Official Luthiers Forum!

Sure Don and range is an issue. Some chemical solutions (Potassium Carbonate is only one solution there are others) produce a RH of around 75% when our range is much less. It's not unusual for the digitals specifically and even some mechanical hygrometers that we have to tested to be deemed good enough in household ranges but WAY off outside of that.

With Potassium Carbonate the only result that you will receive is how your hygrometer does at around 43%. So what about 65% and 30%? Our hygrometers are tools for working Luthiers and as such they need to have some level of accuracy or we need to know the flaws if they don't in both directions of a desired range.

I can do what I showed here in RH of 10 - 90% and have accurate results. You can't do that with chemicals.

Our needs are different than yours too. We stick $10,000 guitars in bags with car wash sponges in them and need to see in the bag an RH of 75%ish so we can close cracks but not above that so we don't do damage. With wet bulb tech we can calibrate a hygrometer to be accurate at 75%.

There are other variables as well with chemical solutions and you can ask him about that yourself if you want I don't remember all his arguments but they were good ones. We like wet bulb tech it's simple, it's physics, it does not change with the day of the week and it is not range influenced either.

Yep absolutely true Jim. Dave Collins would balk and tell you that the test is very sensitive even to our breath and our bodies in close proximity to the measurements. He would argue that taking both readings at the same time is prudent but I see your point too and agree with you, it can be done with one.

I wanted to add I watched ebay for a while and found mine for $35. A bargain and I've had it for over ten years now.

We have a local builder in the area who produced over 250 guitars before he hung up his tools and stopped producing ****.....

His stuff is deeply flawed and the first 100 or so were produced with no RH control. Most of them have been taken to the landfill in that the economics were not there to fix half a dozen top cracks when a new crop would spring up next year if we fixed what happened this year. I have three clients who will never buy a small builder instrument again and openly say so to everyone who will listens because they feel like they were ripped off for $3 to $5K that they paid for their flawed guitar.

One of these was deemed not economically justifiable to repair when it was only three years old.....

So of course and without this being directed at anyone specifically it's my belief, my strong belief that before anyone should attempt to build a guitar they should put in proper shop controls so that what they produce is not a heartbreak for someone else some day.

Darrel I was pretty sure of my self that I would never sell my guitars but things changed and I sold close to 50 of them. You never know if your stuff needs to be ready for prime time or not so why not just use best practices. You know too I have to say this, you do whatever you want which is always my position too

Ken I am in the same climate as you are and I have always been able to do what it takes to have a shop nailed at 45% humidity. My first guitars were built in a suite at the residence Inn in Sunnyvale, CA. I had a dehumidifier and a humidifier in my suite. My next guitars were built in a spare bathroom shop in my condo in Ann Arbor. Same deal I took water out of the air or put it back in as required.

It's not easy, it's not cheap, it's required though if you want to produce anything that you wish to go to someone else at any point without being a problem for them. And to anyone who has any desire to sell an instrument that you produce if you do not do what it takes to build in a controlled RH you are being irresponsible and giving small builders and our industry a bad name when your stuff breaks someone's heart and cracks to pieces...... As a character on a British sit-com that I used to love used to say "I am unanimous about this!"

Again please know I have to always advocate for best practices but I do truly believe that job one in building any guitar is to do what it takes to have a stable and appropriate RH in your building area.

OK, I appreciate the elaboration. As with anything, everybody should use the tools they prefer. But honestly, I don't see anything in the above elaboration that should dissuade a builder who wants to control RH for building (as differentiated from a repair person who has to target an extremely high RH for a repair task) from using the Potassium Carbonate solution method for calibrating hygrometers. There's science behind that method, too.

I own a Psychro-Dyne also, by the way. I've had it for a few years. They can be found for varying prices on eBay.

Well, after dreaming up several schemes for building one I noticed your mention of ebay and realized I should look before I leap. Found a used Psychro-Dyne for $20 plus $12 shipping. Thanks for the nudge.

I hear and agree with you Hesh. In 20 years of building in this climate, no cracks and no stability issues other than a tweak or two and one easy neck reset. If the humidity was ever over 50% when building, I'd expect to see issues though.

The basement has been 40-45% all year for the past couple years that I've had the separate accu rite gauge. Before that, the dehumidifier was just set for 50%. I set it at 45% now.

It is very stable. The only real change is the slow change in temps from maybe 75 in August to maybe 45 in February. The ground is behind the air. It's usually between 55 and 65. HHG is a trick. I just glued cello ribs on the last couple days.

