A velocity modified flowmeter is a single-unit sensing device utilizing velocity, depth, and diameter data to determine volumetric flow in open channels. Special units can be obtained for use in full pipe flows. A typical unit is made up of a steel band to which is attached an electromagnetic flow-velocity sensor and a pressure transducer to measure water depth (Figure 7-9).
1. Operating Principles: Using a plug-in module correlated to the pipe diameter, the processor applies velocity and depth data to the continuity equation (Q = A x V) and computes volumetric flow without need for pipe slope, surface roughness, or empirical formulas. Even though volumetric flow is the principle information computed and recorded, velocity and depth data may be displayed.
METER DESIGNS: A typical unit is a steel band, sensor units, and cabling between the power source and the data transmitter.
1. Steel Band: An expandable , stainless steel band recessed into the flow conduit and expanded against the interior wall. Attached to the steel band is a capsule containing the velocity and depth sensors. The capsule shape is streamlined to prevent fouling of debris.
2. Velocity Sensor: A solid state, electromagnetic sensor attached to the steel band to correlate existing velocities to mean velocity of the flow conduit. These data are transmitted to the unit processor.
3. Depth Sensor: Water depth is measured by a submerged pressure transducer encapsulated with the velocity sensor. Water depth data are transmitted to the unit processor.
LIMITATIONS: The limitations of a typical velocity modified flowmeter are as follows:
  • Conduit diameter range is from 8 to 102 inches.
  • Temperature limits are from -1°C (-30°F) to +32°C (+90°F).
  • Liquids must have a conductivity of at least 5 micromhos/cm.
  • Diameter of pipe or cross-section of channel must be known.
  • 120 VAC, 60 Hz power is required.
INSTALLATION: Velocity modified flowmeters are installed inside pipes using an expandable steel band sized to the pipe diameter. Velocity and depth sensors are an integral part of the band. Cabling is attached to the sensors from a power supply and to recording equipment. Installation q ay be permanent or temporary.
Velocity Modified Flowmeter
FIGURE 7-9. Velocity Modified Flowmeter
MAINTENANCE: Inspect every six months to ensure sensors, recording and processing equipment are calibrated and functioning.
ACCURACY AND RELIABILITY: Typical accuracy of a velocity modified flowmeter is ±2% with a repeatability of ±0.5%. Reliability is high as long as scheduled maintenance is performed.
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