Humidistats and humidity control

Instrument Maintenance
1

Hi, everybody.

I live in the desert, where humidity control is a year-round battle. Recently, I have purchased some new, cheap humidistats to monitor the humidity not just in my music room, but around the house. For those of you with experience with humidistats, I’ve got a conundrum.

I set two new humidistats up next to my harpsichord, and they both registered exactly the same: 44%. This would be fine, except that my old humidistat registered 35%. The obvious explanation: my old humidistat is wrong.

But it’s not that simple. I ran the “salt” test (closing humidistats in bags with salt slurry). Using that method, they are supposed to read 75% after a few hours (or up to 24 hours according to some instructions). Both humidistats, left overnight in two separate bags with salt slurry, read 72%. They are supposedly accurate within 1%-3%, so 72% seems reasonably close to the expected result.

I’m puzzled: if they’re both reading fairly accurately—and identically to one another—using the salt test, then why are they so drastically different when I take them out of the bag and put them in the music room? And how do I know which one is closer to reality?

Additional question:
If you have models of humidistats that you are satisfied with, could you share? I’m happy to invest in a more expensive one for the music room. Cigar people seem to praise the Caliber IV, and I have ordered one of those. But it’s not really any more expensive or sophisticated looking than the ones that I already have…

Thanks!
Jonathan

2

Quite a few yeas ago I studied the mechanical behavior of polymers in different humidity environments. We (my student and I) used a wide range of salts for generating different humidity levels. I could find out what reference we use for our choices, but I am not sure you want to download a scientific paper for that. If your expected humidity is, say, around 50%, you could, perhaps, look up a salt or two for that in the publication

[

engineeringtoolbox.com

](Saturated Salt Solutions - Controlling Air Humidity)
.
The next question is how and where to get the appropriate salt. That depends on your environment. In an academic institution there are typically chemical stockrooms: If you can make the acquaintance of a professor interested in Music or someone in the music department, they might be willing to help (?)

I am in southern California, and my meter rarely goes over 55%, I think.

Wolfgang

| jonathanrhodeslee
July 15 |

  • | - |

Hi, everybody.

I live in the desert, where humidity control is a year-round battle. Recently, I have purchased some new, cheap humidistats to monitor the humidity not just in my music room, but around the house. For those of you with experience with humidistats, I’ve got a conundrum.

I set two new humidistats up next to my harpsichord, and they both registered exactly the same: 44%. This would be fine, except that my old humidistat registered 35%. The obvious explanation: my old humidistat is wrong.

But it’s not that simple. I ran the “salt” test (closing humidistats in bags with salt slurry). Using that method, they are supposed to read 75% after a few hours (or up to 24 hours according to some instructions). Both humidistats, left overnight in two separate bags with salt slurry, read 72%. They are supposedly accurate within 1%-3%, so 72% seems reasonably close to the expected result.

I’m puzzled: if they’re both reading fairly accurately—and identically to one another—using the salt test, then why are they so drastically different when I take them out of the bag and put them in the music room? And how do I know which one is closer to reality?

Additional question:
If you have models of humidistats that you are satisfied with, could you share? I’m happy to invest in a more expensive one for the music room. Cigar people seem to praise the Caliber IV, and I have ordered one of those. But it’s not really any more expensive or sophisticated looking than the ones that I already have…

Thanks!
Jonathan

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Wolfgang G. Knauss
Theodore von Karman Professor of
Aeronautics and Applied Mechanics, emeritus
California Institute of Technology
Pasadena CA 91001

626 395 4524 Phone — Office
626 798 3793 Phone — Home
626 797 0405 Fax — Home

3

Thank you, Wolfgang. Sounds like you have a great deal of experience and expertise with these matters!

My issue is not that the humidistats are inaccurate. It’s that they don’t match when they are out of their plastic bags. In the bags, with the same salt (plain old table salt) in roughly the same amount, with a few drops of water, they are reading exactly the same. Outside of the bags, in the music room that I’m trying to monitor, they are quite far off from one another. I would love to know some way of ascertaining which one is more accurate in the room. Since they are both within their claimed margin of error within the bag, I’m puzzled about how to assess their accuracy in the room itself.

