Reliable Hygrometer Calibration for the Luthier

[woodrat]

I though that I would open a thread about a topic that all luthiers should be interested in. I have been having a discussion with Dr Kevin Blake from the James Cook University in Townsville for a while now. Kevin is the Director of the Advanced Analytical Centre at JCU and is also a member of the ANZLF. We have been talking about humidity and its affects in lutherie and how to be sure that you have an accurate way to measure it. Kevin has done tests with Sling Psychrometers, Electronic weather data loggers as well as several hygroscopic chemical salts and come to the conclusion that using salts is the most accurate way to see whether your instrument is giving you an accurate RH measurement. He has used Magnesium Chloride, Potassium Carbonate, Magnesium Nitrate and Sodium Chloride. These salts give 33%, 43%, 54% ans 75% respectively and when used in an airtight container give a reliable means to be able to calibrate your hygrometer with accuracy. For lutherie purposes we would use Potassium Carbonate as the 43% RH it gives us is exactly in the range we want to be sure out hygrometer is accurate in. Below are some pictures of the tests that Kevin did in lab desiccators in the controlled environment at the University as well as some pictures of the results that I was able to obtain in my workshop using more readily available equipment...i.e. an airtight lunchbox with a clear lid. My results replicated Kevins perfectly and show that this is an easy way for a home luthier to be able to calibrate their hygrometer and enjoy the confidence of knowing that they are working within the parameters of building safety.
My setup,as I mentioned, was able to replicate what Kevin did in the lab . There are just a few things to keep in mind. Patience is necessary as you need to give the salt some time to stabilize the environment in the desiccator (lunchbox or similar). Distilled water is needed to wet the salt so that you don't introduce other salts that may be dissolved in the water that may skew the results. Put the salts in a glass or ceramic dish, preferably a shallow one. Do not use plastic for this.as the salts are exothermic when first wet, hence the need for something like glass or ceramic. Wet the salts out with some distilled water but don't add to much so that you have free water in the bowl. Try to spread the wetted salts out as much as you can to maximize the surface area available to the atmosphere. Put the dish in the desiccator with the hygrometer. Let it stabilize for 24 hours before you take a reading. One way to be certain that you don't have it too wet is to put some more dry salts in a different dish overnight and see if the measured RH comes down from the first reading. If it does then remove the dry salts and allow the wet dish to give up some more of its moisture to the atmosphere in the desiccator. After 12 hours it should be exactly 43%RH in there. Check your hygrometer and calibrate as necessary remembering that it is best to do this when the RH in you workshop is low as when you open the top to make the adjustments you will be handling it in air that is not controlled like you had in the desiccator. You should have enough time to make any necessary adjustment to the hygrometer with a small screwdriver on the calibration screw that is usually accessible through the back or the side of the instrument. The salts can be kept in the sealed container for the next time you need to calibrate your instrument. It would be a sensible precaution to calibrate it every 12 months.
Hopefully this information will help us understand the subject better and give confidence in using this essential lutherie tool

Here is a helpful link to a table that gives the RH of various salts

http://www.omega.com/temperature/z/pdf/z103.pdf

Pics of Calibration using saturated salts.

[Allen]

Very useful demo John.

As Kevin is the expert on this, perhaps he'll comment on whether the temperature that these calibrations are carried out may affect the results.

For instance, would you expect to see a variation on those numbers if they were carried out at 15C as compared to 30C.

[Tod Gilding]

Thanks John,

I remember a discussion with you , regarding placing buckets/bowls of Potassium Carbonate in the work shops controlled humidity area, to assist with keeping the humidity at 43% ,what are your thoughts on this now ? do think it would be worth the trouble and expense for an area larger than a lunch box ?.

[woodrat]

Hi Allen...we are fortunate in this respect with potassium carbonate as according to the table in the link in the post it only varies by 0.04%RH in temperatures between 0 Deg C and 30 Deg C...There is no data above 30 deg C on that table but that is pretty useful for calibration purposes...

