How the Radar Method Determines Flow Rate

The radar measurement method has been gaining increasing significance in the water and wastewater sectors, particularly highlighted by the emerging trend of "Metering outside of the medium." This contactless approach to flow measurement using radar technologies has become a focal point due to its vast array of applications. It offers a dependable and stable measurement system in various scenarios, including pipes and canals that are only partially filled.

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Figure 1. Hybrid measurement system.

Flow Rate Determination

Monitoring water and wastewater volumetric flow rates is crucial for numerous processes. To ensure consistent flow evaluations, a system that enhances velocity detection is necessary for specific applications. The radar measurement method facilitates contactless flow velocity monitoring. This feature is particularly beneficial in environments with substantial sedimentation on the channel bed, or where sensor installation on the channel bottom is impractical.

Unlike other measurement methods, radar technology stands out for being largely unaffected by the measurement medium's characteristics—such as temperature, viscosity, density, or conductivity. Moreover, this microwave-based technique offers ease of installation and requires minimal maintenance.

Measurement Principle

Radar sensors are positioned externally to the measurement medium. The radar sensor emits a signal at a defined frequency, which is reflected back when it strikes the water's surface. This reflection induces a frequency shift detected by the radar sensor, which is then processed via the Doppler principle.

Figure 2. Schematic drawing: Radar measurement principle.

Wave disturbances on the water's surface are fundamental to the radar methodology. The sensor captures the waves’ movements, measuring the water's surface velocity. Utilizing NIVUS's hydraulic COSP model, one can derive the average flow velocity from the selectively measured surface velocities. An additional level sensor, which determines the wetted area A, is incorporated to assess the flow level. The flow Q is computed using the formula:

    Q = V x A

Q = Flow rate
V = Average velocity
A = Wetted area

Formula 1: General flow rate calculation

Radar Technologies

There is a distinction between continuous-wave radar units and pulse radar units. Pulse radar units emit high-frequency impulse signals at elevated power levels. These units receive echoed signals only after reflecting from the target.

In contrast, continuous-wave radar units continuously transmit signals and receive reflected signals in real time, facilitating the ongoing measurement of velocities—particularly useful in the water and wastewater sectors. NIVUS employs continuous-wave radar technology for flow assessment.

Radar Meter System

A standard radar measurement setup includes the innovative NivuFlow 550 transmitter, a radar flow velocity sensor (optional with Ex zone 1 approval), and a level sensor. These sensors capture measurement data for the transmitter, which then calculates the flow Q based on hydraulic models.

Figure 3. Complete radar measurement system: OFR radar sensor, level sensor type i-Series (left) and NivuFlow 550 transmitter.

Set-up and installation of this system are quick and straightforward, as all parameters can be directly configured on the transmitter, negating the need for additional hardware or software.

In wastewater treatment plants (WWTP), both inflow volume and velocity must be established and monitored to prevent flooding of the treatment facility. Due to anticipated dirt loads and sedimentation at the channel base, a contactless, low-maintenance measurement system was requested. Consequently, the radar meter emerged as the most suitable solution, fulfilling all operational criteria.

Figure 4. Application example: WWTP inlet sewer.

Hybrid Flow Measurement

NIVUS is unique in offering a hybrid measurement system alongside traditional radar meter systems. This extended hybrid system combines radar flow velocity detection with ultrasonic cross-correlation techniques. As a result, the configuration includes the NivuFlow hybrid transmitter, two flow sensors, and one level sensor.

Figure 5. Hybrid measurement system: v-sensor 1 OFR radar sensor, level sensor i-Sensor, v-sensor 2 POA sensor, NivuFlow hybrid transmitter.

The transmitter merges the flow velocity measurements to deliver an average velocity. The hybrid meter system has two primary applications: redundancy in measurements for increased accuracy and extending measurement ranges during extreme flow conditions.

Summary

In recent years, radar-based flow metering technologies have become increasingly prevalent. These radar measurement systems are distinguished by their wide applicability in partial filling scenarios. Given the non-invasive nature and minimal maintenance required, radar metering is particularly advantageous in environments with significant sedimentation or turbidity, where traditional sensors may prove ineffective. The effectiveness of radar technology for flow measurement extends through hybrid configurations, enhancing both measurement accuracy and operational range.

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This information has been collected, reviewed, and adapted from materials sourced from NIVUS GmbH.

For more information on this source, please visit NIVUS GmbH.