Hope that makes sense!

Jonathan

4

Dear Jonathan.

I understand that both devices are near each other near the hpschd., subject to the same draft (air movement) environment. Even though I am skeptical of your term “slurry” —we always had a stand of solution above the salt— but then your measurements pretty much agree with the table I gave you in my web-citation which indicates what it should be (about 4% lower). In fact, when I understand you correctly, your two new devices record the same RH once in their NaCL bags (~72 % RH) and then outside the bags, but next to each other near the hpschord, they registering both ~44 %. So, really, the only difference I see, is between the two new devices and the old one, which sounded to me was a Cigar-destined device.
When I started with tracking the RH for my hpschord, I also started with Cigar-destined device, which I soon discarded (stopped using it). It measured notably lower RHs than the device I then purchased over the internet an EXTECH HUMITY ALERT, which claimed to possess NIST-calibration.
( Extech 445815-NIST Hygro-Therm Humidity Alert 14 to 140°F & DP w/NIST).

I have worked with NIST people for many ears and trusted the advertisement (but I would not know, in fact, if the Amazon-seller dared to lie for a device that costs less than $ 200 now).

The fact is that in two test situations your new devices perform identically, but differently from, what I call “your tobacco device”. But you really do not know what the actual RH near your hpschd is. Could it be that it really was 44%? It is not far from what I often experience in the norther LA basin (Altadena, CA).

My original, implied intention for you was to have all your devices in an environment that is close to what each should experience near the Harpsichord. If you know what the humidity should be about, say 55% RH, it would be important to see, if all your devises would record that with a salt solution of Magnesium Nitrate (Mg(NO3)2 in the temperature range of 25 to 35 deg C. Let me offer a guess, without knowing what devices you now bought: Your cigar device is likely to be lower; at least, that was my experience.

Best of luck.

Wolfgang



| jonathanrhodeslee
July 15 |

  • | - |

Thank you, Wolfgang. Sounds like you have a great deal of experience and expertise with these matters!

My issue is not that the humidistats are inaccurate. It’s that they don’t match when they are out of their plastic bags. In the bags, with the same salt (plain old table salt) in roughly the same amount, with a few drops of water, they are reading exactly the same. Outside of the bags, in the music room that I’m trying to monitor, they are quite far off from one another. I would love to know some way of ascertaining which one is more accurate in the room. Since they are both within their claimed margin of error within the bag, I’m puzzled about how to assess their accuracy in the room itself.

Hope that makes sense!

Jonathan

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| wgk Wolfgang Knauss
July 15 |

  • | - |

Quite a few yeas ago I studied the mechanical behavior of polymers in different humidity environments. We (my student and I) used a wide range of salts for generating different humidity levels. I could find out what reference we use for our choices, but I am not sure you want to download a scientifi…

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Wolfgang G. Knauss
Theodore von Karman Professor of
Aeronautics and Applied Mechanics, emeritus
California Institute of Technology
Pasadena CA 91001

626 395 4524 Phone — Office
626 798 3793 Phone — Home
626 797 0405 Fax — Home

5

Dear Wolfgang,

Aha! Thank you! This is most helpful.

All of my devices are in fact of the same type, but certainly no more sophisticated than what you’re called “cigar” devices. They’re all designed for casual room monitoring, resembling residential thermostat controls. I have had my old one hanging above a light switch for several years, and I placed the new ones atop it.

You are quite right that the old one measured lower than the new ones when out of the salt bag. Though since it is the same basic design, I don’t know that that really tells us much; just that it is reading differently than the other two when outside of the bags. (It reads exactly the same as the new ones when it is in the bag.) So I still don’t know which is correct, or indeed what the room’s humidity really is. According to these devices, somewhere between 33% and 44%.

Perhaps the most helpful thing that you’ve provided me here is the link to the EXTECH device. It appears to be far more sophisticated than the cheap things that I have been discovering. (At least, it’s a lot more expensive!) I might just “spring” for one of these and trust its accuracy above my cheapo things. I suppose that I might also find a humidity-controlled environment in my university and take them there. (Alas, our School of Music is NOT humidity controlled, and the instruments suffer badly because of that.)

