HYDREKA - Model BFM 801 -Heavy-Duty Velocity Measurement Instrument

The flat sensor has the sensing electrodes on one surface of the sensor and these should normally be facing uppermost. The volume of water whose flow is being sensed is a cylinder between the electrodes and extending to approximately 10mm above the sensor. This sensor is therefore also suited to shallow applications. Because of the small sensing volume, if the water is turbulent, the real time readings of the flat sensor will fluctuate more than those of the cylindrical sensor.

The sensor should be aligned into the flow and the calibration is determined for +ve flow [flow on to the sensor at the electrode end] with the sensor upstream of the wading rod.

The sensor will detect negative flow and the same calibration is used, although there will normally be some interference caused by the turbulence from the wading rods. The unit is set to measure speeds up to 5 M/sec in both directions.

The system is calibrated as a combination of sensor with electronics in the Control Display Unit, and the serial number of both the probe and the CDU are displayed on the display on start up.

The system will operate in any conducting fluid, and the conductivity does not effect calibration. At veiy low conductivities the signal will, however, become noisier. A simple check is to check the noise in still water.

The CDU has its own internal batteries, and bulkhead connectors for the data interface to PC and the sensor. There is also a waterproof pressure equalising valve, to compensate for changes in temperature and atmospheric pressure that would otherwise cause the display membrane to distort. The comiectors have waterproof pro-caps for when not in use. The CDU is designed for operation in rain conditions and for temporary immersion in water to 0.3m for 10 seconds, provided all connectors or pro-caps are fitted.

The EM system measures the flow twice every second, and calculates the real time flow every second as the average of the half second readings. The REAL T^KM display is updated every second. The average speeds are computed as the average of the one second real time values over the averaging period which has been set [maximum period of 600 seconds]. If an average period is terminated early, then the calculation is based on the time since the average was started.

The 2Hz data from the EM system has been digitally filtered from raw 96Hz data. The filter is a Digital FIR filter with a fixed time delay and no frequency dependent phase shift. The filter -3dB cut off is 0.27Hz, and the delay time is 3 . 5 seconds.

The STANDARD DEVIATION [SD] is calculated from real time samples taken during the averaging period and gives an indication of the quality of the measurements, A high standard deviation indicates either a high variability in the flow, or the probe has not been held steady during the measurement.