought that I wanted a 42-48% RH range AND something like 72F and found this very hard to achieve.... Then someone came along and said that the temp was more for the humans (and glue set-up times) than anything else.

So my shop is always in the range of 42-48% RH but the temp can be anywhere from 70-80F which is much easier for me to maintain.

The 42-48% RH range is close to or exactly what some of the fact*ory makers maintain too. They build in this range to help ward off warranty claims when their products have to live in the real world of forced air heating (very dry). An overly wet environment can also be problematic for a finished instrument too but in my part of the rock what we see most that damages guitars is drying out...

Once I realized that the temp was more for me and that it could vary a bit I was able to stabilize my shop and just dressed to be comfortable what ever the temp was.

Now there is also a belief that if one lives in a certain RH climate and builds an instrument that will never leave this climate and RH range this can be fine too. I have heard of Arizona builders, where it is very dry, building in a drier RH range and not being concerned because the instrument will never leave the Arizona area.

But if you sell your instruments or travel with them the 42-48% RH range is probably a very good idea. It's also a very good idea if you have major seasonal differences in temp which required heating that can dry air.

seeaxe · Post by **seeaxe** » Thu Sep 03, 2009 4:42 am

Allen wrote:
I haven't worked out a way to convice the accountant that I need to put in air conditioning into my workshop before she gets it in the lounge room.

Allen, surely the answer is obvious - move the workshop into the lounge, install the aircon and you will both be happy. Your wife will think its great becasuse she wont have to go so far to bring you all those cups of tea and cold beers.

vandenboom · Post by **vandenboom** » Mon Sep 07, 2009 9:12 pm

seeaxe wrote:
Allen wrote: ...move the workshop into the lounge, install the aircon and you will both be happy. )

...alternatively, move the TV, sink & ironing board into the workshop, install the aircon etc etc
....now I'm asking for it!
Frank

kiwigeo · Post by **kiwigeo** » Tue Sep 08, 2009 11:39 am

During my last lot of house renovations the boss wanted a slide out under bench ironing board in the kitchen so she could watch TV while doing her ironig. When I suggested we just put a TV in the laundry she went apeshit and wouldnt talk to me for a week.

Nick · Post by **Nick** » Tue Sep 08, 2009 1:13 pm

kiwigeo wrote:When I suggested we just put a TV in the laundry she went apeshit and wouldnt talk to me for a week.

Strange folk, women (apologies Lillian, not you obviously!

). I reckon I'll die still wondering what makes them tick.
The guy I work with actually quite enjoys it when he has words with his wife! He takes himself out to his garage grabs a beer out the fridge and sits in his Calaise Commodore with it's luxurious leather seats and puts Pink Floyd on the car stereo....Loud & enjoys himself! Pisses his wife off no end that he does because he's supposed to be miserable

Lillian · Post by **Lillian** » Tue Sep 08, 2009 1:50 pm

Nick O wrote:
kiwigeo wrote:When I suggested we just put a TV in the laundry she went apeshit and wouldnt talk to me for a week.
Strange folk, women (apologies Lillian, not you obviously! ). I reckon I'll die still wondering what makes them tick.
The guy I work with actually quite enjoys it when he has words with his wife! He takes himself out to his garage grabs a beer out the fridge and sits in his Calaise Commodore with it's luxurious leather seats and puts Pink Floyd on the car stereo....Loud & enjoys himself! Pisses his wife off no end that he does because he's supposed to be miserable

No need to apologize. I'm stranger then most in some regards.

Martin, she probably went off you because that is a setup you would have for a domestic not for the lady of the house.

Allen · Post by **Allen** » Tue Sep 08, 2009 5:02 pm

The boss just read these and got a chuckle. She flat out refuses to iron anything. If it can't be shook out, then too bad. At least the humidity up here necessitates the wearing of lots of cotton t-shirts.

Taffy Evans · Post by **Taffy Evans** » Sun Sep 13, 2009 6:54 am

My workshop moisture reading has been off the scale for weeks now, buckets of water make a bit of difference [at least putting a reading on the digital screen] but not enough.