...Hi Tod...I dont think that would be very practicable now that I have a better understanding of the subject....I remember mentioning it to you but it probably falls into the general category of my regular overoptimistic flights of fancy that one...

Cheers

John

[bdunne]

What would be a ready source to obtain said Potassium Carbonate from?

Brian.

[woodrat]

Hi Brian....I have not looked into that yet Brian. I got mine from Kevin but he is not in a position to supply it to any forum members. I punched it into eBay and there is lots of it available.

Cheers

John

[Jim watts]

I recently went through this and obtaining the salts difficult for me, with the exception of NaCl. So I ended up using dry bulb and wet bulb readings, not quite as accurate I'm sure but pretty darn close.

I have a long lab thermometer that's easy to read. I wrote down the ambient temperature, I then wrapped a small piece of wet cotton around the bulb using some cotton thread to secure it. Holding it at the opening of my shop vac while it was running I recorded the wet bulb temperature after it stabilized. I also checked our barometric pressure form our local weather station. The temperature dropped and stabilized fairly quickly, a minute or two. Using an online calculator I was able to get the RH from the two readings and the barometric pressure, or you can use a psychometric chart to read the RH.

I also have an environmental chamber at work and I was able to check my hygrometer against that not long ago and it was within 1%, so I think I'm comfortable with this method. But, if I had easy access to the salts, I'd probably do that.

I thought I'd throw this out there for people who don't have access to the salt. Hope it makes sense!

[Tod Gilding]

It Shouldn't be hard to obtain the salts...Only Some Salts.

as one that can be used is Sodium Chloride (ordinary salt) available at the supermarket, the only difference is you are looking for 75%

[woodrat]

Hi Tod, The reason that we focused on Potassium Carbonate is that it lets us know that our hygro is accurate in the most important range i.e. the range we want to build in. Sodium Chloride (NaCl) certainly does do the same job but only lets us know that our hygro is accurate at 75% which is of less interest than knowing our hygro is accurate at 43%. Also if you are going to use Sodium Chloride I believe that you should use non-iodised NaCl for the test for it to be accurate.

John

[Tod Gilding]

Yeah sure john , I realise that, but, I remember when we spoke about this ages ago I went off searching for a supplier and was unable to find anyone that could or would supply Potassium Carbonate in quantities small enough for calibrating a hygrometer.

I remember a company that would supply a 25kg bag, but would not post it and if they would the cost would have been out of the question for the intended use.

That said, there could be a new supplier now , but I have noticed that no one has yet answered Brian.

[Mike Thomas]

http://biomedcafe.com.au/cartpage/viewp ... 20-%20250g

This is where I got my potassium carbonate from.

[Tod Gilding]

Well there ya go ..Thanks Mike

Sure Beats buying 25Kg

[woodrat]

Hi Mike and Tod...I am not sure about that one....It says it is Potassium Carbonate on the label but in the text it refers to it as bicarbonate. Mike have you used it for the purpose we are discussing? We need clarification on this point as I have a hunch that it is the same thing.

http://en.wikipedia.org/wiki/Potassium_bicarbonate Potassium Bicarbonate KHCO3

http://en.wikipedia.org/wiki/Potassium_carbonate Potassium Carbonate K2CO3


John

PS...as for Brian's question...."What would be a ready source to obtain said Potassium Carbonate from?" ...

...we're working on it!...

[Mike Thomas]

I am pretty sure it is potassium carbonate John, but anyone wanting confirmation could check with the people selling it.

I bought it to test the Taylor hygrometer I got from you a couple of years ago. I have had a digital hygrometer for about 12 years, and the Taylor gave very similar readings, particularly in the 40 to 55 RH range. But they both could have been wrong, hence the potassium carbonate test. The digital hygrometer can't be calibrated, but amazingly, it showed 43.2% RH at 20 degrees c, using the potassium carbonate i.e. it was spot on. The Taylor was also very close, showing about 45% with the potassium carbonate. Since the Taylor can be calibrated, I wound it back a fraction.

