Velocimeter Hardware
Figure 2. Parts of a velocimeter probe. From left to right: cable, conditioning module, measuring probe.
A Velocimeter consists of three modules: the measuring probe, the conditioning module with cable (Figure 2) and the processing module. The waterproof conditioning module, which contains low-noise electronics, is rugged and can be deployed to 30 m in the standard configuration.
The acoustic sensor consists of one transmit transducer and three receive transducers (Figure 1). The receive transducers are mounted on short arms around the transmit transducer at 120° azimuth intervals. The acoustic beams are oriented so that the receive beams intercept the transmit beam at a point located at 50 mm or 100 mm below the sensor. The interception of these four beams, together with the width of the transmit pulse, define the sampling volume. This volume is 3-9 mm long and approximately 6 mm in diameter. All three receivers must be submerged to ensure correct 3-D velocity measurements (except for the 2D/3D probe).
Velocimeter calibration factors are determined by the speed of sound and by the angles between the transmit and receive transducers. To ensure that the correct speed of sound is used, the water temperature and salinity must be entered in the data acquisition software. The calibration angles are measured at the factory and need only be changed when a new probe is installed. Maintenance calibration is not required unless the probe is physically damaged.
The processing module performs the digital signal processing required to measure Doppler shifts. In the Laboratory Velocimeter, this computationally intensive task is implemented on a PC-board that fits any IBM-compatible computer (minimum 386/387) with full-sized slots. Several Velocimeters can be controlled by one computer, but each instrument requires a separate processing module.
In the Field Velocimeter, the processing module is a low power, stand-alone unit with its own microcomputer. This module is intended for use in underwater or power-limited applications where a full-sized computer is impractical. This unit can be interfaced to a user-supplied data acquisition system via analog outputs or to a notebook computer through the RS232 port. Nortek also supplies Windows 95/98/NT data acquisition software that can collect data from many Field Velocimeters at once.
The standard data acquisition software supplied with the Velocimeter provides real-time display of data in graphical and tabular form. The data are recorded to disk in highly compressed binary files which can be easily converted to ASCII format with the data conversion programs supplied with the system.
The Velocimeter has input/output control lines that permit synchronization with other laboratory instrumentation. Analog outputs proportional to the three velocity components are also provided for easy interface to existing data acquisition systems.
For applications which require simultaneous operation of several Velocimeters from the same computer, Nortek offers a rugged data Acquisition Computer that can operate up to eight instruments.
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Figure 1. The acoustic sensor has one transmit transducer and three receive transducers. The sampling volume is located away from the sensor to provide undisturbed measurements. Doppler velocity is derived from signals scattered by small particles. In natural bodies of water (streams, lakes, rivers, oceans, etc.) the natural occurrence of particles is sufficient for proper operation. In model tanks with running water (flumes, open channels, closed pipes, etc.) microscopic bubbles in the water column tend to act as natural seeding. In clean, quiescent water (ship models, tow tanks, and some wave flumes), seeding materials must be added to concentrations of approximately f10 mg/l. Non-soluble seeding material with sensitivity close to 1 and mean diameter of 8-10 m is available from Nortek.