Violin case hygrometers: electronic and dial-type

December 8, 2012 at 04:00 PM · A hygrometer is a device which measures how much “drying-out power” there is in the air surrounding it, a.k.a. relative humidity (RH). String players want to know this because the instruments can suffer from air which is too dry, and a hygrometer, if it’s doing it’s job, tells you when you’re safe and when to worry.

Hygrometers are generally divided into two categories: dial-type mechanical and digital electronic. Mechanical ones use a salt-coated strip balanced against a spring to move a needle on a scale displaying relative humidity; consumer-grade electronic ones use a chemically-treated capacitor as a sensor and show the value expressed directly in numbers.

Dial-type hygrometers have been around a long time, and found their way into violin cases in 1984. Since then, consumer electronics have become cheaper and more miniaturized, and so now digital hygrometers are being installed in violin cases too.

Once would assume that a modern digital hygrometer would “obviously” be much more accurate that a mechanical one, being that they are a product of modern digital technology. However, as our resident expert David Burgess has proven, that’s surprisingly not the case at all. Consumer-grade digital hygrometers can show differences of 30%, and even more.

And that’s not all: hygrometer accuracy varies with changes of temperature and humidity. The same hygrometer can be spot-on at 70°F and 50% RH and be considerably off under other conditions. That’s because they tend to be influenced by changes of the absolute humidity (amount of water vapor in suspension, i.e. in grams per cubic meter). For this reason, even expensive certified-accuracy digital hygrometers state a declared accuracy at only one temperature, generally 68°F (20°C).

Fortunately for string players, this is not a real issue because violins are almost always kept indoors at room temperature, where humidity conditions rarely vary past 25-70%, of which we are only interested in the “danger zone”, i.e. 40% or lower.

Thus, all we basically need is a hygrometer that will tell us whether the violin is OK or if we must use a humidifying device, in conditions that we can generally define being between 60- 85°F (16-30°C) and 25-50% relative humidity. (If you live in an igloo or a steel mill, you may need to choose a different range). Both electronic and good, properly-installed dial-type hygrometers meet this criterion.

The advantage of dial-type hygrometers isn’t only that they won’t suddenly read “Low Batt” while you are on tour in Mongolia by horseback. They have a useful feature that almost all digital ones lack: you can calibrate them yourself to correct for inaccuracy. So I still think that for the safety of your violin, good dial-type hygrometers can be preferable, provided that a few simple steps are taken.

1) Defining the standard. When we say that a hygrometer is inaccurate, it’s inaccurate compared to what? We need a standard to compare it too. One method is to take several different electronic hygrometers (you can borrow them, but they’re also cheap to buy), set them up in your music room, and then average the results. Repeat the test when the air is dryer or wetter. In the end, select the hygrometer which gave the most consistent readings closest to the arithmetic averages, and then write on the back an eventual manual correction (say, +2%) to add or subtract from the readings. This hygrometer can now be your reference, and if kept in the same room without touching it or moving it around, it should provide as close as we can come to an accurate reading in that temperature range without using costly professional equipment.

2) Preparing the hygrometer for calibration. If you’ve purchased a dial-type hygrometer (to hang on the wall, set on the desk, or to install in your case) you can calibrate it to match the readings of the reference hygrometer. However, it must first be acclimatized to the surroundings. After you take it out of it’s package, or even out of another room, it’s a good idea to let it sit next to the reference hygrometer for a full day, to allow it to fully adjust to the same conditions.

3) Calibration. Dial-type hygrometers have a little hole on the backside, where you can insert a small screwdriver. Turning the screwdriver will turn the mechanism on the bezel, thus changing the position of the needle, so you can make it match up with the reading of the reference hygrometer. Tapping the “glass” with your fingernail will make the needle settle to the exact reading. Don’t handle the hygrometer too much while calibrating it because you’ll heat it; don’t breath on it either because that you’ll subject it to more humidity than the surrounding air and knock the readings off. It’s also a good idea to perform the calibration during an “average” day, not a particularly, cold, hot, wet, or dry one.

