Wave Gauge Calibration

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This page focuses on resistive wave gauges

Overview[edit]

These instructions provide a generic overview of the calibration procedure for a twin-wire resistive wave gauge (or probe), in order to determine a calibration coefficient.

Resistive wave gauges are comprised of two parallel steel rods fixed at a set distance apart, which are mounted perpendicular to the flow direction. A high frequency alternating voltage is passed through the rods, and the conductance between the rods is recorded [1]. The measured conductance varies depending of the rod submergence, the measured conductance can in turn can be converted into a water surface elevation by following a calibration procedure. This requires recording the instrument output voltage for a range of water surface levels, and then establishing the linear correlation between water depth and instrument voltage. Wave gauges are normally designed to aid the calibration procedure, for example, they may be constructed with pre-drilled holes along the supporting stem at pre-defined intervals (e.g. 10mm) both above and below the zero datum (see Figure 2). The exact procedure may change slightly for different equipment manufacturers and instrument types, and slight variations of the calibration procedure detailed here may be implemented in some scenarios.

Instrument Installation and Set-up[edit]

Figure 1: Annotated photograph of wave gauge components and set-up, corresponding to Figure 2
  1. Install wave gauge securely in the required location, and connect electrical wires from the wave gauge into the control unit, and the oscilloscope.
  2. It is recommended to clean the twin-wire resistance gauges to minimise the effect of measurement noise from variable electrical properties.
  3. Slide the supporting stem up of down, to position the gauge at the required elevation for data acquisition, and secure in place using the holding pin. Note: zero datum (shown in Figure 2a) is given in this example for simplicity; however, gauges can be set up for data acquisition with a non-central position on the supporting stem if necessary.
  4. The instrument should then be ‘zeroed’ by adjusting the zero datum dial the control box, so that the output voltage reading on the oscilloscope is 0.00V.
  5. Initially, the wave gauge should be lowered using the supporting stem and secured in place using the holding pin. This is position +100mm in the example shown in Figure 2a, simulating a rise in water surface elevation.
  6. The instrument gain can then be adjusted on the control box to ensure that the maximum operating range of the instrument is utilised, which will ensure that the highest resolution measurements are obtained. For example, if a symmetrical number of calibration holes are present above and below the position zero hole, and the wave peak or trough will not exceed this range, then the calibration can be set close to, or at to the instrument operating limit, i.e. ±10V.

Additional Notes:

  • The gain should only be adjusted once on the first occasion, such as upon instrument installation, or the start of a new experiment. If the gain is changed substantially, it is recommended that steps 3-4 should be repeated after step 6, ensuring the instrument remains correctly ‘zeroed’. If repeating steps 3-4, the user should then proceed to step 7 (skipping steps 5 and 6).
  • The wave gauge will have a maximum operating range, for example ±10.0V. Therefore, if the instrument range is ±10V and the gain in Figure 3a is set to +10.0V, then there may be a small region that is uncalibrated, as shown in Figure 3b, and any water surface recording exceeding that position will be recorded as -10.0V.
  • In some research scenarios it may be impractical to calibrate using the calibration holes on the supporting stem, if so, the same principles can be applied by raising and lowering the water level within the facility in set increments.


Figure 2: a) Schematic of wave gauge components and set-up, red and Black dashed lines indicate the electrical wire connections. Wave gauges connect directly to the control unit, where the instrument can be zeroed and the gain set. An oscilloscope can be connected to the control unit in order to provide a live and direct output measurement in Volts. Note: the control unit may also be referred to as a ‘wave probe monitor’. b) Interval spacing of predrilled holes along calibration stem.
Figure 3: Maximum extent of wave gauge positions during calibration procedure, and thus the maximum calibrated range of wave gauge. a) simulates a rise in water surface elevation, such as a wave crest, while b) simlates a fall in water surface elevation, such as a wave crest. Refer to Figure 2a for numbering of holes on calibration stem.


Calibration Coefficient Procedure[edit]


Instrument setup is now complete and a calibration process should be undertaken as follows:

  1. A measurement of voltage output should be recorded while the wave gauge is mounted at each of the calibration positions (i.e. each hole). This will enable the operator to record a table of data containing voltage output for each position – The example below shows data collected for calibration holes with a 10mm spacing (Table 1).
  2. The results of step 7 should then be used to calculate a linear regression equation between water elevation and output voltage from the wave gauge. A plot of this relationship is shown in Figure 4.
  3. Steps 7-8 need to be repeated regularly, and as a minimum, they should be undertaken at the beginning and end of each experimental day. Figure 5 shows an example of an actual instrument calibration. This is necessary to ensure that any change over time due to changes in either water temperature, salinity, or instrument variability can be fully quantified. It is possible that after some time, when the gauge is positioned at calibration position zero, the output voltage may have ‘drifted’ and no longer be exactly 0.00V. For example, the voltage may now read ±0.20V at position zero, it is important to record this, and conduct a calibration procedure to quantify this change. The user should not reset the gain or voltage to zero, as this will incorrectly alter the calibration coefficient, simply repeat steps 7-8.

Note: If the user is confident a linear trend exists along the wave gauge, a lower resolution calibration procedure can be completed by reducing the number of calibration holes at which measurements are acquired. It is advised that a minimum of three measurements are acquired, covering the full range of the wave gauge.


Table 1: Example calibration table for one wave gauge
Date (dd/mm/yyyy) Time (hh:mm) Calibration Hole Water Surface Elevation, η (mm) Voltage (V)
02/11/2018 13:27 1 +100 10.00
02/11/2018 13:29 2 +90 9.00
02/11/2018 13:31 3 +80 8.00
02/11/2018 13:33 4 +70 7.00
02/11/2018 13:35 5 +60 6.00
02/11/2018 13:37 6 +50 5.00
02/11/2018 13:39 7 +40 4.00
02/11/2018 13:41 8 +30 3.00
02/11/2018 13:43 9 +20 2.00
02/11/2018 13:45 10 +10 1.00
02/11/2018 13:47 11 0 0.00
02/11/2018 13:49 12 -10 -1.00
02/11/2018 13:51 13 -20 -2.00
02/11/2018 13:53 14 -30 -3.00
02/11/2018 13:55 15 -40 -4.00
02/11/2018 13:57 16 -50 -5.00
02/11/2018 13:59 17 -60 -6.00
02/11/2018 14:01 18 -70 -7.00
02/11/2018 14:03 19 -80 -8.00
02/11/2018 14:05 20 -90 -9.00
02/11/2018 14:07 21 -100 -10.00


Figure 4: Example calibration graph and linear regression from data in Table 1
Figure 5: Calibration coefficient graph from an actual experimental dataset

References[edit]

  1. Hughes, S. (1993). Physical models and laboratory techniques in coastal engineering. Singapore: World Scientific, pp.472-478.