Testing and Calibration of Contact-Free Radar Type Discharge Sensor

Abstract

There are numerous techniques for measuring flow rates in open channels and closed conduits. The exact and real-time knowledge of the discharge is an important task in the fields of hydrology, water storage management and irrigation. Periodical calibration of the current meter is therefore necessary as per ISO 3455/IS 13371 [1, 2] to ascertain its performance and to account for possible wear and tear of its parts. Installation, performance, reliability and safety for rotating and non-rotating type current meters in flood plain are challenging tasks due to turbulent flow and the presence of debris, etc. To overcome this, water contact-free device is need of time, which can sense flow from distant place. Tehri Hydro Development Corporation has acquired such sensors from Sommer Messtechnik, GmbH, Austria, to measure the continuous river flow and entrusted the calibration to CWPRS, using current meter rating trolley (CMRT) facility. Equipment is based on the principle of the Doppler effect. The instrument is mounted on the CMRT above the tank. The radar sensor transmits a signal with a constant frequency in a specific angle to the water surface. The signal is reflected and received back with a shift in frequency due to Doppler effect by movements of the water surface. In order to simulate river-like flow conditions, three cup type current meters at a specific depth by rigid rod suspension were immersed at front, middle and rear sides of the trolley to propagate surface water currents. The contact-free water level measurement by sensor uses the principle of transit time measurements of reflected signals and transmits a short microwave impulse in the direction of the water surface. This impulse is reflected at the water surface and is recorded by sensor. Calibration of contact-free radar discharge sensors type RQ-30A [3] has been carried out. The result of calibration indicates close agreement between the speed of rating trolley and corresponding water velocity indicated by the sensor, which are well within the specified accuracy of ±0.02 m/s for most of values. The water level measured by the metric gauge and indicated by sensor is also within specified accuracy of ±0.002 m. Periodical calibration of the sensor is very much necessary to check its accuracy and performance.