Incidentally, the speed with which the Taylor hair hygrometer responds to changes in humidity makes calibration difficult if the ambient RH is much different from 43%. It's so quick that by the time you've got it out of the container, and got the screwdriver onto the adjustment screw, the reading's already changed.

[woodrat]

Hi Mike....Yes that is the reason I mentioned doing the whole thing in lowered RH. I do it in my dry room at the the same RH so it makes any adjustments easy as the needle is not running away...

[charangohabsburg]

I'm a bit late, but still: thanks for this detailed presentation John. I have read of that method before, but have never tried it. However, I like the idea. It would make the use of precision hygrometers quite redundant by using two cheap ones. While one hangs on the wall, the other one sits in the salt box, and then swap them every couple of days.

[Bob Connor]

I've just ordered some potassium carbonate to calibrate the two that I have.

The Taylor is reading 10% higher than the new one so it'll be interesting to see which one is reading inaccurately.

My money is on the Taylor being out.

Regards

[woodrat]

Hi Bob....I reckon you might be right!

BTW where did you order you K2CO3?

cheers

John

[Bob Connor]

I got it from the link that Mike Thomas posted above John.

Regards

[Luke72]

I know this is an old post, but has anyway clarified whether or not the potassium BI-CARBONATE (KHCO3)- (the link that Mike Thomas posted) can be used with any success instead of potassium Carbonate (K2CO3)? I have some Potassium Bi-carb so i thought i'd give it a go...and it failed. It came back with readings closer to the 70% mark and i know my 6 hygrometers can't be out that much! Maybe i added a drop to much water or maybe it just HAS to be potassium CARBONATE and not Bi-carb.

[kiwigeo]

If you do a search on hygrometer calibration Potassium Carbonate seems to the compound used.

I know this is an old post, but has anyway clarified whether or not the potassium BI-CARBONATE (KHCO3)- (the link that Mike Thomas posted) can be used with any success instead of potassium Carbonate (K2CO3)? I have some Potassium Bi-carb so i thought i'd give it a go...and it failed. It came back with readings closer to the 70% mark and i know my 6 hygrometers can't be out that much! Maybe i added a drop to much water or maybe it just HAS to be potassium CARBONATE and not Bi-carb.

[blackalex1952]

I set up a watch on eBay for a high quality hygrometer and got a you beaut vintage professional one from the US for an excellent price. Then I borrowed a wet/dry bulb thermometer, I could have knocked one up, but the local school science lab had one. Using that and an online chart for calculation which I cross referenced with an auto online calculator, along with several wet/dry bulb readings to minimise error I tuned it up. The wet /dry bulb readings were very accurate, as the multiple readings were consistent. A lot simpler than sourcing the right salts. Best to calibrate in the ball park of the RH you will use the most, eg 45%. Some luthiers have told me that 30% is good. I repaired an instrument which was living in coastal NSW recently, where the average RH is always rather high, so 50% was my level for that box rebuild. If I'm sending to a very dry climate, I go lower depending how finely the plates are finessed. Factory guitars which have been overbuilt are not as big an issue. This is only what I have done so far, I'm no expert, but that is my thinking. A friend who lives nearby on a high plateau had been making guitars in WA and moved with a few finished instruments assembled at 45% was recently dealing with his handmades that he had brought with him whilst setting up a new workshop where the RH was sitting at 15%! He was constantly setting buckets of water and boiling a kettle to keep them in storage!!! BTW. Thanks for "bumping " this thread, it is a valuable resource this one!-Ross

[Luke72]

https://youtu.be/XhlyzoeA_7E
Just saw this little obrien guitars youtube clip on making your own hygrometer. Looks like a pretty neat little thing. I'll definitely give this a go just for fun!