I wanted to glue up this week, so as the boss is overseas on holidays I turned on the air cooler [evaporative] in the house and brought the content in the workshop from no reading to 52% in 15-20 minutes. My workshop is off the side of the house so it worked well, bloody cold in the house and a nice 26deg in the workshop.

There are 3 doors between the cooler and the building shop so I can control the environment by adjusting these, or just turn the cooler down. I hope it rains before the cook gets home.

Gaby · Post by **Gaby** » Sat Oct 03, 2009 8:36 pm

Allen, just wondering... the relevant humidity may be the same at different temperatures, but the wood may behave differently at higher temperatures (affecting the building process)?

Allen · Post by **Allen** » Sat Oct 03, 2009 8:51 pm

Not that I've noticed Gaby, nor have I heard of any issues unless you're talking about the extremes of leaving an instrument in a closed vehicle that then affects glue joints etc.

More of a comfort level for the builder. Higher temperature does make some aspect of building much easier. Such as using hot hide glue. You have much longer working time when the ambient temperature is in the mid to high 30's than those in colder climates, where careful planning, speedy positioning and clamping are more of an issue.

In my "Dry Room" where the de-humidifier is running, during the summer the temperature is between 40 and 42 degrees C most days. RH is 40 - 45%. While it's quite warm in there, because the humidity is so much less than the rest of the house, it's actually much more comfortable. You do require a beer to re-hidrate after a session glueing braces though.

Bob Connor · Post by **Bob Connor** » Sat Oct 03, 2009 9:20 pm

Allen wrote:You do require a beer to re-hidrate after a session glueing braces though.

Occupational hazard Allen.

Gaby · Post by **Gaby** » Sat Oct 03, 2009 11:16 pm

Life's tough in the tropics!

I meant relative humidity of course (though it is relevant

.

Unfortunately, lots of people subject their guitars to heat and humidity, car boots and even ute trays. Especially during the wet season you see a lot of bridges coming off...

You're doing well keeping the RH that low, you must have a fridge inside your workshop!

Allen · Post by **Allen** » Sun Oct 04, 2009 6:20 am

First piece of essential equipment.

liam_fnq · Post by **liam_fnq** » Sun Oct 04, 2009 10:58 am

You do require a beer to re-hidrate after a session glueing braces though. Laughing

the way its getting hot up here i try to stay hydrated through most of the building process.

Taffy Evans · Post by **Taffy Evans** » Mon Oct 05, 2009 9:34 am

I mentioned in my earlier post that I humidified the workshop from the house air cooler. Well as I feared my wife returned from the UK and found that the house was too cold when the workshop was just right, so I moved to plan B.

I bought a small portable air cooler, to put moisture back into the air and it works fine. It has a control for 3 fan speeds and a control for the amount of water through the filter, so its pretty flexible. You should not hear me complain in the future as I've got a refrigerated A/C, a dehumidifier and an evaporated cooler [for moisture control]. These do two rooms the building shop and repair shop. In the machine area I suffer, But the boss is happy now.[/quote]

Allen · Post by **Allen** » Mon Oct 05, 2009 5:49 pm

Yah, but how about a beer fridge....no mention of that most essential piece of luthiery equipment.

Taffy Evans · Post by **Taffy Evans** » Tue Oct 06, 2009 12:30 pm

Yep, got one of those too. However it's got more small bottles of Superglue than beer in it at the moment. It has plenty of coke in it tho, to go with the JD.

Rick Turner · Post by **Rick Turner** » Tue Oct 06, 2009 12:38 pm

I hope that's Coke with a capital "C"...

A forestry guy once made me a table that showed how much heat it would take in the winter time to lower the RH of my workshop from the RH outside. As I recall, it didn't take much of a rise in inside temperature to make a significant difference to the RH, and thus to the EMC (Equalibrium Moisture Content) of the timbers in my space. That was many years ago...like in 1976 or so, though, and I have no idea where that scrap of paper went. Might be able to Google for the info, though.

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Relative humidity and Actual moisture content

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