Conductivity to Measure Sea Surface Salinity to Estimate Speed through the Water or Leeway: Sea Surface Temperature/Salinity Charts & Depth Measurement Accuracy

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Summary

By measuring the conductivity of the sea surface we can calculate the sea surface salinity. Salinity can be used to improve the accuracy of sonar depth measurements from 1 to 5% and we can construct a chart of sea surface salinity which would help find fish. We already measure sea surface temperature. By measuring the interruptions in the measurement we can estimate turbidity and Speed Thru the Water (STW). By measuring turbidity along several axis, the vector of motion of the water can be triangulated and hence an estimate of leeway can be made.

 

Solved

1) Aid to finding fish by providing a chart, updated in real-time as measurements are made, that will enable the location of changes in salinity, turbidity and temperature that are indications of where fish may be found.

 

2) An estimate of STW could help autopilot programs/systems

 

3) An estimate leeway would be very desirable to professional & amateur racing sailors.

 

Note: Leeway is the amount of side slippage thru the water and traditionally very hard to measure or estimate accurately.

 

Solution

The basic premise is to use an AC conductivity measurement to measure the conductivity of the surface of the water, in units of, for example, micro Siemens per centimeter (μS/cm). This needs to be an AC measurement to avoid the issues of the sacrificial anode. The temperature of the water is measured by a thermistor. Using a photodiode we measure turbidity directly rather than indirectly by inference from

the conductivity.

 

Once you have conductivity and temperature, you can convert that to salinity using the standard UNESCO equation of state of seawater.  The international standard algorithm, often known as the UNESCO algorithm, is due to Chen and Millero (1977), and has a more complicated form than the simple equations, but uses pressure as a variable rather than depth. For the original UNESCO paper see Fofonoff and Millard (1983). Wong and Zhu (1995) recalculated the coefficients in this algorithm following the adoption of the International Temperature Scale of 1990 and their form of the UNESCO equation is:

 

c(S,T,P) = Cw(T,P) + A(T,P)S + B(T,P)S3/2 + D(T,P)S2

 

Cw(T,P) = (C00 + C01T + C02T2 + C03T3 + C04T4 + C05T5) + (C10 + C11T + C12T2 + C13T3 + C14T4)P +

(C20 +C21T +C22T2 + C23T3 + C24T4)P2 +

 

(C30 + C31T + C32T2)P3

 

A(T,P) = (A00 + A01T + A02T2 + A03T3 + A04T4) + (A10 + A11T + A12T2 + A13T3 + A14T4)P +

(A20 + A21T + A22T2 + A23T3)P2 + (A30 + A31T + A32T2)P3

B(T,P) = B00 + B01T + (B10 + B11T)P D(T,P) = D00 + D10P

T = temperature in degrees Celsius

 

S = salinity in Practical Salinity Units (parts per thousand) P = pressure in bar.

Range of validity: temperature 0 to 40 °C, salinity 0 to 40 parts per thousand, pressure 0 to 1000 bar

(Wong and Zhu, 1995).

 

Salinity has a direct effect on the speed of sound thru water and hence the accuracy of our sonar depth reporting, this adjustment can be anything up to about 5% of the depth.

 

Turbidity could be estimated indirectly by the frequency in fluctuations to the conductivity measurement. By knowing the separation of the electrodes, the minimum particle size that can be measured is defined. By direct measurement (using a photodiode) we could measure trurbidity in units of FTU, FNU or NTU units.

 

Given the GPS location, a map of temperature, salinity and turbidity over time can be created and when presented to the user, they can spot where the contours change in the map and these are indicative of where fish may congregate. Some fish will prefer particular environments and extend their range to find food in places within their comfort zone and other fish perhaps from the other side of the confluence, will feed on them. Thus the map helps the User find where fish might be congregating.

 

Knowing how either of the conductivity or turbidity measurements varies over time, gives an indication of the vessel’s speed thru the water (STW), similar to ultrasonic techniques. Furthermore, given several conductivity sensors pointing in different directions, then the STW in particular directions can be measured and thus turn speed into a vector quantity which can be used to measure Leeway. Leeway is of major importance to racing sailors as it is a direct measure of how much speed they are losing to the water, if they can reduce the amount of leeway they can increase their forward speed.

 

Aeration is a factor which we can infer from interruptions in the conductivity measurement. Aeration allows the sonar to know when air bubbles have passed over the sensor, and thus to know when the sensitivity of the transducer has been impaired and the samples for the ping may not have the same characteristics as the pings before or after it.

 

Once we know about Aeration, if the sensor is out of the water for prolonged periods, if say it is mounted on one hull of a catamaran which is currently out of the water, due to tacking, then we can reduce the power in to the sonar transducer to prevent damage from pinging in air; similarly we can prevent damage while the sonar is on the bench during development or servicing.

I160011FL01 Conductivity Measure Salinity and Speed Est Publication