Frequently Asked Questions

1. Q: What is the difference between open channel and full pipe flow?
  A: Open channel flow is flow in any channel where there is a free surface such as sewers, rivers, ditches, etc. Full pipe flow is flow in a completely filled pressurized conduit.

2. Q: How do electromagnetic insertable magmeters operate?
  A: An array of electromagnetic sensors are strategically located on the insertable probe that spans the entire pipe diameter. The insertable probe detects and compensates for shifting profiles unlike spool-piece meters and flowmeters that only provide a single point flow measurement.

3. Q: When monitoring open channel flow, why is it important to monitor both depth and velocity?
  A: The measurement of both velocity and level in the same cross-section is a requirement necessary for accurate flow rate measurement using the Continuity Equation Q = V x A where Q = flow, V = average velocity and A = area.

4. Q: Why is it important to verify flowmeter accuracy with the use of independent testing laboratories?
  A: In addition to the manufacturer’s published specifications, testing done by an independent laboratory assures the potential purchaser that the meters accuracy rates have been verified by a third party source. When shopping for flowmeters, be sure to ask the manufacturer if they have independent test data available. Most manufacturers do not make the costly investment and only perform in-house flowmeter testing.

5. Q: How is radar technology utilized to measure flow?
  A: A non-contact sensor transmits a radar beam that interacts with the fluid and reflects back signals at a different frequency than transmitted. These reflected signals are compared with the transmitted frequency. The resulting frequency shift provides an accurate measure of the velocity and the direction of the flow. Level is detected by ultrasonic pulse echo. Flow is then calculated based on the Continuity Equation Q = V x A, where Q = flow, V = average velocity and A = area.

6. Q: Is there a resource to assist for flow professionals with open channel and full pipe flow calculations?
  A: www.flowcalculator.com

7. Q: How are flowmeter accuracy rates stated?
  A: The accuracy of flowmeters, and other instruments, is usually stated as either a % of full scale (FS) or a % of reading. It is important to understand the meaning of each as they represent very different numbers.

A flowmeter with a velocity accuracy of 1% FS and a range of 20 ft. per second (fps) would have an error of ± 0.2 fps throughout the operating range. At 1 fps this flowmeter has an error of 0.2 fps or 20% of reading. It is only at 20 fps that this flowmeter actually performs to an accuracy of 1%.A flowmeter with its velocity accuracy expressed as 1% percent of reading with the same 20 fps operating range will have an accuracy of 1% regardless of the actual velocity.

8. Q: Why are non-contact open channel flow sensors preferred by most users versus submerged style flow sensors?
  A: The non-contact sensor eliminates accuracy problems inherent with submerged sensors including sensor disturbances, high solids content and distribution of reflectors. The non-contact sensor can also be installed and removed from street level virtually eliminating confined space entry concerns that include expensive equipment and manpower costs.

9. Q: What methods are used for open channel velocity profiling?
  A: A particle of water near the conduit wall will not move as fast as a particle toward the center. To understand this, take the molecules of moving liquids as an example. The first layer of molecules stick to the wall of the conduit. The next layer will move by sliding across the first layer. This happens throughout the flow with each successive layer moving at a faster velocity. The change in velocity is greater near the conduit wall than it is toward the center. If velocity measurements of each layer could be taken, a velocity profile would be produced. Velocity decreases near the surface. Because most flows fit this profile, this is called the typical profile. There are, however, situations which will cause other profile shapes and it is usually more difficult to calculate flow with these shapes.

To calculate flow, an average or mean of all the varying velocities must be determined. Because it is not practical to measure the velocity of each layer of molecules, methods have been developed by which a mean velocity can be determined from velocity measurements taken at a number of positions in the flow. Follow this link for a detailed tutorial on open channel velocity profiling:
www.marsh-mcbirney.com/classes/oc_profiling/

10. Q: Is there a resource for flow monitoring professionals that includes up-to-date industry, educational and product information?
  A: Yes, the free monthly e-newsletter Focus On Flow. This newsletter helps flow professionals keep the edge on solutions, analysis and innovations critical to their success. It is located at:

www.focusonflow.com

11. Q: What is the minimum solid contents for velocity measurement with a submerged Doppler (ultrasonic) velocity sensor?
  A: The answer depends on the Doppler sensor’s ultrasonic frequency. The higher the frequency, the better the performance in cleaner water applications. A 1 MHz Doppler sensor would be able to reliably measure velocity in flows that are as clean as 50 mg/L (ppm) of total suspended solids (TSS).

12. Q: Why isn’t there a standard to present accuracy (both level and velocity) of flowmeters? e.g. % full scale vs. % reading
  A: The accuracy of flowmeters (and other instruments) is usually stated as either a % of full scale or a % of reading. It is important to understand the meaning of each as they represent very different numbers.

A flowmeter with a velocity accuracy of 1% of full scale and a range of 20 feet per second (fps) would have an error of ± 0.2 fps throughout the operating range. At 1 fps this flowmeter has an error of 0.2 fps or 20% of reading. It is only at 20 fps that this flowmeter actually performs to an accuracy of 1%.

A flowmeter with its velocity accuracy expressed as 1% percent of reading with the same 20 fps operating range will have an accuracy of 1% regardless of the actual velocity.

13. Q: What type of flowmeter would be best used on a parshall flume that is exposed to the elements? The channel will surcharge during high flows.
  A: I would lean toward using a submerged pressure area velocity flowmeter in the upstream position of your parshall flume. The submerged pressure area velocity sensor would be my top choice as it will be the least affected by the backwater condition you will occasionally see with a flume downstream of it. We have many customers that employ this approach with great success. They often use their flume readings when it is not in surcharge and then use the area velocity readings when they are in surcharge

14. Q: Could you comment on the use of submerged Doppler sensors as flows in a pipe change from open-channel flow conditions to full, surcharged pipes?
  A: The transition occurs transparently. What I mean by this is that as the level increases and goes into surcharge, the data logger (which is programmed with the channel dimensions) realizes that the pipe is now full and the flow calculation holds the wetted area to that of full pipe even though the depth measurement can and often will continue to read increasing depth as the surcharge depth increases above pipe invert.

The above is a list of Frequently Asked Questions supplied by the Flow measurement zone sponsor, Marsh-McBirney. Please check back periodically for updates!