Wind can be defined as the air under pressure that flows around the instrument. It is measured in two ways; pressure and flow.
Wind pressure is measured in organbuilding by “inches of water” (or “millimetres of water”). This term comes from the traditional instrument used for pressure measurement, the ‘U’-tube manometer. Although more sophisticated methods are available today, the ‘U’-tube manometer filled with water is accurate, simple, reliable and easy to make. Pressures are sometimes expressed as “inches of water” or “w.g.” (water gauge).
It is critical that the wind supply to a particular rank of pipes is maintained at the correct pressure – this is the pressure that the pipes were designed to speak on and voiced at.
Most theatre organ pipework operates on pressures between 6″ w.g. and 15″ w.g. with some ranks on 25″ inches of water on very large instruments. Classical organs may use pressures between 2″ w.g. and 6″ w.g., with large reeds (eg in cathedral instruments) on higher pressures. There are exceptions to all these, of course.
Wind flow is more difficult to measure, partly due to the compressible nature of air. Fortunately in practical organ building there is normally no need to measure air flow except when specifying a blower, where wind flow is critical in order that the organ is not ‘short of wind’. Flow is usually measured in cubic feet per minute (cfm); various rule-of-thumb equations are used to determine the necessary flow, based on the number of ranks being fed.
As well as pressure and flow, temperature and relative humidity of the wind is important. Temperature affects the tuning of pipework, and humidity (too much or too little) can adversely affect the many wooden components inside the organ. It is important that the wind supply system avoids temperature build-up and rapid humidity changes. It is not unknown for some ‘money-no-object’ organ installations to have the wind fed via huge air-conditioning units to assure a stable temperature.