4) Installation. If you install the hygrometer in the case, make sure you don’t cover the little holes that allow air to flow freely through it. I’ve seen flush-mounted hygrometers in cases that are very elegant but don’t allow air in – the needle might just as well be painted onto the bezel. If you hang it on the wall or set it on your desk, keep it away from doorways or windows, and make sure that direct sunlight can never hit it. A hygrometer exposed to direct sunlight, even through a window, will quickly overheat and may require subsequent re-calibration.

At this point, you can use your calibrated hygrometer to observe the relative humidity which will be displayed relatively accurately. If it shows 45-50% or more, you don’t have to worry about the ravages of dry air; below those numbers a humidifier should be used, according to the relative instructions. Some hygrometers in fact do away with the numbered scale entirely, leaving just a safe-or-not-safe indication, which in the end is all you really need to know.

(Of course, if your hygrometer came pre-installed in your case, it’s probably not possible to remove and re-calibrate it, but one would hope that the case manufacturer did all of this already.)

Note however that if you keep your calibrated hygrometer within the case, it will measure the relative humidity there, not in the room, unless you leave the case open all the time. Some cases are in fact lined with hygroscopic materials (i.e. moisture-absorbing) to help maintain a stable microclimate within it, and the readings will reflect this.

Lastly, both electronic and dial-type humidifiers should be re-checked from time to time to make sure they are still working, as both types are subject to increasing inaccuracy owing to dust and dirt. A quick test to see if it’s working at all is to breathe directly onto it for a minute or so and see if the numbers go up.

Questions, anyone? (there will be a short quiz next period) ;-)

Replies (8)

December 9, 2012 at 02:31 AM · Dimitri,

Sorry but I kind of take issue with your method of primary standardization: Averaging readings obtained from a "random sample" of instruments.

By analogy, supposed you wanted to determine the population of Mexico City. You could walk out onto the street, ask the first ten people you see to estimate this quantity, and then take the average of those.

The problem with "taking the average" is that you have to assume that individual errors are evenly distributed about the mean.

Determining relative humidity "for real" is not a terribly easy thing to do in practice, but the old wet-bulb dry-bulb approach at least has a sturdy basis in theory and has the advantage that the measurement tools are thermometers, which themselves are fairly easy to calibrate.

The next problem is that calibration with a single data point is hardly a robust method.

Fortunately there is another possibility that is not necessarily absurd: Accuracy is not that important. Maybe it is enough to say, if your lips are chapped a lot and you find you've slipped a peg more than once in a week's time, then you need to put in your Dampit or set up a room humidifier where you keep your instrument. Maybe the little gadget is really just a gimmick.

December 9, 2012 at 03:00 AM · I have a Musafia case, and there seems to be no way to remove the hygrometer for calibration.

However, reading the humidity really shouldn't take ANY kind of gauge. Is it really that difficult to feel that it's dry or humid?

December 9, 2012 at 07:30 AM · To Paul: I simply indicated “one method”, which I think is the easiest and sufficiently accurate, and explained how to do it. There are better and more sceintifically accurate methods but they are more complicated, and in this case there is the Law of Diminishing Returns because accuracy as I mentioned isn't really that important. I personally use a professional certified hygrometer for this purpose, but not everybody needs one! BTW I don’t quite understand your analogy about population estimation, unless the ten people you stop on the street are all demographers.

To Scott: most experts agree that a hygrometer in the proximity of your violin is a good idea. They even put them in cigar cases! RE your own case, please contact me privately.

December 9, 2012 at 09:03 PM · Dimitri, I have tested "certified lab-grade" hygrometers which were way off, compared to both sling psychrometers, and saturated salt solution calibration (which I currently believe is the most accurate). Don't want to mess with you too much beyond that, because I think you're trying to do a good thing.

