Official Luthiers Forum!

Here is the latest Luthier Tips du Jour video - In this video I talk about how to calibrate your hygrometer.
This video as well as all my other videos are available via my website, http://www.obrienguitars.com/videos , LMI's website or on youtube.

Good information on calibrating your hygrometer

62° in the shop!! No wonder he is always wearing that sweater

Ed

Good stuff, as usual. Thanks, Robbie!

For those interested, there is another method explained by Mario in an old thread over on the MIMF. (Hey, Grumpy, we miss you!) I have a sling psychrometer and have employed the "salt method" but I find this useful as well: http://www.mimf.com/phpbb/viewtopic.php ... 583#p15583

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George :-)

I got a question for the sling psychrometer folks.

Years ago in a conversation w/ Bill Collings he said he gave up on slings because when ever he reversed the wet sock and put it on the other thermometer he always got a different reading. Anyone experience this?
As a result of that conversation I've always used Potassium Carbonate but always wondered if his troubles w/ sling psychrometers were just an anomaly.

If you switch the wet sock you have to swing the psychrometer backwards while using the opposite hand.

Just kidding.

I've never tried that and don;t know why it might lead to different results.

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George :-)

I tried using Mario's dew point calculation method several times, but found that in my hands the results were very sensitive to things like the ratio of ice to water volume in the container and the thickness of the side wall of the container. I found it difficult to get reproducible results with replicate measurements sometimes varying by as much as 10% RH. I wanted to be able to make that method work because it's fast and simple, but I wasn't able to get results I could trust. Others may have better results with it though.

I've been using the potassium carbonate method as described here: http://www.anzlf.com/viewtopic.php?f=1&t=6123#p69813. I like it because it is simple, cheap, and reproducible. The disadvantage is that it takes at least 12 hours.

The advantage of potassium carbonate over sodium chloride is that it equilbrates at 43% RH instead of 75% RH. Calibrating an hygrometer to 75% and then using it to measure RH in the ideal luthier range of 45% carries a risk since it assumes that the hygrometer's response is linear over the RH range. It should also be noted that wetting the salts is best done with distilled water so that you don't introduce additional salts that may affect the measurement.

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

I got one of those Psychro-Dyne psychrometer's for pretty cheap. That's a nifty little DIY set up but those Psychro-Dyne's are good instruments.

Sling method too sensitive to temperature measurement errors and fiddly, salt OK if you want your shop around 75% RH - linearity (as Jay mentioned) is poor in most less expensive hydrometers
Potassium carbonate method for me.
If it takes 24hrs, no worries - I have 3 in my workshop and another in the spray shop.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Just calibrated my three that I use in my condo. I used my psychodyne which is a dedicated, lab quality implementation of wet bulb testing only with a motor, fan, housing, cool lights.... etc. I do this every year and likely should do it twice a year... My results were telling....

Caliber III POS.... Digital 22% low...... this is the most drift I have ever measured note the "POS" designation... and YES it has a brand new battery....

$8 "BestAir" mechanical 1% low!!!!

$165 Abbeon that is my go-to hygrometer 9% low.

The Abbeon is adjustable and has been calibrated now. The Caliber III is getting pitched or shot, can't decide which yet.... and the BestAir is again one hell of a bargain and you can put these all over the place at this price. Think I may be buying some more of them.

Note: My remaining Caliber III is ten years old. Some of you guys swear by these things and at me when I post this stuff... but I have been telling you for years now that the digitals often rely on an electrochemical reaction where the chemicals break down over time. This Caliber III which is not adjustable at 22% off is useless.... but again it is 10 years old....

Something that is very important to note when calibrating and/or evaluating hygrometers is that many of them, particularly the digitals with an electrochemical reaction as the engine... may be reliable in a specific range and WAY off in other ranges. For example you may find with wet bulb testing that at 45% actual RH your digital may be 4% off but at 60% it may be 10% off. Range is an important consideration. Ideally we would want the most accuracy between 40 - 50%.

