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Fiberglass packaged metering manholes are an innovative solution to the problem of needing to measure piped flow below grade.

Packaged metering manholes factory integrate a flow element (usually a flume, but weirs and magnetic flow meters can also be integrated) directly into a fiberglass manhole.  The resulting structure is lightweight, watertight, corrosion resistant, and ready for immediate installation!

At typically 1/10th the weight of comparable concrete manholes, fiberglass packaged metering manholes cost less to install, are easier to maintain, and have longer service lives than other options. 

The monolithic nature of a packaged metering manhole means that the unit is watertight, with each element laminated together – resulting in a continuous, unbroken exterior fiberglass laminate surface.  There are no joints to leak, seals to maintain, or parts to assemble.  

fiberglass packaged metering manhole with domed top

Corrosion Resistance

The fiberglass construction is perfectly suited for the corrosive environment of raw sanitary sewage.  What would corrode concrete or even lower grades of stainless steel doesn’t pose any problem for a packaged metering manhole.

In industrial or landfill leachate applications, where the standard corrosion resistant isophthalic polyester resins might not be sufficient, flow elements can be fabricated from even more chemically resistant vinylester resins; providing the ultimate in corrosion resistance.  


The smooth flow surfaces and manhole barrel walls are virtually maintenance free and resist the build-up of biological growths and are easy to maintain even if high concentrations of fats, oils, or greases (FOGs) are present. 

Other than the periodic inspection of safety elements (like the OHSA compliant fiberglass ladder), there is little in the way to maintain.

Single-Source Responsibility

As the unit comes as a single, factory-integrated structure, fiberglass packaged metering manholes give the peace of mind associated with singe-source responsibility.  By directly integrating the flow element (flume / weir / mag meter) into the manhole, users can be confident that their installation will work and work correctly. 

For contractors, factory-integrated structures allow them to concentrate on what they do best and not having to worry about the ins-and-outs of trying to correctly install a flow element into a manhole or vault. 


The monolithic nature of a fiberglass packaged metering manhole means that installation is greatly streamlined

Installation is as simple as six easy steps:

  • Excavate the site
  • Pour a slab
  • Unload the manhole
  • Stop flow / cut the line
  • Set the manhole
  • Backfill

For industrial clients this means that critical plant downtime is kept to an absolute minimum.  By using a precast slab, the time to install a metering manhole can be even shorter. At a prepared site, installation shouldn’t take longer than a morning and actual line downtime can be as short as a 30 minutes.

Installations can be scheduled for weekends or previously scheduled plant shutdowns without worry.

Integrated Structures

Unique to fiberglass construction, a key advantage of the packaged metering manhole is the ability to integrate devices and structures much larger than the manhole’s barrel. 

This is particularly important when integrating a Parshall flume (the most common device for measuring open channel flows).  As Parshall flumes get larger in size, they grow in length.  A 6-inch Parshall is 60-inches [152.4 cm] long – and this excludes any end adapters (in fiberglass) or formed channels (in concrete) to transition the flow into and out of the flume.  With end adapters, flumes on average double in length – quickly making them too long to fit into concrete manholes (and many concrete vaults).

Parshall flume measuring industrial effluent in a fiberglass packaged metering manhole

With fiberglass construction, any portion of the flume that is larger than the manhole barrel is simply allowed to extend upstream / downstream as necessary.  Reinforced, watertight covers are then laminated to the flume and the whole structure is then in turn laminated to the manhole barrel.  The result is a seamless, watertight, and monolithic unit.

A wide variety of flow elements can be integrated into fiberglass packaged metering manholes , including:

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