Pressure gauges are used in liquid-powered systems to measure pressure to maintain efficient and safe operating levels. Pressure is measured in psi. Flow measurement may be expressed in units of rate of flow—GPM or cubic feet per second (cfs). It may also be expressed in terms of total quantity—gallons or cubic feet.
Pressure Gauges. Figure 2-18 shows a simple pressure gauge. Gauge readings indicate the fluid pressure set up by an opposition of forces within a system. Atmospheric pressure is negligible because its action at one place is balanced by its equal action at another place in a system.
Meters. Measuring flow depends on the quantities, flow rates, and types of liquid involved. All liquid meters (flowmeters) are made to measure specific liquids and must be used only for the purpose for which they were made. Each meter is tested and calibrated.
In a nutating-piston-disc flowmeter, liquid passes through a fixed volume measuring chamber, which is divided into upper and lower compartments by a piston disc (Figure 2-19). During operation, one compartment is continually being filled while the other is being emptied. As a liquid passes through these compartments, its pressure causes a piston disc to roll around in the chamber. The disc’s movements operate a dial (or counter) through gearing elements to indicate that a column of fluid that has passed through the meter.