Analog Hygrometers - how?

You may have noticed that every time I say “it’s time for a simpler video” it’s a lie. For the record, it doesn’t start as a lie it’s just… well, I start writing the script at the beginning. Seems to make sense to do it that way. And so, I’ll say “ah, this is a simple idea! It’ll be simple! Simple!” and then it just snowballs into a five part series. Not today! We are just gonna talk about this thing. Back when I made the humidifier  video I showed one of these. It’s a basic hygrometer. It has a

dial and a pointy bit and what the pointing bit points to is, roughly, what the ambient relative humidity is. Simple. Some of you asked how this works. And honestly, yeah that's pretty intriguing, isn’t it? It doesn’t take any batteries, and if you look through the back you can kind of get a peek at a coil-like thing. That looks an awful lot like a bimetallic strip! What’s a bimetallic strip, you ask? It’s a strip made of two metals. That’s useful because different materials  have different the

rmal expansion properties. Virtually everything expands a bit when it gets  warmer, and metals are no exception. But how much they expand depends on the specific material. When you take two metals that are not the same and sandwich them together in the form of a strip, the mismatch in thermal expansion coefficients between them will cause the strip to bend slightly as the temperature changes. Bimetallic strips are in all sorts of  places, from the world’s greatest toaster, to blinking Christmas

lights, to the thermostat of space heater, to, well, thermostats in general. But a classic and simple application of the bimetallic strip is in a thermometer like these. And let’s just get it out of our systems, yes these numbers are meaningless to many of you out there and maybe one day we’ll stop using this temperature scale ‘round these parts but I can’t think in Celsius any more than you can think in Fahrenheit and besides, a zero to one hundred scale that  correlates well with the range of

tolerable ambient temperatures is a more pleasing way to do it from my perspective and - hold on this is a tangent, now. I want to show you something. See this? This is called a search engine. There’s all sorts of them out there these days. And most of them let you do this: 70 f to c. Blam. There it is. You have a smartphone, right? It probably has a search bar in a very convenient place. Oh and maybe you even have a voice assistant you can literally just ask what whatever temperature is in what

ever other scale you like better. It really is that easy. Now, when it's appropriate, I’ll continue to provide conversions here on this YouTube channel because it would be careless of me not to, but when I dare to offhandedly tweet something and use one of our silly units I don’t wanna hear it. You’ll spend way more time composing your complaint to me than you will just googling the conversion yourself. We are all very aware of how silly the aversion  to the metric system is but we can’t help it

. Oh, and yes I am on Twitter. It’s mostly puns and other dad jokes. And snark like this! OK, let’s get back on track. In a thermometer like this, the bimetallic strip is formed into a coil with the outside end secured in place and the inner part of the coil attached to a pointer. Although the amount the strip bends  when the temperature changes is slight, forming a coil compounds that effect with each rotation. You end up with a coil that contracts as it  gets colder, and expands as it gets war

mer. Or potentially vise versa. And so, the center of that coil rotates somewhat as the temperature changes. Attach a pointer to the center of that coil, and with a little experimentation and calibration you’ll have created a simple device  which indicates the current temperature. The effect is surprisingly linear. The spacing between these numbers is consistent through most of the travel, although you can see it vary somewhat at the ends. So, if we take this thing apart through The Magic of Buy

ing Two of Them we see what appears to be two bimetallic strips. One for the tiny, mostly useless  thermometer they give as a token gesture, and the other for the hygrometer. But this one is actually a bimetallic strip tease. Sure, it’s essentially the exact same idea, but how can two dissimilar metals give you a humidity reading? Well, they can’t. In fact not bimetallic, but it is bimaterial. Take a closer look at it and you’ll see that one side of the coil appears to be covered in a sort of pa

per. And that’s because it is. This style of hygrometer, fittingly named the Metal-Paper Coil hygrometer, functions identically to that of  a bimetallic thermometer. The key difference is that the salt-impregnated paper changes based on humidity. This is one of those things that seems puzzling at first but then absolutely obvious once you know it. Like, you know what happens to paper on really humid days. It gets all floppy and gross. Same sort of thing goes on here. The paper side of the coil w

ill  deform slightly as humidity increases, changing the overall tension on the coil. That makes the needle move. Watch what happens when I put a bit of water directly on the paper. The coil quickly deforms and the needle shoots to “very damp.” And here you’ll notice that the scale is very much not linear. Low humidities are far apart, indicating that the effect of humidity on the paper diminishes as humidity increases. And this clock is why I decided to make this  little video. I bought it when

I bought the other clocks for the last video because it would actually be useful to me and I had a perfect spot for it outside. But as soon as I realized the humidity scale was linear, I got suspicious of this thing. Sure enough, it’s crap. It agrees with this one around 50 percent or so, but this won’t ever read higher than maybe 80 percent? This one read 90 and this only read 75. No good. In fairness, metal-paper coil hygrometers aren’t super accurate to begin with. This website I found says

they’re only good to about 10%. And it’s not like I could expect the hygrometer in a $10 Walmart clock to be any decent to begin with. But if you ever see a linear scale on a hygrometer like this, it’s definitely not one of the better ones. OK, well probably not - I haven’t yet tested this one at the time of filming. I’ll pin a comment with the final verdict. This melding of paper and metal isn’t the  only way to go about it, of course. In fact, it seems to be a rather new-fangled way to do it,

presumably it’s just very easy to manufacture inexpensively. All you really need to make a hygrometer like this is some sort of spring that’s pulling on something else which will deform based on humidity. The first hygrometers that worked on this principle used actual human hairs. The hair would lengthen with increased humidity, reducing tension on a spring or pulley, and thus allowing an indicator of some kind to move. As it dried out, the opposite occurred and  the indicator moved in the other

direction. This style of hygrometer is still made, but  it’s a rather specialist sort of thing. However, they can be quite accurate when calibrated, with one manufacturer claiming accuracy to within one percent at room temperatures! However, I found other sources which aren’t quite so optimistic. The worst thing about these cheap  hygrometers is their accuracy. While the hair-tension method  might be a little bit more accurate, it may still not be as accurate as you can be  with other means. Ot

her means? What other means? Well, the humidistats in the humidifiers I showed  you are electronic. The actual component that becomes affected by humidity is simpler than the circuitry that interacts with it. Typically there will be a polymer or salt of some sort that's exposed to air, and the electrical resistance of this material changes with the moisture absorbed in it and thus the ambient humidity. Measure its resistance, compare that to a lookup table, and now you know the relative humidity

within a few percentage points. These days this rather basic task  can be done at the component level, allowing a simple interface between a sensor  and whatever device you’re putting it in. There are other techniques involving materials  which exhibit changes in their capacitance, but the essence of all of them  is in the material which changes in some way depending on ambient humidity. It can be something we measure electrically, or it can be something that moves a pointer like  a piece of pa

per or a human hair. What we’ve done with these devices, including the thermometers, is quite simple but also quite profound. Figuring out how environmental conditions affect different  materials allowed us to work backwards and define those environmental conditions using the material as a reference. Those humans, so clever! While it may be more technically impressive to  do that using electronicals and hook it up to a microcontroller which can beam god-awful blue numbers straight into your reti

nas, that’s just not as satisfying as these simple things. Turning the physical properties of something into a, well, turning motion is never gonna get old to me. I told you it’d be simple! Now I’ll have fun looking at the YouTube studio and seeing my watch time metric tank. It’ll be great. Oh yeah, sorry. No music or  credits today. Just… this. Take a closer look, and you'll see that one of zah elbledebuh be deh. plerh