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The difference between the pressure found on the inside of a boiler and the pressure on the outside of it is indicated by a steam gauge. This is a gauge pressure reading and should not be confused with absolute pressure. A steam gauge is used to measure the steam pressure at the top of the boiler. Generally a reading of 12 psi (pounds per square inch) indicates a dangerous build up of pressure in low-pressure steam heating boilers. The boiler should be shut down before the pressure exceeds this level.

If the steam gauge is operating properly, the needle (or pointer) will move with each change of pressure inside the boiler. Shut off the steam and the gauge needle should drop to zero; turn on the steam and the needle should rise to the correct reading. It is very important that the steam gauge be regularly checked to insure that it is operating properly.

The steam gauges used on boilers operate on the bent-tube principle; that is to say, the tendency of a bent or curved tube to assume a straight position when pressure is applied. One end of the curved tube is attached in a fixed position to a pigtail (connector tube) which is, in turn, attached to the boiler. The gauge needle is mounted on a rack-and-pinion gear attached to the free end of the curved tube. The pressure in the curved tube causes its free end to move slightly in its effort to assume a straight position. This slight movement is multiplied by the rack-and-pinion gear, causing the needle to rotate and indicate the steam pressure.

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