Any device available that can *maintain* humidity in a room?

September 19, 2022, 7:26 PM · I know there are many solutions on the market for humidifying or de-humidifying a room. These either do one or the other, but not both.

Are there any devices which actively *maintain* a specific humidity within a room, by adding water to the air when it drops below a a certain level, or removing it when it rises above a certain level?

Note: I am already familiar with Boveda packs, which use saturated salt solutions to maintain an exact humidity inside a case. I'm looking for something scalable to a larger space.

Also: if this device doesn't exist already, do you think there'd be a market for it? I certainly would buy one.

Replies (25)

September 19, 2022, 8:03 PM · Check out "room humidifiers" on google search.
Avoid those that spray water mist.
September 19, 2022, 8:14 PM · Hey Andrew, humidifiers only add water to the air, but don't remove it if the humidity goes over a certain point. I'm looking for something that maintains a specific range (for example, 40-50%) relative humidity at all times, without much human management.
September 19, 2022, 8:19 PM · There are humidifiers and dehumidifiers on the market with humidistats. I have no experience using them.
September 19, 2022, 11:19 PM · I have whole-house humidifiers and dehumidifiers, but unfortunately switching between the two is manual. It's not that big a deal since the switch is largely with the seasons.
September 20, 2022, 7:47 AM · As Andrew said, a humidifier with a humidistat is what you want.
Edited: September 20, 2022, 8:02 AM · I used a basement dehumidifier when I lived in the Washington DC suburbs and a full-house humidifier (with humidistat) when I lived in the California desert. What Lydia said is correct - unless your room is moving rapidly between climates - also, air conditioners function as dehumidifiers when they recycle room air.

However I purchased those devices about 60 and 50 years ago, respectively, so it is likely design upgrades may have been made.

September 20, 2022, 9:46 AM · Any time this subject comes up -- which it does with seasonal regularity -- I recommend consulting David Burgess's website. He has made a thorough study of this topic including principles and recommendations for gear.
September 20, 2022, 1:14 PM · @Erik I have not seen a device that serves both functions. It would be very convenient. I have a humidifier for my studio in the winter when the air is dry. But it does not do a great job of dispersing the atomized water over a large area.
September 20, 2022, 1:47 PM · Eric asked:
"Also: if this device doesn't exist already, do you think there'd be a market for it?"

I don't think there would be much of a market, particularly if the price was high. Most people are content with switching between a humidifier and dehumidifier twice per year as the seasons change. In some cases, this can be as easy as flipping a switch to turn one off, and the other on.

September 20, 2022, 4:01 PM · Dang, well I guess I'll just stick with my current inventions.

As a side note: I will mention that one way of maintaining a very consistent humidity in an area is to take a bowl and fill it with Potassium Carbonate, and then dampen the salt. Ideally, it should be in a pyramid form, so that water extracted from the air can run down the sides. This is basically the same mechanism of action as Boveda packs use, but much less expensive in the long run, and can be scaled up for larger vessels.

(note: Potassium Carbonate is dangerous in dust form, so probably do the wetting outside.... also don't touch it. I recommend a PTFE bowl to reduce salt creep)


Here is a table of different saturated salts and their critical relative humidities:

https://www.researchgate.net/figure/Saturated-salt-solutions-and-its-standard-relative-humidity-value-for-the-measurement_tbl1_238085499

September 20, 2022, 8:23 PM · As a chemist I recommend keeping potassium carbonate away from your children, your pets, your hands, and especially your eyes. Potassium carbonate is not particularly toxic, but it is quite an alkaline substance. I really can't imagine having a "bowl" of it "arranged like a pyramid" sitting in my room and expecting that to do much for the relative humidity. Humidity control inside a violin case where there might be, at most, a couple of cubic feet of interior volume is perhaps controllable by using various salt mixtures with known affinity for moisture. But a whole room? I'm skeptical.
September 20, 2022, 10:01 PM · In the summers, when Maryland becomes rather swamp-like, I run a standalone dehumidifier in my music room, in addition to the whole-house dehumidifier.
September 20, 2022, 10:01 PM · In the summers, when Maryland becomes rather swamp-like, I run a standalone dehumidifier in my music room, in addition to the whole-house dehumidifier.
September 20, 2022, 10:02 PM · Last time I did it Paul, I swear I had like an extra 4-6 oz of water every day that the salt "collected" from the air. I did have a fan running over it, though, so maybe that influenced it. It was also very humid, which is why I did it at that time. In addition, it was a fairly large amount of KCO3, perhaps 12 dry oz.

But, most of the time in the past, I put the bowl inside of a display case that held about 6 violins, rather than trying to use it to maintain the humidity of an entire room. I, too, am skeptical about the ability of salt to maintain the humidity of a large room.

