When the boiler is priming or steaming very fast, the gauge glass may show a false water level, hence reading should be compared with that of the test cocks.
If the water gets too low, the first parts of the boiler to be injured will be the top of the flame box, or the combustion chamber, and the top row of tubes, because they are the first surfaces that the water will fall below and leave exposed to the heat without having water on the other side.[68]
[68] See [page 370], Vol. II., on low water in boilers.
The pressure in the boiler is shown by a steam gauge, pressure gauge, or dial gauge as it is promiscuously called.
A Bourdon dial gauge or pressure gauge consists of a dial casing, containing a hollow thin brass hoop, oval in cross section, which receives steam from the boiler.
This hoop is fixed at one end, while the other end is closed and free to move. The free end is connected by a small link to a toothed sector, which gears or engages with a small pinion fast upon the spindle of the pointer or needle. When the steam is admitted into the hoop, it straightens out or expands in diameter to an amount that is proportionate to the amount of the pressure within the hoop, and thus causes the needle or index pointer to revolve, and denote from the markings or readings of a dial plate the amount of pressure within the hoop.
If the pressure within the hoop is released, it will move to its normal or zero position. In the course of time, however, the hoop is apt to get a slight permanent set and not indicate correctly. It may, however, be approximately tested for accuracy by testing its readings with that of the safety valve.
The working parts of the gauge, and its casings also, are made of brass, so that they shall not corrode, and to prevent the heat of the steam from permeating the gauge and impairing the action from expanding the parts, a small quantity of water interposes between the gauge and the steam, the construction being as follows:
Outside the gauge casing the steam pipe is bent into a loop forming an inverted syphon which is to contain the water.
At the lowest point in the bend of the syphon a small cock is inserted, which lets the water out of the leg of the syphon nearest to the boiler, because water in that leg would, from its weight, cause the gauge to show a pressure higher than that in the boiler.