What do we mean when we say a certain organ stop is voiced on so many "inches of wind?" ...
This refers to inches of water in a column; the higher the wind pressure, the taller the column of water that it will displace.
The instrument which measures this is called a water manometer.
A typical manometer is simply a tube shaped in a "U" configuration; this is filled part way with water or some other fluid, such as mercury (Hg); pressure being measured is applied to one of the "U" openings and the other opening is typically left open to the atmosphere.
Commercial manometers usually have an adjustable scale that can be moved up and down a bit, which makes the scale easy to zero with no pressure applied to the tube.
If we don't want to bother with zero adjustment, we can simply figure out the total distance between the tops of the water column on both sides and calculate the difference (photo).
Technically, one does not "measure a water column" with a manometer or a gauge.
The "inches of water column" term is a pressure measurement value just like "pounds per square inch (PSI)."
The term "inches of water column" is simply a very low pressure value; one inch of water column is equivalent to only about 0.036 PSI when the water is at 68 degrees Fahrenheit.
Since PSI values for low pressures aren't very practical, organ builders prefer to use the term "inches [of water]" to describe the air pressures applied to the pipe work.
It's possible to actually make a water manometer for one's self using flexible 1/4-inch clear hose with inches marked in ink; before measuring, a "U" shaped bend is made in the tubing (like the drain pipe under a sink) so as to keep the water from flowing down into the wind chest upon which the pipes sit, but this procedure is risky (water could get into the wind chest).
Many professional organ builders still prefer to use this type of device (with features to prevent getting water into the wind chest) to measure wind pressures provided to the pipe work.
For those who are not conversant with different ways to measure pressure, one atmosphere (atm) is equivalent to 760 mm (approximately 30 inches) of mercury (Hg) in a manometer and to 33 feet (approximately 400 inches) of water, and measures 14.7 pounds/square inch (PSI).
Organ stops are voiced on all kinds of wind pressures; a wind pressure of 50 inches of water would be extremely high; many organ stops are speaking on only 2-3 inches.
This means that the pressures at which most organ pipes operate are easily within the capability of human lungs to supply.
While voicers (people who tune and adjust the tone) might be able to build the pressure simply by blowing into some of the smaller pipes, they don't have the capacity to provide air for a 16-foot or 32-foot.
Moreover, blowing into a pipe is bad for the pipe; the human breath contains too much humidity that condenses inside the pipe; this isn't good for metal or wood, and reed pipes especially are affected by this; in addition, the breath heats the pipe and detunes it slightly.
One should really never blow into a pipe unless it's intended for the scrap heap or for demonstration purposes only; even then, it's not a good idea.
Many metal organ pipes contain lead; we don't want to ingest even a tiny bit of that; even trained organ technicians, if they decide they must blow into a flue pipe when doing on the spot corrective voicing (such as when the languid has fallen to check if the pipe is on speech or not, if it is quick or slow, etc.), they're trained to use their hand as a sort of "bridge" between the pipe and their mouth.
Getting a reed stop to blend with the chorus is a tricky business for the voicer, even when the reed is tame; and the louder the reed is, the trickier it becomes.
Four ear-splitting chorus reed stops on the Midmer-Losh Atlantic City Convention Hall organ are on 100 inches of wind pressure; their names are Grand Ophicleide, Tuba Imperial, Tuba Maxima, and Trumpet Imperial; and they are loud, very loud, and noisy; any of them all by itself can cut through the fullest ensemble like a hot knife in butter, but none of them can be used in ensemble.
This organ operates off 625 horsepower worth of turbine and compressor engines; when just 3 of the 7 blowers start up and you're anywhere nearby, you can literally feel the thrust.
This is waaaaay, waaaaaaaay more wind and power than any pipe organ should ever need; no chorus reed stop, no matter how big the space into which it speaks, needs to be on 100 inches of wind when 25 inches or less would be sufficient (the only advantage about having a "100 inch reed" in the organ, if advantage it may be called, is to set the instrument apart from any other.
From the organist's perspective, having a big reed this loud in the organ is a little like owning a bulldozer for getting around town when you have to blend with traffic:
Is it more powerful than any car or truck on the road? ... yes.
Can we take it on the road? ... well, no.
When it comes to driving (performing at the organ), we have to leave it at home (ignore it).
Organ builders sometimes supply an otherwise wonderful pipe organ with an unenclosed reed stop like this, usually in the Great or Solo division, sometimes just labeled "Trumpet" but often with a fancy name (Pontifical Trumpet, Celestial Trumpet, Tuba Ultima, etc.), which is voiced to carry a line over the top of the full organ; the premise with this may be good, but the result can be so loud and blaring that the stop is completely unusable in the chorus, for solos, or even for fanfares ... it can't be used for anything in fact; any organist who tries to do so will create a nasty uproar in the audience every time and, if they keep at it, can be expected to lose some of their own hearing.
Drawing a powerful, unenclosed reed like this simply and deceptively labeled "Trumpet 8-foot" and trying to use it in ensemble before an audience can be a horrendous surprise, if not a painful revelation.
In the grand symphony orchestra we never hear of the strings and woodwinds playing at every dynamic level to nuance the music with wonderful tints of light and shade while at the same time the brass can't do anything except blast away at an insufferable double fortissimo (ffff).
In just the same way, a big reed in an organ finds the greatest use when its enclosed; its placement in a swell box does nothing to destroy its tone ... it may even temper it to a very desirable extent ... and imparts a degree of flexibility and expression which would increase its utility by at least ten-fold [See blog, Horizontal Trumpets].
Organ chamber wind pressures generally range from 2-3 inches on the low end up through 5-8, 10, 12, 15-17, 20, or even 25 inches for some of the big reeds [See menu bar, Photos 3 subpage, Kimball History/Spec].
Different divisions of the instrument, even different stops within those divisions, may be voiced on different pressures.
In the auditorium of the Cathedral of the Scottish Rite in Saint Louis, Missouri, where there is an historic IV/53 Kimball Symphonic organ, for example, the entire instrument is voiced on 10 inches of wind save for the Vox Humanas in the Swell and Echo divisions (which are on 7-1/2 inches) and the Solo Tuba Mirabilis and Pedal Bombarde (which are on 25 inches).
This entire instrument of 9 divisions and 143 speaking stops operates off one 30 horsepower blower.
The illustration depicts a hypothetical water manometer reading for a stop voiced on wind of 7 inches.
One PSI is equivalent to 28 inches of water, which also equals 0.07 atm (i.e. less than 1/10th of the air pressure we breathe).
This isn't very much, and yet, if we witness a major wind leak or take a pipe out of its pipe chest hole, the rush of air we notice can be greater than a common house fan on high speed.