One reason why I originally endorsed, and then abandoned dial-type hygrometers:

The laminated material controlling the dial will change according to how many times it is flexed. Think of it like hair spray application. Your hair is really stiff at first, but after you run your hands through the hair a few times, it changes.

I tried cycling the dial-hygrometers multiple times to get them to hold a good reading, but finally gave up.

Not that there aren't also huge challenges with the digital units. For reasons I don't yet understand, they need to be powered up for several days before their readings will start to stabilize. After that, the calibration process starts.

Go forward to the first battery replacement on a calibrated digital unit, and maybe as much as 5% accuracy may be lost.

But most of that is beside the point. I think a lot of us in the fiddle trade are doing the best we know how to do, in the area of fiddle preservation, and your contributions are much appreciated.

December 10, 2012 at 04:53 AM · Dimitri, the point of my analogy is that if you take the average of ten things you know are flawed but don't know why, then you will end up with an average that is hardly more defensible than one of the individual measurements alone.

I think I maintain my violin's environment pretty well and honestly I don't look at the little dial in the case because the chance that it is correct is just so vanishingly small. I use the methods I outlined in my previous post.

December 10, 2012 at 06:44 AM · I have two digital hygrometers/thermometers that agree with one another within about 1ºC and 1 or 2% humidity. They also agree on temperature with an alcohol thermometer I have, so my guess is that they are close enough to get a good idea of the environment my instrument is in.

Case hygrometers, on the other hand, don't seem to work at all... most of them don't seem ever to change their reading even a little bit. That includes the hygrometers in my two Musafia cases; I love those cases, but the hygrometer seems to merely be a decoration.

December 10, 2012 at 08:49 AM · Katherine, I ordered two batches of digital thermometer/hygrometers from the same manufacturer, about 20 each time. The hygrometers on the first batch were within about 2 percent of each other, with average readings 5 percent low. The second batch were within about 2 percent of each other, with average readings 15 percent low.

These were a type which couldn't be recalibrated, and I didn't think I could sell the ones that were 15 percent low. Anybody want some inaccurate hygrometers really cheap? LOL

One reason I think accuracy is important is that it takes energy to humidify and dehumidify. If you take the humidity 15 percent lower than you need to with a dehumidifier, it will use a lot more electricity.

Another reason is that if the humidity is actually higher than you think it is (say 80% rather than 60%), you're getting into the danger zone where instruments permanently distort under the string pressure, like the fingerboard height dropping over time.

The thermometers are usually quite accurate, but it doesn't seem to be related at all to the accuracy of the hygrometers.

December 10, 2012 at 11:58 AM · Thank you, David, for adding your input. The thought of possible “fatigue” of the movement had occurred to me too.

Slightly over a year ago I initiated a test of five calibrated dial-type hygrometers which I decided to torture far beyond normal music room conditions: keeping them on my dusty workbench would have been bad enough (!) but I also routinely expose them to direct sunlight, bang them around rather uncerimoniously, put them outdoors in the elements and so forth. To date they have gone through temperatures between –5°C (28°F) and 37°C (99°F).

Just last week I left them out in the courtyard overnight, in fog (100% RH) and sub-freezing temperatures. After bringing them back to my workshop at room temperature, within three hours all five hygrometers returned to within 5% of the correct ambient value. (I can post pics if anyone is interested). This is a year and a week after I calibrated them.

I should also add that these five hygrometers weren't all from the same batch: they were hygrometers that got scratched or had other aesthetic issues for which they never got installed in cases and started to pile up on my workbench. At least I've put them to good use!

A year-long test isn’t such a long time – cases are supposed to last much longer than that – but, hey, it’s a start. The test will be continued indefinitely: I’m curious to see how long the hygrometers will last under this kind of stress, and what may eventually happen.

Just to clarify, by posting this I’m not trying to prove anything to anyone, I’m simply sharing my experiences with those who may find them useful or interesting. To each his own.

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