Hydra Expanded Description

Contents:

1. Introduction

2. Velocity Sensor

3. Serial Channel

4. Analog Channels

5. Multiple Burst Sampling

1. Introduction

The heart of the SonTek/YSI Hydra is its datalogger. The central processor (CPU) controls the operation and precise synchronization of data from all sensors, even at the high 25-Hz sampling rates used by the SonTek/YSI ADV and ADVOcean Probe. In addition to the velocity sensor, the Hydra can accept one additional serial instrument and up to four additional analog sensors. The Hydra is also capable of multiple-burst sampling schemes.

Figure 1. Hydra Data Integration

2. Velocity Sensor

Velocities for the Hydra system can be measured by an ADV or by an ADVOcean Probe. The standard configuration includes compass/tilt, temperature, and strain gage pressure sensors mounted into an expanded bell cap on the probe's signal conditioning module. For detailed information on the operational principles of the velocity sensor, please see the ADV or ADVOcean Probe descriptions on this web site.

3. Serial Channel

To date we have written two drivers for the additional serial channel -- one for the Paroscientific quartz crystal pressure sensor, and one for the SeaBird MicroCat Conductivity/Temperature sensor.

While the standard strain gage pressure sensor is an excellent sensor for wave studies in relatively shallow water, the higher accuracy and stability of the quartz crystal sensor is necessary for long-term tidal studies. The Paros is limited to a maximum sampling rate of 4 Hz.

The SeaBird MicroCat CT sensor is designed for the measurement of slowly varying salinities and temperatures. The MicroCat is a highly accurate sensor that gathers data at a maximum rate of 0.2 Hz.

4. Analog Channels

There are three analog channels into the Hydra system, two of which are typically taken by the strain gage pressure sensor and the measured battery voltage. If a Paros is installed on the serial channel then the strain gage analog channel becomes free. The battery voltage measurement allows the user to develop a voltage vs. remaining battery power for their specific instrument configuration, but it can be bypassed if the channel needs to be used for another sensor. One of the most common additional analog sensor(s) integrated into a Hydra system is an Optical Backscatter Sensor (OBS) for turbidity studies.

5. Multiple Burst Sampling

The Hydra can support up to three independent sampling strategies to run simultaneously. A typical example would be to gather a five-minute average for mean properties, followed by two minutes of data at 4 Hz for wave studies, followed by a burst of 30 seconds at 25 Hz for turbulence studies, followed by a sleep period to conserve power.

• Hydra Overview
• Hydra Applications
• Hydra Specifications
• Hydra Principles of Operation
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