[blackalex1952]

The laminated veneer hygrometer is not very accurate and should really be taken as an indicator. It is quite possible to just seal one side of a single veneer to make one as well. But this is a great demonstration of some of the issues surrounding guitar construction and stability. I have double glazed sliding cedar external doors, big ones, at my house. On the instructions for the sealer that I use it is emphasised that the same number of coats of finish on both the inside and outside of the doors needs to be applied, in order to prevent the doors from warping, causing jamming or in the worst case scenario, the glass breaking. Because different species of timbers are used in guitar necks, it is also important to understanding neck movement. It has much to do with the different moisture take up of the timber species or kind of finish on each side of the neck. Different species absorb moisture at different rates, and with a neck, the fingerboard has no finish and the back of the neck does. And neck finishes vary as well. The finish isn't usually a great problem, but in some instances is an explanation of why one neck is less stable than another. Some makers put a sealer cote on the inside of the guitar to stabilise the moisture take up either side of the plates, in particular, the soundboard. With high quality responsive guitars, I think this is important, and so I seal the inside of my guitars with a couple of thin coats of shellac as a precaution. Also because I also am into Selmer Maccaferri style instruments, these were always sprayed with nitrocellulose lacquer both inside and out. The number of coats and finish thickness on each side of the plates is not so big an issue. But I feel a couple of coats on the inside of the box at least slows down the rate of wood movement, given that guitars are often packed into the case at high humidity level in one environment like a hot moist room after a gig, then transported to a lower RH climate eg a cold environment outside then later either left in the car or taken into a house or storage which creates quite a variation in RH that the guitar is subject to. The reverse is also true, eg pulling an instrument out of a case which keeps the instrument at one RH level into and environment which is drastically different. I have found a lot of variation in some instances, where the RH in a wardrobe is higher than in the house. I think it's important to avoid sudden changes in RH. This is one extra safety precaution. I also use humidifiers in the cases and use common sense, depending in my case, on the time of year and where the guitar is going to be used. If, when the RH where I live is relatively low and the instrument is being transported, say, to coastal QLD which has been sitting at high RH, I use the humidifier. I also use them when transporting by air as there can be dangerous rapid changes in RH throughout the trip. The case helps a lot to slow this down, but can also trap warm air, which when it cools reaches the dew point to where this causes condensation. With electric instruments the metal fittings usually oxidise because this activates the build up of acids from perspiration. I spend a lot of time freeing up the height adjustment and intonation screws of adjustable electric guitar bridges in my repair work. Particularly on the bass side where a players hand comes into more contact with the bridge. That's often where the screws bind due to rust and verdegris. This moisture activated acid also reduces string life and causes string intonation issues. That's why I always clean the strings with a cloth after playing, paying particular attention to the underside of each string as well. Crap on the strings also changes the slick feel of strings under the playing hand and can be a cause of fret buzz. A dirty fingerboard feels to me rather sticky too. I always wash my hands before I play an guitar. So understanding RH is not only important for soundbox assembly.-Ross

[blackalex1952]

I wrote an email to the company in the link to Biomed Cafe in this thread re the potassium carbonate. Here is their answer, copied and pasted from the email:
"Our potassium Carbonate is of food grade and I would suggest in your situation to obtain laboratory grade suitable for calibration."
So another commercial supplier of a small quantity of the stuff of laboratory grade needs to be posted, hopefully, here.
Given that the potassium carbonate is not polluted by the distilled water required for calibration, it's probably worth a try to see if the local high school could help with a "lend" of a small amount, or involve someone like a luthier either pro or amateur by testing in a classroom situation, ie a prac related to the school science teaching agenda. Also, given that the potassium carbonate is not polluted by the distilled water required for calibration, if I find some I could post it to anyone who wants to use it who contacts me, after I have used it. It might get to "tour" quite a few interesting workshops if the recipient users do the same!..I'm referring to people on a tight budget in Aus here.
-Cheers!Ross