• 4129 Varden Crt

  • Mississauga, ON L5L 4A7

  • Phone: (905) 607-2728

  • Fax: (905) 607-9180

  • E-mail: tqi@sympatico.ca

Flow Monitoring

A/V Meter vs Weir – Flow Monitoring Considerations & Typical Study Progression

Most flow monitoring contractors utilize Area/Velocity (A/V) type flow monitors which are commercially available from various manufacturers. These employ a depth sensor to measure flow depth which is then converted to flow “area” based on the pipe geometry. They also measure velocity which is adjusted to “average” velocity before being multiplied by the area to get flow rate (Q=V*A). This type of flow monitor can provide accurate results when used in sewers with appropriate flow conditions. A/V meters are intended to be able to measure flow during surcharge conditions but unless the velocity sensor has been calibrated at near surcharge conditions (which is seldom feasible), such flow measurements are not very accurate.

Standard, commercially available Area/Velocity (A/V) type flow monitors seldom provide reliable flow data in small sewers. There are many reasons for this including :

  • Pressure type depth sensors have a non-linear response at low flow depths resulting in poor accuracy at low depths of flow/small pipes.
  • The sensor block size is significant relative to the flow cross-sectional area at low flow/small pipes. This impacts accurate measurement of flow area in such conditions.
  • In steep small sewers a standing wave often forms just upstream of the sensor block where the velocity is measured. This causes the flow area where velocity is measured to be different from that where the depth/area is measured resulting in significant under measurement of flow rates.
  • In small sewers and low flow depths there is often insufficient depth of flow to allow accurate measurement of velocity.
  • Precision of velocity(& thus flow) measurement at low velocities is poor. The minimum velocity for flow measurement is generally considered to be 0.06 m/s(0.2 ft/s) and as rates reduce close to that, accuracy “drops off”.

These factors are significant enough that on many occasions some of our competitors have retained us to assist them in measuring flow in small pipes with our proprietary Custom Compound Weir (CCW) flow monitoring system. We routinely work with only 2 such firms in Ontario for whom we have great respect.

This system was recognized by the ACEC with an Award of Merit in their annual national engineering awards program and has been used at over 1200 monitoring stations across Canada.

Our CCW monitoring system consists of a compound weir (which has been shown to have excellent self cleaning properties), custom built for each location and instrumented with a 2.5 to 3.0 psi 0.1% non-linearity level sensor and data logger. This sensor has a precision over twice that of 5 psi level sensors used in A/V meters. I can provide a copy of our corporate brochure which shows examples of our CCW monitoring system and our proprietary flow plotting package if desired. Our CCW system typically costs more than an A/V meter system to set up but is more economical to operate and costs typically become comparable after a duration of 2 months.

Standard, commercially available Area/Velocity (A/V) type flow monitors seldom provide reliable flow data in small sewers. There are many reasons for this including :

In situations where surcharge is expected, our CCW can be instrumented with an A/V meter. The weir maintains a minimum flow depth sufficient that it can be accurately measured and the A/V meter depth sensor is then utilized with our weir. During conditions of surcharge the velocity sensor is utilized in conjunction with the depth sensor to directly compute flow (Q=A*V). During high flow but non-surcharge conditions before and after a surcharge incident both flow measurement methods are valid and the velocity sensor can be calibrated by the weir. This provides more accurate surcharge condition flow measurement than would otherwise be the norm. This unique capability was behind our being selected on a sole source basis for a massive flow monitoring project (over 40 monitors utilized) covering selected combined sewers in Hamilton in 2010.

Our flow monitoring work typically progresses as follows :

  1. 1. An initial meeting will be held with yourself if desired to review the intended purpose and location of each monitoring station. Our CCW system typically measures outflow from a manhole while A/V meters are normally set up to measure inflow to a manhole. After the meeting we locate and assess the different manholes.
  2. 2. A senior hydraulics person will then enter each station’s manhole to fully assess the hydraulic situation, install weir supports and accurately measure the manholes for weir design/fabrication if necessary.
  3. 3. We will then design and fabricate custom weirs for each location at our office/shop.
  4. 4. A second trip will then be made to install the weirs and instrument them. The stations will be visited twice per month for data retrieval and system checks. At each visit entry is made to take manual readings to compare with the monitors.
  5. 5. The data is reduced to flow rates and available as “preliminary” flows as the project progresses.
  6. 6. At the end of the monitoring period all the stations (including weirs) will be removed and the whole period data reviewed by myself before being finalized.
  7. 7. The data will be assembled in a database and multi-colour weekly plots produced covering all data. Examples of our plot format may be found in our corporate brochure or can be provided separately.
  8. 8. Interval averaging will be carried out to determine daily average flows and these will also be plotted.
  9. 9. Short duration “event” plots will be produced covering rainfall events of interest to facilitate analysis of system response to rainfall.
  10. 10. All work carried out will be documented in a brief report (3 copies) which will include full sets of weekly, daily and event plots.