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Hi All:

I recently found a good article on the calibration of hygrometers...
http://ezinearticles.com/?Prevent-Mold! ... &id=602308

I work for a lab and was able to purchase both Sodium Chloride and Magnesium Chloride Hexahydrate from a scientific supply company. I followed the method using the Sodium Chloride first and found the hygrometer was bang on, only about 0.5% low at 75 % relative humidity. When I used the saturated Magnesium Chloride Hexahydrate solution to check a second point, I found that it showed 45 % relative humidity when the solution should have produced a 33 % environment.

Obviously the hygrometer had been calibrated at the higher range from the factory.

My question centers around the fact that since we build at 45 % and the hygrometer was reading about 10 % high in the lower range. Will I run into any problems with the guitars built at what was probably closer to 35 % relative humidity? I'm assuming that I should now calibrate the hygrometer to the 33 % value produced by the Magnesium Chloride Hexahydrate since it is closer to the humidity range we use to build.

Any Comments ???

Greg

Greg buddy good job with the testing.

David Collins has in the past brought up the idea that when we calibrate our hygrometers it's important to calibrate them for the relative "workable" range that we operate within. For example a hygrometer may be well calibrated at 75% RH but off a great deal in the 45% range. This is why a wet bulb is a better calibration method then the salt tests.

In addition, even though I have done it too, many of the digital hygrometers will specifically warn you in the directions that people like me never read against doing a salt test. They say it can damage the electronics.

A guitar built in too dry an environment will fair better if it encounters a dry environment in the future. We like 45% because some of the standard bearers like Martin, Taylor, etc. have found that 45% RH is a pretty good range for where the majority of the guitars that they manufacture, speaking in terms of a bell curve, will end up.

But guitars that will end up in dryer or wetter environments and stay there would IMHO be OK built to a RH closer to the median RH of where ever that is.

One thermometer

One shop vac or dust collector

One sling psychrometer RH chart from anywhere on the internet

That's all you need to make yourself a dead accurate, vacuum powered sling psychrometer.....

Sling Psychromter?

That's a joke right? If it isn't it really made me laugh!!

Greg

Hey, pssst. Greg. grumpy doesn't make jokes on this stuff.

Search the archives. He has a post about it.

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now known around here as Pat Foster
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http://www.patfosterguitars.com

Didn't mean to mock you Grumpy

I really had never heard of such a thing. I just did a quick search for it on the net.

Thanks for the tip, I'll see if I can come up the materials required to double check the salt method.

Greg

Wasn't "Sling Psyco-hromter" a great movie with Billy Bob Thorton? I just thought that I would ax the question.....

If anyone else needs the tables for a sling psychrometer, Environment Canada has a good set of Tables

http://www.on.ec.gc.ca/skywatchers/logs_e.html

Look at the .pdf version

Greg

Hesh, you shouldn'a dun 'at. He's jes a boy.

A couple more points here:

What type of hygrometer are you calibrating? In MANY tests with my old Abbeons (easily found on ebay), a sodium chloride calibration at 75% agrees within 1 point of a sling hygrometer calibration at 42% - 45%. I would not trust any digital hygrometer unless it is lab grade. Even if accurate now, the cheap ones can go wildly off as they age.

When you run the salt tests, you have to make sure that there are no significant temperature variations during the course of the test. For example, if the temperature in the room drops 10 degrees (setback thermostat or whatever) then it will take the salt solution a while to re-stabilize at that temp.

Brook

Brook

I'm using a high quality mechanical hygrometer from LMI...

http://www.lmii.com/CartTwo/thirdproduc ... Hygrometer

I was very careful about temperature swing while I was performing the calibration with the salt solutions.

I just re-calibrated the unit using the Magnesium Chloride Hexahydrate solution (33 % RH) and I'll be setting up the Sling Psychometer tonight to check how they correspond. I'll post back tomorrow with the results.

Greg

Update...

Took a measurement with a vacuum-powered sling psychrometer (as Grumpy suggested) last night with the hygrometer calibrated to the 33% RH salt solution.