September 21, 2022, 2:45 AM · Eric, 4-6 ounces of water removal per day doesn't seem like it would be adequate. During very humid conditions, my dehumidifier will need to remove at least a gallon per day to keep the relative humidity level under 60% in a single room.
September 21, 2022, 11:21 AM · Calcium chloride is the usual chemical to use. This is what is in commercial products such as DampRid. It is not for dehumidification of a space that is large and not relatively airtight. In a room that's humid and not airtight all you'll end up with is a bowl of salt solution.
September 21, 2022, 11:55 AM · In the summer, I typically remove 2 gallons per day from my whole basement, and the temperature is around 75 F, and the humidifier is set to shut off at 50% RH. I was curious about David’s estimate of water removal so I decided to perform a quick calculation, mostly for "entertainment purposes." Let’s say we’re talking about a 12 x 12 x 8 room, that’s 1152 cubic feet or 32600 liters. And let’s say it’s summer and we’re maintain the room at 77 F (25 C). At this temperature the vapor pressure of water is 23.8 mm which is 0.031 atm. So at 100 %RH, I calculate that the entire room holds 744 g of water vapor. I realize that I’m reporting these values to a needless level of precision; I’m doing that in case anyone wants to check my figures. At 90% RH, I get 670 g of water in the whole room. And at 50%, the total moisture content is 370 g. Thus, dehumidifying a room that is at 90% RH to 50% RH at 77 F requires the removal of 300 g of water, roughly ten ounces. Now, there is a GIANT assumption underpinning this result – that the room is a closed system – that there is not simultaneously any input of moisture. Obviously, that assumption is totally absurd unless your home is really weird, with rubber gaskets around the interior doors, etc. The level of continuous input would be very hard to estimate. You could measure it approximately – by seeing how much water collects, per day, in a dehumidifier that is set for the desired humidity level. Then you’ll know whether your little pile of K2CO3 can handle the load. I’m curious, though, whether Erik has to replace his potassium carbonate continually, or whether he recycles it by drying it somehow.
September 21, 2022, 12:20 PM · Following up on Ann's comment about calcium chloride. My dad worked his whole career at Wyandotte Chemicals (which was swallowed up by BASF) in Wyandotte, Michigan. One of their chief products was sodium carbonate by the Solvay Process, which generates an enormous volume of calcium chloride as a byproduct. (The chemistry is described on the Wikipedia page for sodium carbonate.) One of the technical challenges they faced was storing the calcium chloride, because it turns out to be extremely corrosive. And it's not easy to find customers for calcium chloride because there's a glut of it, and it's useless for just about everything except road salt, which is seasonal. They had a pavilion to shield their calcium chloride inventory from the weather, only to watch pretty much the entire structure disappear into piles of rust. So they ended up building a huge storage shed entirely out of wood -- including old fashioned wooden pins instead of steel fasteners, like a pioneer's log home.
September 21, 2022, 2:50 PM · Ann, I use potassium carbonate because it keeps relative humidity at around 45%, which is near optimal.

I'm in California, where humidity really isn't that much of an issue. It's crazy that you guys are pulling literal gallons out of the air!

Paul, when I was using the salt solution in a relatively closed space (display case for violins), it reached equilibrium pretty quickly, and so I would just occasionally either add water or dump a bit out. Of course, dumping it out would also get rid of some of the salt, so over time I would need to add more.

However, during the time period that I used it in a larger room, it would gather so much water that I had indeed pondered the idea of "Refreshing" it by boiling, rather than constantly replacing the salt. But I recall that was at a particularly damp time.

Based on your calculations and the size of my room, it does seem plausible that the salt alone might be enough in my case, for most of the year here.

September 21, 2022, 3:47 PM · A humidor box or room would maintain a humidity level. Some cigar stores have walk-in humidors.

September 21, 2022, 8:39 PM · Erik you seem to have missed the GIANT disclaimer on my calculations. I think you should run your dehumidifier for a week in that room and see how much water it collects per day during your humid season.
September 21, 2022, 11:30 PM · I don't measure my dehumidifier output, but the room one alone (much less the whole-house one) pulls enough water out that it has a hose that runs directly into a bathroom drain.
September 22, 2022, 4:09 AM · At rest, the human body puts approximately .8 liters (27 oz) of water into the air per day from respiration (breathing) and evaporation from the skin. More as the level of physical activity increases.
September 22, 2022, 4:50 AM · I believe that it is much more than that.

I have had the experience of living in some desert countries with RH of 30%. If you would close the door of the room and without any ventilation of conditioning, you would be able to see the hygrometer move up just by your being there, until it would reach around 70%, when you would start to sweat (The saturation slowed the sweat evaporation); by 85%, you would be soaked. That was a process of no more than one hour... It reminded me the book Dune.

September 22, 2022, 5:11 AM · True, it highly depends on a number of factors. .8 liters is just a baseline.

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