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Building a Vacuum Gauge

If the repairman does not wish to invest in a dial-type gauge such as can be secured from the piano supply houses, a simple gauge of this type can be made quickly and easily. One or the other type is a necessity for proper regulation of reproducing pianos.

F is either clear plastic or glass tubing, with an inside diameter of at least 1/4 inch. It is formed to a U-shape and fastened to the board by any convenient means; if glass is used a rubber tube can be used to form the U at the bottom. D is a yardstick or other scale marked off in inches. One end of the tubing is left open to the atmosphere, and the other is connected to a piece of rubber tubing, as at E, which is used to connect to the vacuum source being tested.

The tubing is filled to its midpoint C with water. If it is colored with ordinary vegetable coloring matter, found in most household kitchens, it is easier to read.

When a vacuum is introduced at E, the water will rise in that side of the tube, and the total amount of vacuum is indicated by the number of inches between the tops of the water columns, as A. This will be the difference between the numbers of inches on the scale D.

A direct-reading gauge can be made by marking a scale with half-inches indicated as inches, and making sure that the 0 inch mark always coincides with the level of the water at rest, either by making the scale adjustable, or by making sure the amount of water is such that it always comes to the 0 point at rest. Such an arrangement is easier to work with, as the readings can always be made directly such as at B.

Use the fact that one inch of water in a water column is equal to 0.036 pounds per square inch of pressure; this is your conversion factor. Fom the example, 10 inches of water (10" WC) is equivalent to 0.36 pounds of pressure (0.36 psi). Looking at it another way, a player piano that's in decent shape will play music loudly on about ONE pound of air pressure (or 28" WC).

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