The psychrometer indicated a 49% RH and the Hygrometer read 40%. I also set up the hygrometer with the 75% RH salt solution and it read 67% after stabilizing.

I'm going to try re-calibrating the hygrometer with the 75% RH salt solution and again compare the reading with the psychrometer result.

I did find one reference from the Government of Canada that indicated the unreliability of sling psychrometer readings... http://www.hc-sc.gc.ca/ewh-semt/pubs/ai ... ces_e.html

"Sling psychrometers are inexpensive and simple to use; however, the results are unreliable. The instrument should be calibrated frequently against a primary standard, and the wick must be kept moist and clean. Powered psychrometers are more expensive but provide a direct and more accurate readout of relative humidity."

They don't say what a "primary standard" is, but do you think the vacuum-powered version of a sling psychrometer would be considered a "powered psychrometer"? Weigh in on this Grumpy, if you're out there.

I noticed that if the temperature differential between the wet and dry readings had been even one degree larger, the correlation between the two instruments would have been bang on.

Comments?

Greg

Yes, the shop vac pulling air through the wet bulb is indeed a powered psychrometer. The slings are unreliable because not everyone can spin them fast enough ad long enough to get the right readings.

Is your thermometer fine enough to read to a half degree? Mine are lab thermometers, and wile there aren't any lines in between say, 20 and 21C, I can easily see it in 1/4 degree increments. also, I re-wet the bulb after a minute with a little hair color bottle with room temp water in it. I let the wet bulb stabilize, write down the reading, squirt more water onto the wick, and let it stabilize again. If the readings are the same, I'm good to go.

As long as my hygrometers are within a few percent of the psychrometer, and the hygrometers read within one percent of each other, I'm happy.

Thanks again Grumpy:

I should be able to read my thermometer to at least 0.5 degrees maybe 0.25. The tip about re-wetting the bulb and taking a second reading is a good idea too.

Any idea what they're taking about with the "primary standard" reference?

Greg

No idea. I always assumed the wet bulb -is- the primary RH indicator. Thermometers are accurate and don't "drift" like a mechanical device can....

Here is an interesting article on the difficulties a manufacturing company discovered in trying to accurately measure RH:

http://www.veriteq.com/download/practic ... iences.pdf

Brook

Way to go on that article, that was very insightful...

Now I don't feel quite so much like an idiot about what I thought should be an easy process, after all...

"How hard could it be?"

It was also very helpful to know what is considered to be a "primary standard" for calibrating RH.

Great find, one for my archives. I might get in touch with Veriteq Instruments to see if they offer calibration services to the average Joe.

Thanks...Greg

We've been using an Extech digital psychrometer. We've been pretty happy with it.

I've been hanging my sling psychrometer in line with the exhaust of my air cleaner and that seems to work well. Also hung it in front and the results were the same. Very close to the Abbeon as you would expect. The cheaper digitals that have been around the shop a while are reading 12-15 percent low.
Terry

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It's not what you don't know that hurts you, it's what you do know that's wrong.

Update...

I re-calibrated the hygrometer to a 75 % RH salt solution and made measurements with the vacuum-powered sling psychrometer.

The psychrometer measurements indicated 50.5% RH and the hygrometer reading was 52.5 % RH. It looks like pretty good agreement even though the hygrometer was calibrated to the 75 % RH salt solution.

I performed duplicate measurements (as Grumpy suggested) with the psychrometer and averaged the two measurements. They were 13.75 degrees and 13.25 degrees, with the average being 13.5 degrees celcius. The dry bulb temperature was 19.5 degrees for a temperature depression of 6.0 degrees celcius. On the psychrometer charts this would correlate to 50.5 % RH.

The hygrometer is supposed to be accurate to +/- 3 % over the entire range of 0 to 100 % RH. So the reading from the hygrometer was well within the acceptable error for this device.

This should be close enough for the work we do, no?

Thanks... Greg