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A BRIEF OVERVIEW TIDAL MEASUREMENT TECHNOLOGY

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A tide gauge is an instrument used to measure changes in sea level mechanically and automatically. This tool has a sensor that can measure sea level which is then recorded and stored on a storage system.

Previous tidal measurement technology used the Float and Stilling Well Gauge (figure 1). A Stilling Well is a vertical tube (- can be PVC) with a hole in the bottom that can be used to drain seawater. The water level inside the tube will be the same as the open sea outside, but the effect of waves will be dampened. Inside the well is a float that rises and falls with the water level, and is attached through a wire on a pulley to an accurate clock-driven chart recorder. The rise and fall of the water level are recorded as a line by a pen on graph paper that moves with the rise and fall of the sea level, and the operator digitizes the graph.

Figure 1. (left) Schematic of float and stilling well tide gauge (source: bidstonobservatory.org.uk) and (right) float and stilling well tide gauge unit installed at the jetty (source: qyudos documentation).

Currently, there are several tide sensor mechanisms developed, the most popular types of sensors are Pressure Tide gauge and Radar Tide Gauge. Pressure Tide Gauge uses a pressure sensor that utilizes the principle of hydrostatic pressure, and Radar Tide Gauge measures the distance between the sensor and the sea surface by utilizing electromagnetic waves.

Pressure Tide Gauge

In this discussion, we will focus more on explaining the pressure sensor. Then how does this tool work? The answer applies the principle of hydrostatic pressure.

In this case, the sensor inserted into seawater is a pressure sensor (figure 2). Sensors that are in water will experience hydrostatic pressure, so the relationship between pressure and depth is linear because the value of density and gravitational acceleration is fixed.

The density of fresh water under standard conditions at 15 °C is 994 kg/m3 and sea water under standard conditions at 15 °C is 1025.97 kg/m3. While the acceleration of gravity is 9.81 m/s2. So that the water level to the sensor can be obtained from:

Figure 2. The pressure sensor of the Valeport Tidemaster unit is inside the housing so that it is protected from interference (source: qyudos documentation).

Valeport Tidemaster Calibration

The Valeport Tidemaster ‘user calibration’ method requires the operator to input the gain factor (distance per dBar) and offset (distance of the sensor from the datum or usually zero bar). Valeport Tidemaster measures pressure in deciBar units (1 Bar = 105 Pascal; 1 dBar = 104 Pascal). The relationship between depth and pressure is as follows:

Gain Factor

The gain factor means that every 1 dBar change represents a change in sea level by a certain number of meters. This value is different for freshwater and seawater.

  • Fresh Water

Using a standard density of 994 kg/m3 (pure water at 15°C) and a gravitational acceleration of 9.81 m/s2.

Then 1 deciBar = 1.025 meters of water depth. Thus the gain factor is 1.025.

  • Seawater

Using a standard density of 1025.97 kg/m3 (standard seawater at 15°C) and a gravitational acceleration of 9.81 m/s2.

Then 1 deciBar = 0.993 meters of water depth. Thus the gain factor is 0.993.

Offset

The distance of the sensor to the datum generally uses the zero point of the tide palm so that the offset is zero (figure 3). If there is an offset, the inputted value is positive if the sensor is above the datum and negative if the sensor is below the datum (figure 3).

Figure 3. The offset value is measured from the sensor to the zero palm, offset = 0 (source: qyudos documentation).

Reference:

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