The 2-inch Parshall Flume is the second smallest of the Parshall Flumes.  Like the 1-inch, the small size means that it is somewhat limited in use.

When used to measure sanitary flows, the flow entering the flume must be well screened, or it will clog.  The 3-inch Parshall Flume is the smallest Parshall that can be used on unscreened sanitary flows.

Applications

While somewhat limited in its uses, the 1-inch Parshall Flume can be found monitoring:

• Dam seeps
• Clean industrial pre-treatment flows
• Acid mine discharge
• Smaller surface waters
  • Creeks
  • Furrows
  • Ditches

Configurations

Openchannelflow's 2-inch Parshall Flume is available in different configurations:

• Standard
  • Transitions by others
• End adapters
  • ​Connecting to pipes and flanges
  • Stubs and flanges up to 10-inches [25.4 cm] can be accommodated
  • Staged end adapters to fit into existing manholes
• Enclosures
  • Factory mounted into an above-ground enclosure
• Metering manholes
  • Integrated into a fiberglass manhole
  • 2-inch Parshall Flumes can be integrated into manholes 4-feet [1.22 m] and larger
• Wing Walls
  • Span rectangular channels

Flume Accuracy

Under lab conditions, a Parshall Flume can be accurate to within +/-2% but practically speaking, once approach conditions, installation errors, and construction tolerances are considered, the flume's free-flow accuracy usually is closer to +/-5% (ASTM D1941).

A universal flow equation is available to correct for the effects of submerged flow.

If the flume shifts out of position, a correction for lateral / longitudinal settling has been developed.

Flume Dimensions

No two Parshall Flumes are identical in scale.  Openchannelflow recommends against using intermediate or untested sizes.  

Smaller Parshall Flumes (1, 2, 3-inch sizes) do not have standardized sidewall heights.  Openchannelflow's 1-inch Parshall has a standard sidewall height (at the primary point of measurement - H_a) of 9-inches [22.86 cm].

Free-Flow Equation

The free-flow equation for Parshall Flumes is:

For the 3-inch Parshall flume, the values are:

Minimum Head (ft)	0.05	Minimum Head (m)	0.0152
Minimum Flow Rate (cfs)	0.0033	Minimum Flow Rate (l/s)	0.0921
Maximum Head (ft)	0.70	Maximum Head (m)	0.2134
Maximum Flow Rate (cfs)	0.1945	Maximum Flow Rate (l/s)	5.507
Equation (cfs, ft)	0.338*H1.55	Equation (l/s, m)	60.36*H1.55

Discharge Table

A free flow discharge table for the 2-inch Parshall is available for download.  

The table provides:

• A plan view showing the primary point of measurement (H_a) 
• Discharge equations in a variety of Imperial and SI units
• Flume accuracy
• Submergence Transition (S_t)
• Table source

Materials

The 2-inch Parshall Flume is available from Openchannelflow in four different materials: 

• Aluminum
• Fiberglass (FRP)
• Galvanized steel
• Stainless steel

Of these materials, fiberglass is the most easily customized.

Submergence Transition

The Submergence Transition (S_t) of a flume is that point at which the downstream conditions sufficiently reducer the discharge out of the flume that the indicated flow rate needs to be corrected.

The Submergence Transition for the smaller Parshall Flumes (1, 2, 3-inch) is 50%.  At this point and the discharge needs to be adjusted to reflect the actual flow rate.