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Cambridge Organists

No. 10 Under pressure  

Organ wind pressure is measured in inches of water - measured by the U tube device conveniently filled with water. Most church organs run on about 4 to 5 inches. The small organ built by the webmaster and his friend ran on 1 and a half inches. We used two “Woods” fans designed to cool the triode valve computers of the early ‘60s in series. There was a lot of “chiff”. See “Stopped Diapasons” for a picture.

 A Wurlitzer cinema organ housed in an old water mill which was my introduction to the world of organs ran on 10in. A lot of time was spent sticking gaffer tape on junctions in the trunking as the escaping air caused significant noise.

Large cathedral organs run on several pressures - higher for reeds than flues. The Bombardes in Liverpool Anglican Cathedral are on 30 in and the Tuba Magna on 50in!

This sounds a lot and indeed it is difficult to convey such wind pressure without leaks and noise, but 50 inches of water is a mere 1.8 pounds per square inch.

Your blood pressure is about 75” of water (that’s why arterial bleeding can make such a mess), your gas supply is 289”, car tyres are about 830” and High Pressure gas mains are above 2800” (the pressure varies as they are for storage as well as transmission).


Organ Notes

Author’s gauge. Red lines are at half inch intervals with maximum reading of 5 inches. The left hand limb is connected to the chest so  water rises in the right side as it falls in the left, the difference in height being measured against the half inch divisions.

Stops & Pipes Gallery 6