Let's design and build a better hygrometer, once and for all..!

[David King]

Once the fine condensate forms won't all those minute droplets want to joint together and form much larger droplets that will take much longer to evaporate? I'd think a strainer or teaball to pull the ice cubes out of the glass as soon as the surface has fogged over would be useful. With ice cubes in, the water will keep chilling until it get's close to freezing.

[Ron Belanger]

I really like Mario's approach, not only to this question but to all of his lutherie. Simple, accurate and not over-thought. Thanks for sharing your ideas and insights.

[Mario Proulx]

David, I like your "ice cubes in a strainer" idea! Right now, I scoop them out with a spoon, for the exact reason you mention.

Repeated tests(I can't help but play around with this...) have me seriously asking if the newly-formed dew/fog reaches the dewpoint temperature almost instantly. Everything I pull from the fridge is at 40(°C), yet within seconds, the fogged-over surface would read 45-46°. Since the shop was at the low end of the RH range, I cranked the humidifier up a notch, and within the hour, the readings jumped to 46-47°. The hygrometers had all risen to 43-44% by that time, also. And when I took everything into the house, the temp readings were all in the 40-41° range. Knowing that the house should be much drier(no humidification) than the shop, this was expected. What I didn't do in the house was the glass/ice/thermometer test; I guess I should try that, if for nothing else than to make sure that what I've come up with works in RH ranges outside of the luthier's workshop range. And for the sake of sharing all observations, the dew that formed on items in the house was "thinner" and harder to see, such that there may be a limit as to its use in very dry areas. But then again, if it's -that- dry in your workshop, without question, it's too dry! <bg> So on this note, I'd appreciate if many of you could try to replicate my findings, so that we can be reasonably certain that this method truly works, before giving it two thumbs-up. If it does work well for most others, then I'll make a clear set of photos and step-by step instructions in a new thread and ask the mods to sticky it.

Ron, thanks! KISS isn't just a cute phrase, but for me, my life goal. If nothing else, reducing any task/method to its simplest form at least eliminates as many possibilities for errors, and as mere humans, we're all prone to making errors!

[Mario Proulx]

Once the fine condensate forms won't all those minute droplets want to joint together and form much larger droplets that will take much longer to evaporate?

Correct. Note that when you pull out a cold drink on a hot day, it instantly fogs-over, and after a minute or so, droplets have formed and begin to run-down toward the bottom. The finer dew droplets join together, as you say, but what we want to measure is the temperature at the first sign of dew formation, which is the initial visible "fog" of fine droplets.

If I simply leave the ice cubes in the glass and don't stir it, dew forms at the top, first, because ice floats therefor the water cools faster up at the top, and if left alone without stirring, by the time the dew reaches the bottom of the glass, large droplets have formed at the top and are running down. As well, the temperature at the top of the glass will be several degrees colder than at the bottom. By constantly stirring, I can see the dew form across the full length of the glass, almost instantly. Best observed by having something dark-colored behind the glass, also. It's -very- interesting to see it all happen, because it does happen near instantly!

Again, a large, thin-wall beaker on a stir plate, with a suspended thermometer would likely be ideal, and I'd love to at least try such a setup side-by-side against the glass/ice/thermometer(GIT? <bg>) method, to see if the GIT method is flawed enough to warrant the expense and complexity of the stir plate and beaker. Anybody here work in a lab??

[Steve Senseney]

I appreciate the work you have put into all of this Mario!

[Bob Gramann]

I like the way Mario thinks. I still like really simple. I've just ordered some Potassium Carbonate. I'll report back after I run the experiment.

[Chuck Baleno]

For those that have not seen this; http://biomet.ucdavis.edu/frostprotecti ... fp003.html it confirms Mario's method, but uses a "shiny can" instead of a thin glass.

Chuck

[Mario Proulx]

Well I'll be...! I swear I didn't see that before coming-up with the GIT method!

Must now try the shiny can method... Metal is another very poor insulator, and might be a wee bit more precise, though at this point, methinks the GIT method is already as precise as the thermometers will allow, and plenty precise for us to use to check, and calibrate, our shop's hygrometers against.

[Murray MacLeod]

I'm getting terrified by this thread ...

I am in the process of building my first (acoustic) guitar and have absolutely no idea what the RH is in my shop. So I ordered three, reasonably inexpensive "air humidity meters" today from a very obliging German firm, which should arrive in 7 days, but I can't help wondering what's the worst that could happen ...obviously the RH is low at the moment, being winter and all, but when summer comes round and it starts raining ...will there be any adverse effect on the guitar (hopefully completed long before then) other than the soundboard raising and requiring a slightly lower saddle ? And maybe the fretboard will develop a slight backbow and I have to tweak the trussrod ...but other than that ...what ?

I do get that gluing up in a high RH environment can lead to serious problems further down the line, but vice-versa ??

[Barry Daniels]

Murray, Don't you think there is a reason why everyone has been talking about controlling humidity here for like...years? Yes, it is important. Yes, your situation is not as bad as the reverse but negative consequences are still possible.

When I got my humidity under control, my precision took a big leap. Wood plates stay flat. Braced plates stay radiused (per design). Neck setting was repeatable. Life is good. Seriously.

[Halgeir Wold]

Everything I pull from the fridge is at 40(°C), yet within seconds, the fogged-over surface would read 45-46°. Since the shop was at the low end of the RH range, I cranked the humidifier up a notch, and within the hour, the readings jumped to 46-47°. The hygrometers had all risen to 43-44% by that time, also. And when I took everything into the house, the temp readings were all in the 40-41° range. Knowing that the house should be much drier(no humidification) than the shop, this was expected.