I don't run the dehumidifier all the time in the summer, If it takes more than a day to even get half full, I turn it off. It isn't a huge unit, just the cheap middle size one in a 1200 sq. ft. space. It's not on at all in the winter. It used to feel damp down there sometimes (20 years ago) but now it never does. But now the yard is the wettest that it has ever been. I haven't dumped the thing in 2 days, and it is only 1/2 full.

Apparently the water table doesn't have anything to do with it.

This morning it was 42% and 61 degrees in the basement.The dehumidifier is off. Outside it said 36 degrees and 70%. My kindle said that it was 80%, but there wasn't any frost, and not even any dew. In the house it is 67 degrees and 49%.

I just always expect the cooler basement to be more humid than the upstairs in the summer and the winter. But it isn't. So it never makes sense to me. Doesn't cooler air have higher RH?

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Why be normal?

With that climate, you probably don't need control at all. I'm always frustrated with humidity discussions because nobody ever seems to have any tales of guitars that were built too dry, yet demand everyone build at 45% despite that being too wet to survive winter in most of the USA without humidity babysitting.

Not to mention the lack of concern for hysteresis, which in my experience makes at least a 10% RH difference. That is, wood acclimated from high humidity down to 40% remains as wet as wood acclimated from low humidity up to 50%. That's one reason I use a wood strip hygrometer, because it shows the condition of wood in the room, not the condition of the air (at least for wood that's been here a while; anything newly brought in may be acclimated in the opposite direction from the hygrometer). It's also the reason newbies so often have braced plates turn into a potato chip despite the room humidity being the same as when they braced it. I have braced plates that have sat out for years in uncontrolled humidity, and they only flatten out during the extreme dry times when the outdoor temperature drops below 0, and then return to normal when back to bracing humidity.

Dennis, we are in the same part of the world.

I use the wood strip hygrometer to tell me its okay to brace. I use the sling hygrometer to tell me I need to check the humidifier or de-humidifier depending on the time of year.

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Say what you do, Do what you say.

We were using the Bovida two-way 49% packets for calibration the last year or so I was at Greenridge. We used 2-4 packets for a Tupperware tub large enough to hold both Lufft-made, Abbeon-branded analog hygrometers, the digital units used for tie-breaking and in the humidity chamber, and a remote sensor for the lab grade unit we were using for calibration.

With fresh Bovida packets from sealed containers, we did not see more than a percent difference after 24 hours with the laboratory 'chilled mirror' humidity sensor 'borrowed' from the engineering think-tank where one of the boys works versus the labeled value of 49%. I saw one of those units for sale on a used industrial equipment site and could never justify the cost ($6K!!!) given my shoe habit, but a used Psychro-Dyne at about $100 might just fit in the shop budget.

Any information on the difficulty of rebuilding a used Psychro-Dyne instrument, as well as where calibration services might be found would be useful.

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A constellation only takes shape when one maps the whole.
- Beth Brower

The formula for RH is:

RH = The actual amount of water in the air / the amount of water that the air can hold * 100

Temperature is not a part of this equation but it has an affect. If you raise the temperature than that given body of air can hold more water in it. Now your denominator is bigger so your RH value gets smaller.

So yes if you lower the temperature it has the opposite effect and the RH would increase.

When I first started building in a basement I had a real hard time getting the RH right. I'd say you are lucky to have a basement space that stays at 45% year round. Concrete floors typically let in a lot of moisture. The fact that this is Virginia might have a lot to do with it.

My shop now is sealed with spray foam insulation and it's very easy to deal with now.

Meanwhile, it’s spring in the southern hemisphere so I thought I’d follow Dennis’ suggestion and make a wood strip hygrometer to complement my mechanicals…

I can’t offer specific advice but I guess it will be as accurate as the thermometers are. No idea whether thermometers of the kind used in a psychrodyne branded unit drift or become inaccurate over time? I suspect they wouldn’t..

Don't know of any specific source but the devices are not very complicated. Simple electric motor and some batteries and a chambered area for the two thermometer bulbs. If you need anything that I can provide with specs or pics of mine I'm very happy to provide same. Mine has a few issues but still works fine. The motor is noisy as can be and probably needs to be cleaned.

Our SE Alaska rainforest is in a deep freeze and it's way DRY. Usually the shop/warehouse RH is 50-60% and now cold dry weather it's down to 22%. The top board on just about every stack has cupped as the exposed face dries over the wetter interior. I periodically flip the tip board over and it acclimates and settles down. Time to do it again.
Looking forward for our January thaw and warming temps to the 40's and a welcome rain.