Air at any given temperature (and pressure) can hold a certain maximum amount of moisture, which is arbitrarily called 100%. Relative to this amount, the air in any given locale will be holding some per centage of maximum, which we call relative humidity and state as a per cent figure.
For low humidity conditions, say below 35%, human mucous membranes are excellent indicators. No electronics necessary. Canadian winter will teach anyone a lesson in humidity. (Cold air holds less moisture than warm. Breathe cold air through the nose, and we increase its temperature, hence its ability to hold moisture, hence a decrease in its relative humidity, unless we supply moisture as we heat it. The nose and respiratory mucous membranes supply the moisture, and we experience dryness.)
The concept of relative humidity relates the human experience of moisture in the air/not in the air to measurable external reality. Irrespective of temperature or pressure, the RH measurement will give us a very good idea of whether we will feel dry, wet or comfortable. Relative humidity is really an indication of how readily our bodies will give up/not give up water to the environment.
Humans usually feel comfortable when humidity is above 40 per cent and below about 70-75, although there is a huge variation in peoples' tastes. A darkroom maintained at about 50% would probably be fine. Mine varies over the course of the year from 30-35% with no humidifier operating in winter, to 45-60 in winter with the humidifier, to summer humidity of 50-65 with the dehumidifier (Toronto climate!) operating and 70-85 if not.
Photographic paper changes its curl depending on humidity in the darkroom. At about 50-55 per cent in my darkroom, RC paper will lie absolutely dead flat under the enlarger with no easel. Somewhere below that figure, depending on the brand of paper, the corners will lift as the emulsion side shrinks from giving up its moisture. Breathing on a corner will make it lie down.
I hang my FB prints from a line to dry, and I find if the humidity is too low the paper will curl severely, possibly even cracking the emulsion. At about 55-60 per cent or higher, the sheets dry very close to flat.
Humidity too high for too long promotes the growth of mildew, which has eaten up not only numerous showers and bathtubs, but enlarger lenses as well.
In some places, such as where I live, year-round humidity management is a must in the darkroom. I find maintaining humidity between 45 and 60 all year cheap and close to effortless. Keeping it tightly controlled, say 49-51% year round, would be expensive.
The term absolute humidity refers to the amount of moisture (measured in grams, for example) contained in a given volume of air (such as one cubic meter), whatever its temperature and pressure.
Lloyd's Dissertation on Humidity Management Apparatus:
Devices to put moisture (water) into the air of our homes and workplaces are everywhere ("humidifiers"). Nearly as common is the device to remove moisture from the air when we feel there is too much ("dehumidifiers"). A device to measure the relative humidity is less commonly seen, but essential to managing the humidity of a darkroom.
There is a commercially made humidifier designed to be attached to a forced-air heating system (common in North America), and also to the water supply. The unit I am familiar with is sold in Canada under the manufacturer's name, "Wait-Skuttle". Often they label their products "Wait".
The machine is very simple: a plastic tray of water is installed in the largest part of the heating duct, where it first leaves the furnace. This tray has a small valve on it, which works like the one that controls the toilet; the tray stays filled with water (it is hooked up to the cold water plumbing with a small copper or plastic pipe.) Since it can constantly be re-filled, the actual amount of water present is probably no more than a couple of liters. A small electric motor turns a 'water-wheel' device in the tray. It is covered by a piece of foam plastic, so some of the water is always being lifted out of the tray on a large, porous sponge surface. Of course, the hot air of the furnace is blowing by, and through, at all times. So some of the water evaporates, providing humidity. The device has a 'humidistat' that senses the air humidity wherever you place it, so the system only operates to your preset level.
There is a special chemistry they sell in small overpriced packets of powder to keep minerals from the tap water supply in solution, to prevent the machine seizing up. Unfortunately it is worth the price they charge to avoid the bother of cleaning a humidifier caked and crusted with hardened deposits from many gallons of evaporated tap water. This chemistry makes it a simple matter to clean the unit, at least annually. The forced-air-furnace-humidifier should be installed in such a way that the tray can easily be removed for cleaning in the sink, bathtub or dishwasher. Also, the valve fails every few years, so it should be easy to remove and replace. The same for the sponge material.
In Canada, one of these machines probably costs less than C$ 100. Installation by some outside self-proclaimed expert probably would triple the cost.
Very effective air cleaners can be installed in similar fashion, but they are expensive.
In Canada (and much of the USA) the heating season is so long that dry air becomes a major problem. It is dry so long each year that peoples' health can be affected. So an industry has grown up to supply humidity!
In my experience, the most effective humidification method, which is also the most controllable, and the cheapest, is to use the type of humidifier that looks like a piece of furniture and stands on small wheels in a corner of the room. Usually they are covered in stick-on plastic that looks like wood-grain.