Part of the beauty of the psychodyne by the way is it addresses to some degree that sensitivity to humans breathing, air moving, etc. that Colin rightly brought up. Since it's incased it's less sensitive but still somewhat sensitive.

Lastly the Caliber III's that I've owned, tested, and given away have a specific warning against doing the salt test in the instructions. Don't know if this is BS to be a hedge against the "range issue" that I brought up or if indeed the salt test may be corrosive to the chemicals used for the electrochemical reaction.

And really lastly just got a big powered furnace humidifier installed and am looking forward to no water schlepping this year!!!! In prior years I ran three outboard humidifiers to do my home.... Looking forward to not having to do that anymore...

I do this with one lab grade thermometer.
1) take the ambient reading.
2) wrap a wet piece of cloth around the base of the thermometer.
3) draw air across the wet cloth using a shop vac until the temperature stabilizes.
4) take the wet bulb reading.

This very quick and easy and eliminates any error between the two thermometers.

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Jim Watts
http://jameswattsguitars.com

I picked up mine pretty cheap on ebay and use it yearly to calibrate my 2 Abbeons. Well worth the money. Since then I haven't bothered to get a new battery for the Caliber III.

Bob

That's exactly how I do it. I'm careful not too touch the thermometer with my hands, trying not to influence temperature readings. And it's useful to have a thermometer that has (at least) half degrees well marked (I'm using centigrade) in the scale.
In the end of the day, my 2nd hand abbeon is never more than 1% off.

I still do the salt test from time to time , just because of the nerdiness...

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member of the guild of professional dilettantes

Potassium Carbonate in the closed container is the way to go. The equilibrium is a humidity level that is relevent to the shop environment. Sodium Chloride at 75% is just too high. A hygrometer that is accurate at 75% (or calibrated to that) can be way out at 50%.

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Jim Kirby
kirby@udel.edu

???? I don't Follow Jim. If two thermometers respond differently to a test,getting rid of one won't make the other more accurate.

David, Of course you are correct in stating that getting rid of one doesn't make the other more accurate, but it can help to take the error out of a two thermometer set up.
If your using two different thermometers and one of your thermometers reads "x" degs for dry ambient and the other reads "y" for dry ambient then you will have a built in error due to the differences between the thermometers, which I believe is what your referring to in you reference to Bill Collings. Of course to get an accurate result you need an accurate thermometer, which you can verify by checking both ends of the scale with some boiling water adjusted for altitude/barometric pressure. and a glass full of melting ice.
Also it's more convenient for me as I don't have to have two thermometers which is actually why I do it this way.

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Jim Watts
http://jameswattsguitars.com

My impression was things can go wrong despite thermometer accuracy. I find reliable information about practical humidity testing hard to come by.

Has anyone with a wet/dry bulb tester compared results with a potassium carb test?

I hate threads like this, they tend to cost me money.

Just took my Abbeon in to work today to check it against Dave's psycrodyne because it bothered me how much my Abbeon had drifted in a year. Turns out my wick is not getting as wet as it used to and my readings were way off....

Getting older sucks....

[quote="Hesh"]Just took my Abbeon in to work today to check it against Dave's psycrodyne because it bothered me how much my Abbeon had drifted in a year. Turns out my wick is not getting as wet as it used to ...... and my readings were way off....

Getting older sucks....[/quote

Hey, who's is?

Alex

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"Indecision is the key to flexibility" .... Bumper sticker

Just noticed after 3+ years of salt testing, corrosion is showing up on the outside of my Abbeon. None in the previous 15+ years. I wonder what's getting corroded where I can't see.
Hmmmm.

A note to Canadian builders:
I found it difficult to purchase potassium carbonate for hygrometer calibration. The usual Canadian chemical suppliers refused to sell other than to certified compliant laboratories. I eventually found mine through eBay.ca from APC Pure, based in the UK. While the shipping by Royal mail was more than the chemical itself, it still worked out to be less than pricing quoted online from the non-cooperative Canadian suppliers. I'm very happy.


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