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 V.com 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) ;-)
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