How the Radar Method Determines Flow Rate

The radar measurement method plays a progressively important role in the wastewater and water field. This is specifically revealed by the "Metering outside of the medium" trend. Flow measurement by means of radar, which allows contactless measurements, has become a main focal point. It features a wide range of applications and therefore is a beneficial stable measurement system for several applications with part filled pipes or canals in the wastewater and water field.

Figure 1. Hybrid measurement system.

Flow Rate Determination

With regards to monitoring wastewater or water volumes, flow rates determination is vital for many processes. However, to guarantee continuous flow determination, a measurement system that enables improved velocity detection is needed for the according application. The radar measurement method enables contactless flow velocity metering. Hence, it is ideal for applications with strong sedimentation on the bottom of the channel, or if sensors cannot be installed on the bottom of the channel or within the medium because of several reasons.

The radar metering, in contrast to other measurement systems, entails the benefit that it is largely independent from the properties of the measurement medium such as temperature, viscosity, density, or conductivity. Additionally, the microwave-based method stands out from the crowd of other flow measurement methods because of easy installation and low maintenance.

Measurement Principle

Radar sensors are installed outside of or above the measurement medium. A signal with a certain frequency is transmitted out by the radar sensor. This signal is reflected when it impinges on the water surface. Once the signal is reflected from the water surface, a frequency shift is created. The radar sensor detects the reflected signal, which will be assessed through the Doppler principle.

Figure 2. Schematic drawing: Radar measurement principle.

Wave formation on the water surface is the precondition for the radar technique. The sensor measures the waves’ movement and therefore the surface velocity of the water. A single velocity is selectively measured on the water surface. With the help of NIVUS’ hydraulic COSP model, it is possible to calculate the average flow velocity from selective single velocity. An extra level sensor, which allows the determination of the wetted area A, is used to measure the flow level. Flow Q is calculated from the wetted area A and the average velocity as follows:

    Q = V x A

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

Formula 1: General flow rate calculation

Radar Technologies

A difference is made between continuous-wave radar units and pulse radar units. Pulse radar units release high-frequency impulse signals at high power. The radar sensor, once reflected, receives the signal as echo. New signals will not be transmitted before an echo has been received.

Continuous-wave radar units (CW radar units) convey continuous signals. Hence, reflected signals are received permanently which allows for te permanent measure of velocities, for instance, in the wastewater and water field. Utilizing the radar technology, NIVUS relies on the latter technique for flow measurement.

Radar Meter System

The standard radar metering system consists of the new NivuFlow 550 transmitter, a radar flow velocity sensor (optionally with Ex zone 1 approval) and a level sensor. Both sensors provide the measured data for the transmitter which in turn calculates the flow Q by considering hydraulic models.

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

System set-up and installation can be performed rapidly and easily, as all measurement place parameters can be directly set on the transmitter. Thus, extra hardware or software is not required.

In a WWTP, inlet sewer flow volume and flow velocity have to be established and logged. The measurement is used as control measurement so as to prevent flooding the treatment plant. The customer demanded a contactless low-maintenance system due to partially high dirt loads and hence sedimentation was anticipated on the bottom of the channel. Due to this arrangement, for instance, a water-ultrasonic sensor on the bottom of the channel was not preferred. A radar meter was the most perfect measurement system for this measurement site, as all conditions were met.

Figure 4. Application example WWTP inlet sewer.

Hybrid Flow Measurement

NIVUS is the only provider who delivers a hybrid measurement system for flow measurement apart from pure radar meter systems. This hybrid system is an extension of the radar measurement system. Besides flow velocity detection using radar, the flow velocity is also detected by using ultrasonic cross correlation. Thus, the measurement system is supplied with the NivuFlow 7550 hybrid transmitter, two flow velocity 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 7550 hybrid transmitter.

The transmitter completes or unites the measured flow velocities by realizing the average flow velocity. Based on the filling level and the sensor installation positions, there are two crucial applications for the hybrid meter system: hybrid metering as redundant measurement to enhance accuracy and hybrid metering as an extended measurement range, for instance, during flood conditions.

Extended Measurement Range: Flood Sensor

Standard discharge measurement situation:

The ultrasonic wedge sensor is fixed securely on the pipe ceiling. In this system configuration, usually only the radar sensor measures the flow velocity.

Figure 6. Hybrid measurement system; Standard discharge measurement situation.

High level measurement situation:

Set-up slightly out of the channel crest, the ultrasonic sensor starts to measure shortly before the flood situation is reached based on the installation point. In this situation, the ultrasonic sensor and the radar sensor measure parallel. The example illustrated in the image has a very small range of measurement because the ultrasonic sensor is fixed securely to the ceiling and starts to measure only before the dead zone is reached. Based on the local flow velocities identified by the ultrasonic sensor, the hydraulic model can be enhanced for the whole measurement situation.

Flood measurement situation:

Once the sensor’s dead zone is reached, measuring is no longer possible with the radar sensor. From this point on, the ultrasonic sensor fully takes over the measurement task.

Figure 7. Hybrid measurement flood condition.

A reliable measurement is assured over the whole range of measurement because the level during such situations is measured by means of the ultrasonic sensor’s pressure cell.

Redundant Measurement for Increased Accuracy

In this hybrid measurement method, the ultrasonic sensor is arranged below the minimum filling level. Thus, a redundant flow velocity measurement using ultrasound is performed continuously in parallel to radar metering. Local velocities in up to 16 layers are detected by the ultrasonic cross correlation sensor. It is possible to create an accurate hydraulic model of the measurement situation along with the flow velocities established by the radar sensor. Based on this model, the flow rates and the average flow velocity can be established very accurately.

Figure 8. Redundant flow metering.

Summary

In the past few years, flow metering using radar systems is progressively becoming popular. Radar measurement systems stand out for a wide range of uses in various part-filled applications. As a result of the ease of maintenance and contactless measurement, radar metering is especially suitable for applications that feature sedimentation or dirt loads even if it is not possible to arrange sensors inside the medium. The use of radar technology for flow measurement is realized through the options offered by hybrid measurement systems, owing to extending the measurement or increasing the accuracy.

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

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