As published in WATERWORLD Magazine, September 2005
By Patrick L. Stevens, P.E.
There are only two depth of flow technologies that are in practical use today, Pressure transducers and Ultrasonic sensors. Velocity sensors fall into four different categories as discussed below.
Pressure transducers are simple strain gauges with the “strain” coming from water pressure pushing on a membrane. An air tube offsets the effect of changing air pressure. Pressure sensors are commonly found on the least costly flow meters and they are simple to operate. Two weaknesses of pressure transducers are that 1) the air tube must be protected with a desiccant to prevent moisture from entering the tube and destroying the strain gauge and 2) the sensor is subject to drift.
All pressure transducers are subject to drift, with the most expensive sensors exhibiting the least drift. Pressure sensor drift is tolerable to agencies that are interested in only a rough idea of how flows vary over time. Drift is intolerable in applications where absolute values are needed such as billing, I/I calculations, and modeling.
Ultrasonic depth sensors are timing devices that measure the time for an ultrasonic “pulse” to return back to the sensor after reflecting off the water surface. These devices are generally not subject to drift and measure with greater precision than pressure transducers. They are more costly and appear on the more expensive, and more accurate, flow meters. Agencies who measure flows in order to bill satellite agencies almost exclusively use ultrasonic measurement either in flumes or area-velocity meters.
All but one manufacturer uses a “down looker” mounted at the top of the pipe to measure the time for the pulse to travel down and back. The “up looker” is mounted in the flow at the bottom of the pipe and the signal reflects off the surface. It is important that these sensors are mounted level such that the signal travels vertically.
The measurement “dead band” is a weakness if a single transducer is used to both transmit and listen for the return echo. The time required for a single transducer to switch from transmit to listening mode prevents a measurement in the dead zone, which can be up to 18 inches. To address this issue, ADS has developed the quad-redundant sensor, which uses four transducers. This increases reliability and reduces the dead zone to ½ inch.
The greatest discriminator among flow monitors, in terms of accuracy and reliability, is the method for measuring depth. The specification sheet for the both ultrasonic and pressure sensors on the market today all claim to be accurate to within a fraction of an inch. However, pressure sensor drift interferes with long-term reliability of pressure sensor data.
Drift commonly ranges from 1 to 6 inches, but can often exceed 36 inches if uncorrected. Drift can occur in a single day or over a month. Flow monitoring service providers using pressure sensors must budget for weekly visits to the site to spot and correct for drift.
There are four velocity technologies in use by the six major manufacturers. 1) electromagnetic, 2) continuous wave Doppler, 3) gated Doppler, and 4) time of transit. All of these technologies rely on the Continuity Equation which requires that the average velocity be known. This is very difficult to do because of the velocity profile of moving water. The velocity near the pipe wall is lower than at the center and the velocity profile can vary widely from pipe to pipe. Manufacturers each employ different methods and algorithms to convert velocities sensed by their technologies into a calculated average velocity, but the method is seldom included in specifications.
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