The air chamber, combined with connections for discharge pipe, relief valve, and hose valves, should be carefully designed to make the whole weight as small as possible. Keeping this weight down makes the pump run steadier and brings less strain on the flanges at high speeds.
An air chamber of hammered copper and warranted tested under a hydraulic pressure not less than 300 lbs. per square inch is a little better than cast iron as it holds air better, and being lighter it wrenches and strains the pump less when running fast and shaking, but because it costs from $25 to $50 more than cast iron, it is not often adopted.
b. The vacuum chamber must be attached to the pump in the most direct way practicable, but provision must be made for attaching it in such manner as not to prevent readily taking off the cylinder heads.
c. Every vacuum chamber should be provided on one side near the top with a 1⁄4-inch pipe hole plugged. This to be used for attaching a vacuum gauge if desired.
41. Pressure Gauge.
a. A pressure gauge of the Lane double tube spring pattern with 5-inch case, must be provided with the pump, and connected near to inboard side of air chamber, as shown in [Fig. 12], by a 1⁄4-inch cock, with lever handle.
The dial of this gauge should be scaled to indicate pressures up to 240 lbs. and be marked “WATER.”
This kind of gauge is used on locomotives and is the best for withstanding the vibration which causes fire-pump gauges to be often unreliable. Moreover, this double spring form is safer against freezing.