Which instrument is best to determine the hydraulic grade line in a distribution system?

• A. Flow meter
• B. Thermometer
• C. Pressure gauges
• D. Water meter

Answer: (C)

The hydraulic grade line represents the total head available at a point, which is the sum of the elevation head and the pressure head (z + p/γ). To determine this line, you need to know the pressure at various points along the system, because the pressure head is the component that reflects the energy available to push water through the network.

A pressure gauge provides the actual pressure at a location. From that reading you can convert to pressure head (p/γ) and add the known elevation (z) of that point to obtain the hydraulic grade line. The other instruments measure flow, temperature, or volume, none of which directly give the head information needed to map the HGL.