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I read online somewhere that to calibrate your hygrometer, you place it in a sealed bag with a cup containing 1/2 cup of salt mixed in 1/4 cup of water, then after 8-12 hours it should read 75% RH. Is 75% really what it should read? I put three of my hygrometers in the bag at the same time and they read 66, 66, and 68. I was wondering if perhaps they are supposed to read 65%, not 75%.
Does anyone know?

I normally put salt about 1/3 of a medium/small coffee cup. The water should be added until all salt is wet, but it does not make a lake on top of it. Think wet sand at the beach. You can pour out the excess of course. After it is sealed, you should wait several hours, 8 -12 best to make sure it is settled. Also not sure about 3 in a bag, I am testing just one with as little air as practical. I don't squeeze it or anything, but theres not a half room in there either. It should read 75% in the end...If not, adjust the hygro to read 75, but make it real fast. Once back in dry air, it goes down at a visible rate. If you can't make it fast, better to record the difference and make the adjustment a few hours later when the hygro adjusted back to the room level...

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Build log

The laws of physics and absorption will prevail and the salt & water calibration process is fairly accurate though not absolute. I think the sling psychrometer is the best and most accurate gauge. Here is a link to various types of calibration procedures:

http://www.theweatherstore.com/cayohy.html

http://asd-www.larc.nasa.gov/SCOOL/psychrometer.html

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Peter

What Peter said AND some digital hygrometers will specifically say in the instructions that the salt test will ruin them...... My Caliber III's say this in the instructions.

I did the salt test with a Stew-Mac digital and it was half a soda bottle plastic cap with salt and several drops of water, nothing like 1/2 a mixing cup.

The other issue with digitals is that you can see how they do in one range, like say 75% but that does not mean that they will be accurate in another range, say 50%ish.

There is an excellent thread in the archives from a couple of years ago where I did the salt test and it took a couple days for the stinkin thing to reach 75%. I'll try to find it for you.

About a year ago I bought an Aberon hygrometer because I have too much at stake to be guessing with these plastic fantastics digitals and low and behold my results were all over the place..... A month ago I sent in my Aberon for calibration and when I compare it to my digitals now the digitals are slightly off.

What Peter suggested is the only way to really know for sure.

After following the link Peter gave, how come the sling psychrometer isn't used to check shop humidity? Just a pain in the rear?

Secondly, it mentions not putting the precision hygrometer outside as it will be exposed to airborne dust and contaminants that can affect its accurace. Won't the dust from a shop environment (excluding Hesh's shop ) have the same affect? Is that one of the reasons for recalibrating?

Darrin

Better than a sling psychrometer is a powered psychrometer. The setup is the same, with a dry bulb and a wet bulb, but you draw air across both evenly with a dust collector or a shop vac. This is what I do in my shop, and it takes only a couple of minutes.

I have a digital hygrometer as well for quick reference (really I should have at least 2) but I don't trust it. I always double check with my psychrometer before any critical glue-ups.

By the way, I bought my psychrometer from McMaster-Carr for about fifty bucks. It works great, has a convenient chart attached, and saves the hassle of making my own. Just make sure you draw air across in a powered fashion. This will be more accurate than the method suggested in the instructions (which aren't very good at all).

-Clint

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-Clint Beacock

Clint my friend thanks for that! I think that I would benefit from doing what you are doing with the powered psychrometer.

The cost of sending my Aberon back a couple of times a year for calibration is about $30 a pop so I could use a psychrometer to calibrate my Aberon and it would pay for itself pretty quickly.

I checked out McMaster-Carr and I am not sure which model you have. Would you please be so kind as to let us know?

Thanks

The site is http://www.mcmaster.com/. The part number is 3970K9. For anyone who's never dealt with Mcmaster, they have an amazing array of stuff, and the quickest service/delivery i've ever seen anywhere.

-Clint

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-Clint Beacock

I have a digital type , but find my homemade one to be more accurate and reliable than any.
It's made using 6 strands from my daughter's hair ( 9 inches long ). The same principle as the most expensive out there. My cost ,,,, 5 cents
Human hair is found to be the most reliable medium of measuring humidity.

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CRAIG LAWRENCE of AUSTRALIA
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That last one is sure interesting! Thanks for all the replies. I took my hygrometer out of the zip loc and put it in tupperware and sure enough, they were all within 1% of where they should have been. Close enough for me!
Thanks all...

Craig, that looks interesting. Are the graduations 10% intervals? Can you give us more details regarding the underlying principles of your system?\

-Clint

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-Clint Beacock

The same principle as the most sensitive ( and expensive ) hygrometers used in labororatory situations etc ( human hair )
The thread of hair reliably increases and decreases in length with humidity changes. It's been found to be the most reliable and consistent medium known to man ,for measuring humidity .
I suppose I could make a more swish model with a longer indicator and larger dial with more increments , but I haven't bothered . I don't think we should concern ourselves too much about building to an absolute and exact humidity level . A 'Number ' ,if you will. Once the instruments leave the workshop into the real world they'll suffer all sorts of humidity changes . A few percentage points whilst building isn't going to change things much . I'm happy with between 40 and 50 % .I'm reckoning that those early master builders did about the same .

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CRAIG LAWRENCE of AUSTRALIA
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That's interesting, Craig. Do you happen to have any instructions or links on how you determine the graduations. I'm assuming that it's related to the 9" hair length. Presumably, you'd want to 'calibrate' it (which in your case is where you stick the tape ) beginning with an accurately known reading from a trusted hygrometer?

-Clint

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-Clint Beacock

You can make your own using 2 scientific grade, mercury-filled Celsius thermometers for less than $10. http://www.4physics.com/catalog/product_info.php/manufacturers_id/14/products_id/280

Pretty simple ...and cheap.

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JJ
Napa, CA
http://www.DonohueGuitars.com

Here's my less-than-$10 Psychrometer. One thermometer measures the ambient air temp while the other measures the evaporative cooling of a wet gauze covering placed over the bulb. It was positioned next to a fan and the temp was observed after a few minutes.

In this case, dry bulb temp was 19*C and the wet bulb read 13*C. Read the chart located here http://www.eduplace.com/science/hmxs/es/pdf/5rs_3_2-3.pdf In this case the differential is 6*C and therefore the RT = 51%





You can spend $50 on the commercial instrument or about $8 for two thermometers. Actually you can even get by with only one if you want to save even more.

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JJ
Napa, CA
http://www.DonohueGuitars.com

Back when I took my Thermo class, we made one by tying a thermometer to a piece of string. Record the temp then tape damp gauze or cotton around the bulb and twirl the temp probe around while it cools the bulb during evaporation. Once it stabilizes you have the "wet bulb" temperature. Use the chart already linked to determine the relative humidity.

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Formerly known as Adaboy.......

Essentially the same thing Darryl. With the sling psychrometer, you supply the moving air by twirling the thermometer. In my case, the fan supplies the air movement. Both methods speed up the rate of evaporation...both methods use the same chart.

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JJ
Napa, CA
http://www.DonohueGuitars.com

JJ bro where do you get the thermometers? I went to the local drug store, two in fact, and all they had were oral and, well you know.... and they looked much shorter - thank heavens......

Assuming you don't need an oral thermometer, the ones at the site where I recommended are large enough for someone with your reputation! Enjoy!

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JJ
Napa, CA
http://www.DonohueGuitars.com

Hesh, old pal, JJ had a link in his first post in this thread to the thermometers on line. Boy are they long !

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Waddy

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