Just a question...... 40 deg C from the fridge ?????????? Hopefyyly you mean F, - or I'd scrap that fridge

[Jason Rodgers]

So, a shiny pop can would fit that bill (diet Coke, or similar silver can): aluminum is a very good conductor, and would well-represent the temperature of the contents, right? Better than glass?

[David King]

What if you just took a chunk of polished silver, would that accomplish something?
I guess you'd still need to take it's temperature, hence the thermometer stuck in water.

[Kyle Upton]

Maybe this defeats the purpose since it is a purchased item. Extech makes a nice RH meter that is +/- 3% accurate. Grainger and other industrial supply companies carry them. I have used them in my work, and compared them to instruments that are calibrated and certified. They are usually within the 3% specified accuracy range. Also, a good HVAC supply vendor probably has them also. Unless you have a fairly sophisticated humidity control system, 3% should be close enough. We use RH sensors with our automation systems, and 2% is as accurate as we ever use, and those are for cleanroom type environments. For about $120, they are relatively inexpensive, accurate, and durable.

[David King]

The Extech RH300 is $94 from http://www.amazon.com/Extech-RH300-Humi ... B000BF5400 + or - 3% isn't anything to brag about but it probably is good enough for most of us.

[Mario Proulx]

Kyle, we all own multiple hygrometers(IE: RH meter, as you call it). But even a $200+ Albeon needs re-calibration every now and then, and checking/testing against a very precise standard, often(otherwise, how do you know where it is reading?). None will hold a reading forever. Measuring RH means measuring a gas, a thin gas, and it takes a very delicate device to do so, accurately, and especially in our dusty environments(our workshops are NOT cleanroom environments.. <bg>), they are all notorious for "drifting" out of calibration.

Not to mention that +/- 3% means an allowable difference of 6%, and we're attempting to maintain our workshops within a 5% range(42-47%). Let's say that the unit is reading 51%, and you think to yourself "that's fine, it's only a few percent over the ideal", but if the device you trust your work and livelihood to is reading low, then the actual RH could be anywhere from 52-57%. And a +/- 3% specified range is rare. Any device that is "certified accurate" will state very clearly that it must be sent back for re-calibration on a regular basis.

Now, you think 57% RH ain't too bad?

If you build a guitar at 57% RH, it stands a --very real-- chance of splitting, the fret ends will for sure stick out and slice the players hand, the action will drop, etc.... You'll have one very unhappy client, especially if he/she took good care to maintain a RH of at least 40% for the guitar, yet all of that happened to it. You're stuck with the repair bill, and your reputation just took a big hit. All because you trusted a device to be reading correctly, at all times. A device that everyone, but you, understands needs to be re-calibrated at least yearly....

I have perhaps $50 invested in all my hygrometers. I calibrate them often, and check them against one another daily. I have 3 in the shop, all placed together. As long as they're reading within 1% of each other, I can be pretty damned sure that everything is fine. When one of them begins to read different than the others, then it's time to check them, clean them, and recalibrate them. If you only have one, you have NO WAY of knowing, day to day, if it's reading correctly. I don't care if you paid $100,000 and it was personally blessed by the Pope, you have NO WAY of knowing that it is reading correctly.

Starting to understand?

[Mario Proulx]

David, read the last paragraph above.. <bg>

[Clay Schaeffer]

Hi Murray,
Humidity control is important, but humidity in a heated shop will vary throughout the day, as a function of the temperature varies outside the shop. Although it is nice to have precise readings and knowledge of the humidity you are building in, a more general knowledge and less precision still gives workable data. Instruments do not live in humidity controlled conditions, for the most part. Building in relative humidities of 40 to 50 percent generally gives the best results, and is what most of us shoot for. It won't save an instrument that is subjected to 0 % humidity or one that lays out in the rain, but for normal conditions gives a good chance for it's survival. Some days an instrument sounds great and some times not as good - it could be the humidity!

[Kyle Upton]

Mario,

I understand the need for accurate monitoring and control of humidity. My shop is currently without a means of humidity control so I really can't do much in the way of instrument building for much of the year. Of course, I don't make a living at it so it's an inconvenience, not a major problem like it is for a professional.

I agree with using multiple devices to compare so that you have confidence in your accuracy. I am much more familiar with the electronic RH monitors/meters/sensors than hygrometers or by measuring dew point. Assuming your hygrometers are within 1% accurate (and I have no reason to doubt you), what type devices are used to actually control the RH?

I mentioned clean rooms because that's what I'm familiar with. I work for a building automation company. Our systems are used to monitor and control humidity, temperature, pressure, etc in a wide range of environments. I have monitored and controlled humidity and temperature in operating rooms, clean rooms, and art museums, all with stringent environmental requirements. You are right about instruments drifting. With a full blown automation system, you can just make adjustments to the controller via a PC. Of course you still have to have a standard to compare against to know when and how much to adjust. I have found that the electronic devices such as the Extech or similar are really pretty reliable so long as they are treated properly. When the situation calls for really tight control, we do have to use certified instruments, and yes, these instruments do have to be re-certified periodically. I guess I'm saying I agree with you and I do understand the problem.

Because I have access to instruments that I have confidence in, I was offering a solution that may not apply to someone in your situation. If you don't access to instruments with up to date calibration certifications, I can see your point in using the old tried and true methods. I am curious about the actual means of control though. Do you use some sort of PID controller?

[Bob Gramann]

A few years ago, I bought a brand new Abbeon hygrometer ($125) with a calibration certification guaranteeing it to read within + or - 3% of truth. Against my psychrometer, it read 55% when actual was 42%. I have no idea how long it was in the box before I got it, but it had that certificate. I have two of them--once I calibrate them, they stay pretty true. I check them at least every year. I don't believe certificates. We need our own means of calibration.

I have several cheap electronic hygrometers including one advertised to be accurate. None of them are near true. The "accurate" one reads 24% when actual is 42%.