Calibrating Hygrometers - Luthier Tips du Jour Mailbag
In this episode, luthier Robbie O’Brien demonstrates how to make a simple sling psychrometer and use it to measure the relative humidity in your shop, and how to calibrate hygrometers.
Video Transcription
[on-screen text reads: Luthier Tips du Jour Mailbag]
Mailbag question: How can I be sure that my digital hygrometer measurement is accurate?
Robert O'Brien: Today's Tips du Jour Mailbag question comes to us from New Jersey. "Robert, I'm using a digital hygrometer in my shop to measure relative humidity. How can I be sure the measurement is accurate?" Paul in New Jersey.
Paul, first of all, congratulations for realizing that when building an instrument, relative humidity is very important. The materials that we use to build the instrument will swell when they take on water or will shrink when they release that water. Therefore, it's important to control the RH levels or the relative humidity levels in your shop as you build the instrument. You want a relatively constant level of relative humidity. Now, let's talk about what exactly is relative humidity. Water vapor is the gaseous invisible form of water in the atmosphere. It's also known as humidity. When the air in the atmosphere contains a large amount of water, we say it's humid. When the air in the atmosphere is void of moisture, then we say it's arid or dry.
If the air holds the maximum amount of moisture, then dew or frost will begin to form, and or droplets of water will begin to form as clouds. If the clouds become saturated with this water or this moisture, then it begins to fall to earth because it can't contain it all. This is known as snow or rain, and this is a hundred percent humidity. Relative humidity is the amount of moisture in the air relative to the amount of moisture that can be in the air at a given temperature.
Now, let's talk about ways to measure this. The most accurate way that I know of is to use what's known as a sling psychrometer. Let's go over to the bench and I'll show you what this instrument is, and I'll show you a very easy way to make one.
How to make and use a sling psychrometer
So here's a very simple sling psychrometer that I've made. Now, a sling psychrometer consists of two thermometers, and I've got two that I've just taped together here. On one side I have a dry bulb. On the other side, I have a wet bulb. And all I've done is just wrap some cloth or some gauze around there, and then I can add moisture to it. I then put the two thermometers taped together onto a dowel, and that way it's able to sling around like this and take the measurement. Now, when water in the air evaporates, a certain amount of heat is required to convert the air into water vapor. Therefore, a cooling effect takes place as the water evaporates. The dry bulb thermometer takes the reading of the surrounding air, while the wet bulb thermometer here on the other side records the amount of cooling that is required for the water to evaporate at that specific temperature.
If the air is very humid, the differences between the dry and the wet bulb thermometers will not be very large because there's not very much evaporation. However, if the air is arid or dry, a large amount of evaporation takes place between the two measurements, which causes a cooling effect on the wet bulb thermometer, and the differences between the two thermometers will be greater.
So let me show you how I use this. So I'm just going to take a small pipette and add some moisture to the cotton or the gauze that surrounds one of the bulbs on the thermometer.
Once that's done, I'm going to start slinging it around to take the measurement. So now I just take my two thermometers that have been attached to my dowel, and just start slinging it like this. And you want to do this for a good 30 to 45 seconds, maybe even a minute, so that the wet bulb, the moisture around it has time to evaporate. As the evaporation occurs, then the thermometer on that side, the temperature should be dropping. When I stop, I'm going to make a quick note of what each thermometer reads as far as temperature. Okay, it's been about 60 seconds. So now I'm going to immediately read what the wet bulb temperature says, and it says 52. And I'm going to look at the actual temperature in my shop, which should be the dry bulb side, and it says 62. Now, what do those two measurements mean?
Using a psychrometric chart
In order to decipher this or interpret these measurements, I need to go to the internet and download what's known as a psychrometric chart. Those are easily available. A lot of weather sites have those things. Now, the barometric pressure, in other words, the altitude that you're at also comes into play here, and I'm sitting at about 5,300 feet above sea level. So that's going to give me a lot different measurement or a lot different reading than if I were at sea level. So when you go to the internet and download the psychrometric chart, download one that takes into consideration your actual height or your altitude above sea level. I'm at 5,300 feet above sea level, so I've downloaded one that will take that into consideration. So there's some pretty fancy psychrometric charts out there. This one is relatively easy to read. It's got your wet bulb temperatures across the top and your dry bulb temperatures across the left side here.
So my dry bulb temperature was 62 degrees, and my wet bulb temperature was 52 degrees. And where those meet, it tells me that I have 45% relative humidity in my shop. So I've shown you how to make a very simple sling sling psychrometer, and you can use this to measure the relative humidity in your shop. You can also use that to calibrate your hygrometers.
There's also another way to calibrate your hydrometers, and that's using ordinary table salt and water. Now, an ordinary table salt and water mixture will stabilize or normalize at 75% relative humidity. So let me show you how to use table salt and water to calibrate your hydrometers. So here's a variety of hygrometers that I have in my shop. This one here is reading 23% relative humidity. This one is reading 44% relative humidity. That's a huge difference. I really would like to know which one is the most accurate.
How to calibrate your hygrometers
So let me show you how to calibrate your hygrometers. So I'm going to start just by taking an ordinary cap off a bottle of water and filling it up with salt. This is just normal table salt. I'm then going to add water to the mix [Robert uses a small plastic pipette to add water to the cap]. Now, I don't want to saturate this and get it all soaking wet. All I want to do is just have it kind of like a wet sand. You don't need much more water than that. Once that's done, I'm just going to take it large Tupperware container here like this and place this in it. Then I'm going to place all of my hygrometers inside.
You could also use a Ziploc bag or something, but you want it to seal completely. This is a very tight-fitting lid, so I should be fine with this. Now, I'm going to leave that overnight. Some people say to leave it as much as 24 hours, but usually about eight to 10 hours or overnight, it's probably good enough. So when you come back tomorrow, you can open the Tupperware or open the Ziploc bag, wherever you've stored your salt mixture and your hygrometers. And if your hygrometer is calibrated correctly, it should read 75%. If it doesn't, then you can recalibrate your hygrometers or you can mark on their plus three or minus three or whatever it needs to be. So you know when you read that, add three or subtract three to get the correct measurement. So Paul, congratulations once again for realizing the importance of controlling the relative humidity in your shop. I've talked about this a lot in my videos because it is important. Now you have an accurate way of measuring what the relative humidity is in your shop, and now you have an accurate way of calibrating your hydrometers.
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