Inside they have a plastic bucket that you must fill every day, and a wheel, covered in foam plastic, rotating through the water. A fan blows air through the foam, which is of course wet! Darkroom humidity can be increased in minutes with this type of machine. (A piece of air-conditioner filter material taped over the air inlet on the back of this type of machine helps keep it clean inside.)
These machines are easily repairable forever; used ones often sell very cheaply because people find it distressing to see how dirty they become with accumulated minerals from tap water. The Salvation Army stores are good sources of dirty old humidifiers that have years of life left in them. They sold me seven for twenty-two dollars once!
The same packets of powder chemicals described above for the Wait-Skuttle type of machine will keep this type of humidifier from sludging up. The wheel (it is easily removable) and its foam cover can be cleaned once or twice a season with cold water; in the bathtub might be a good place. Plain white vinegar cleans them with plenty of effort, but shouldn't be necessary if the powder has been used. Never leave vinegar in the tank if chlorine bleach will be added later.
In normal operation, a SMALL amount of chlorine laundry bleach [10-15 ml] every few days or weekly will prevent the growth of smell-producing micro-organisms. Too much chlorine bleach in the humidifier water will make your whole place smell. An eyedropper is a good tool to dispense the bleach every few days. But do be careful with bleach around acids. This combination releases deadly chlorine gas; in the humidifier water (or washing machine!) we use the chlorine to kill micro-organisms. While we're on the subject, never mix ammonia (another common household chemical) with bleach either.
I keep one of these machines next to my enlarger in the darkroom, so the paper won't curl in the winter. In dry conditions, the emulsion shrinks and pulls the face of the print up. Humidity swells the emulsion, allowing the paper to lie flat. Of course, too much moisture in the emulsion swells it so the paper curls away from the emulsion -- as it does after processing, when it is wet.
Filling these machines becomes a chore in the winter. They can easily evaporate a gallon of water per day. In my darkroom, I have a length of garden hose leading from the sink to the humidifier, so all I have to do is attach it to the faucet and fill it up. Not so easy in the living room!
I have found the counter-top types of humidifier to be useless. They tend to be short-lived.
The type that is "ultrasonic" scatters fine white dust: it breaks the water into droplets and disperses them, along with whatever might be dissolved in the water. It does not evaporate the water and keep the minerals inside the machine.
The other type of counter top unit has a small heater and boils the water. There are actually two types that do this, neither of which is a good idea. The heating element only lasts a few seasons, and is a very expensive little part. Most of these machines are built in such a way as to make it impossible for the owner to repair them. So it is off to Mr. Overpriced Repairman for Overpriced Parts and Installation. The older ones are small and made for evaporating medication when children are sick, not for constant service.
The dehumidifier is the other side of the coin, a necessity in places like Toronto, where I live. A dehumidifier costs more than a humidifier, but is less bother to have around. They tend to have very long service lives, so used units may be a cheap alternative.
The higher cost comes from the refrigeration unit it contains. An energy user it is, too, but a reliable one. It really is quite amazing how long-lasting and repair-free refrigeration units in refrigerators, air conditioners and dehumidifiers have become.
The dehumidifier has a fan that blows air past the cooling coils of the refrigeration unit. Water condenses out of the air onto the coils, and drips down off them. The cleverly placed bucket (inside the attractive wood-grained-plastic housing) catches the water. Usually there is a connection on the bottom to attach a drain hose so you can forget about emptying the thing. If it's possible to stand the machine over a basement floor drain, it's perfect.
Some people capture this water and store it, because it is relatively clean. In contrast to water in the humidifier, the water condensed out of the air is very free of impurities (compared to municipal tap water, at least!) They use it for mixing developer, in place of distilled water. Real purists put this water through a water distilling machine. Let your conscience be your guide. At least cleaning the dehumidifier is not a chore.
My dehumidifier is over twenty yeas old. It has absorbed very little money or effort in upkeep. I had to replace the fan motor recently (twenty years old, eh?) but it works much better now, and more quietly, too. I have taped an air-conditioner filter across the back of the machine where it draws air. I have to clean it once every couple of years, but seeing the stuff it collects disappear is a pleasure.
Relative Humidity Readout ...
The key to using the humidifier/dehumidifier is the relative humidity indicator. Usually they're a large-numeral LCD device, and often also have a temperature display. If they would only show the time as well, they'd be perfect. The kind they sell at Radio shack are fine. I've had one for many years now, and it is actually more accurate than strictly necessary. It closes the feedback loop and tells you where to set the far-from-exact "control" knobs on the humidifier and dehumidifier.
I also have a regulation sling psychrometer (the whirling kind) which gives a much more accurate and precise indication. I used it a few times until I was convinced the electronic one was at least close. It's a lot closer than I thought it would be.
Using these simple devices, it is possible to maintain darkroom humidity within fixed limits cheaply and effortlessly.