Let's Talk Calibration!

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Diane Kauffmds
Posts: 3246
Joined: Sun Mar 09, 2014 8:13 pm

Let's Talk Calibration!

Post by Diane Kauffmds »

I’m probably the forum Harpie when it comes to humidity control. Short of someone dropping a guitar off of a bridge, or some other catastrophic damage, too much or too little humidity will do considerable damage to a guitar. The majority of damage that I see come into my shop, is due to humidity. Too much and the wood swells; too little and it shrinks. Cracks appear, glued joints and plates come apart, bridges come loose.

There are myriad humidity control methods on the market, but today I’m focusing on environmental control. For those of you who keep watch over your instruments with hygrometers, whether in the case, or in a room or shop, kudos!

But, how many of you have thought about your hygrometer’s calibration? Most hygrometers, whether they be digital or analog, can be calibrated easily and should be checked at least yearly. This is really easy to do and doesn’t require any special equipment.

A salt and water slurry will produce a known 75% rh in an enclosed environment. All you have to do is make a slurry of salt and water, put it in with your hygrometers into an airtight container, and leave them for about 8 hours. Read and adjust. This is how I do an accurate calibration of my hygrometers.

1. I make the salt/water slurry. There is no real ratio here. I put about 1 tablespoon of salt into an open little dish or even a lid, and add water, just to cover. Leave it for a couple of minutes. It may be hard to see in this white dish, but the salt is covered with water.
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2. To make the slurry, which means there should be no excess water, I drain the excess water off of the salt.
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3. There will still be excess water, so I use a paper towel, and I scrunch it on top of the mixture, to absorb the rest of the excess water. This will leave a good slurry, not too wet and not too dry.
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4. Place your little dish or lid into an airtight container. Here, I’m using a rubbermaid container, which is large enough for the salt and all of my hygrometers. I lay the hygrometers in the container.
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5. The problem with this is that you need to be able to read the humidity level on your hygrometers, without disturbing the humidity in the container. But, I have to open the lid to read the levels. I’ve solved this little problem by sealing the top of the container with clear plastic. I tape all around the plastic, to the container. Then, I put the lid on the container.
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I allow it to sit for about 8 hours, to ensure homogeneous humidity in the container.

6. I open the lid, without disturbing the plastic and I read all of the hygrometers. Ideally, they should read 75%. I make a note, on the plastic with a sharpie pen, above each one, indicating the calibration needed, ie +3, -2, etc.
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7. NOW, I break the plastic seal and I calibrate each hygrometer, per the amount I wrote. They react to the outside air very quickly, so just setting them at 75% won’t work. Since I’ve noted how much I need to calibrate each one, it doesn’t matter what they read. I simply move the needle + or – the amount I noted, no matter what they read in my hand.


8. I put them all back into the container, reseal the clear plastic, and put the top back on for another 5-8 hours. After the allotted time, I take the lid off and read them again.

Voila! All are at =/- 1% of 75% humidity and I can live with that!
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Diane Kauffmann
Country Roads Guitars
countryroadsguitars@gmail.com
jread
Posts: 560
Joined: Wed Feb 13, 2019 9:52 am
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Re: Let's Talk Calibration!

Post by jread »

thanks for sharing! I got 3 identical, new hygrometers and calibrated using your method. They were all within a % but I trued all three. These all match the RH controller on the dehumidifier

Then this darn Boveda for Music | 49% RH Humidity Control just doesn't want to cooperate. I suspected that it was raising the RH in my case well above 50.
It seems to be a 62% RH pack if you ask me because that's what my case was and this plastic bag sealed in an airtight crockpot (dry).
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Diane Kauffmds
Posts: 3246
Joined: Sun Mar 09, 2014 8:13 pm

Re: Let's Talk Calibration!

Post by Diane Kauffmds »

jread wrote:thanks for sharing! I got 3 identical, new hygrometers and calibrated using your method. They were all within a % but I trued all three. These all match the RH controller on the dehumidifier

Then this darn Boveda for Music | 49% RH Humidity Control just doesn't want to cooperate. I suspected that it was raising the RH in my case well above 50.
It seems to be a 62% RH pack if you ask me because that's what my case was and this plastic bag sealed in an airtight crockpot (dry).
If you bought that from Amazon, I'd send it back. If not, contact the manufacturer.
Diane Kauffmann
Country Roads Guitars
countryroadsguitars@gmail.com
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