Thanks again!

JRL

6

Hi @jonathanrhodeslee this topic arises frequently. I have written various brief screeds on it in the past. Wolfgang has experience with this measurement scientifically, but let me say the result of my experience and research. Domestic humidity meters - you’d be lucky to get anything close to 10% accuracy. Also, the salt bag test only measures on specific humidity level and these devices are notoriously nonlinear, which I think is what you are seeing. The response is nowhere near a straight line compared to the bag test, and domestic ones you cannot adjust anyway. I do not call them humidistats as that would imply they are device that manages and maintains a constant humidity by analogy with the word thermostat. I think hygrometer is better, corresponding to thermometer.

Now. from my delving deeply into this subject, the only reliable way to measure humidity is with the combination wet and dry bulb thermometer and associated chart that you have with a sling psychrometers. This clever device is not something that gives a continuous reading sitting in a room but it will give you a very accurate reading. The sling psychrometer has been around for a hundred years, and you can get quite cheap ones most anywhere. Impractical, but reliable. This is because relative humidity is calculated from the wet and dry bulb temperatures and the dew point.

https://www.instrumentchoice.com.au/news/how-does-a-sling-psychrometer-work

As to cigar hygrometers, just because cigars are very expensive and aficionados are wealthy doesn’t mean they are really accurate. There’s a brand called Inkbird and they have a cigar type one from $15. It’s OK, but not a scientific instrument. It’s as good though as any of the others, expensive or not. I should mention that Inkbird also make quite nice bluetooth temperature and humidty data loggers, quite compact and also pretty cheap.

Now, for my violins, here where it is super dry in Australia all the time, I use the Stretto humidifier devices, small bags of a gel like substance that you initially soak in water, and it holds water and releases it very slowly. specially designed for valuable violins. You can get a kit from Stretto which also has a small battery powered hygrometer. I velcro it inside the case and it gives max and min and current humidity. My opinion is that it is pretty good. I always choke to see violin cases with little builtin humidity meters. These 20 cent units just seem to sit at 60% and never budge in most of the ones I have. Dreadful rubbish. The Stretto bags are not going to humidify your harpsichord, but you can buy the meter separately.

See the Stretto Digital Hygrometer in the shop page.

Also, I have made my own design custom boards for Raspberry Pi’s with very accurate analog thermometer sensors and hygrometer sensors and 12 bit ADC’s, which I can calibrate. Still the best I can get out of the humidity sensor (that I can afford) is about 5% accuracy.

Finally, I buy a lot of electronic components from Aliexpress. But I offer a warning. You can get thermometer/hygrometer handheld units that are similar in appearance to multimeters. There are thousands of them. Do not expect any of the them to be even remotely accurate. For all the excellent electronic devices on Aliexpress it appears Nature compensates by selling an equally large number of total garbage meters. Caveat emptor. If you want something accurate and reliable in this type of device you would need to go to this sort of level:

7

My experience (a decade or two ago) with building electronic hygrometers from component level up matches other’s findings reported here. There are several sources of sensor chips, over a wide price range, all are somewhat nonlinear, some barely work. They all need a bit in the way of supporting electronics, but the necessary details for the cheaper ones tend to be unspecified by the manufacturers, so the people who make the cheap ‘instruments’ may be relying on guesswork.

Apart from electronic versions, the hair, and the sling psychrometer, there is another alternative: not sure if it has a proper name – call it a “bent stick hygrometer”. Two long, thin strips of wood glued together (non-creeping adhesive) one with grain running parallel with the long side, and the other transverse. As wood users I am sure we can see how it works. A timber species with high coefficient of expansion wrt humidity is desirable. I found beech worked well. I no longer have the one I built, but it was more or less the proverbial “afternoon’s work” (I seem to remember spending more time rummaging through the scrap box than anything else).

The key point, I suggest, is that as long as the RH near the instrument is (1) reasonable and (2) stable, its exact value is not important. We just need to know “make it wetter” on “make it dryer”, or (happy days) “leave it alone”. If the climate is very unstable and the process is to be automated, it still does not matter whether the actual RH is known, as long as whatever sensor we use is stable over time.

Frank.