Salinity sensors

It is recommended to read this article in conjunction with the article Salinity.

Practical Salinity

In the course of time, various methods have been developed to determine the salinity of seawater, see the article Salinity. The most practical method currently used is through electrical conductivity. Because this is an indirect method, an accurate relationship has been established between the conductivity [math]C[/math] and the salinity [math]S[/math] as a function of temperature [math]T[/math] and pressure [math]p[/math]. The salinity determined in this way is called the Practical Salinity. The accuracy with which the conductivity of seawater can be measured is limited for reasons listed later in this article. Possible measurement errors can be eliminated, at least partly, by considering the relative conductivity [math]R[/math], the seawater conductivity [math]C(S,T)[/math] relative to the conductivity [math]C(35,15)[/math] of a standard saline solution at 15°C containing 32.4356 g KCl in a mass of 1 kg. The salinity of this standard solution is exactly 35. The relationship between the salinity [math]S(T)[/math] and the relative conductivity [math]R=R(S,T)[/math] reads:

[math]S(T) = S(15) + \Delta S(T) , \qquad R=\Large\frac{C(S,T)}{C(35,15)}\normalsize , \qquad (1)[/math]

[math]S(15)=0.008-0.1692\,R^{1/2}+25.3851\,R+14.0941\,R^{3/2}-7.0261\,R^2+2.7081\,R^{5/2} , \qquad (2)[/math]

[math]\Delta S(T)= \Large\frac{T-15}{1+0.0162(T-15)}\normalsize (0.0005-0.0056\,R^{1/2} -0.0066\,R-0.0375\,R^{3/2}+0.0636\,R^2-0.0144\,R^{5/2}) , \qquad (3)[/math]

for [math] 2 \, \le S \le \, 42 [/math]. If R=1 we have S=35.

For simplicity, the equations (1-3) are written for atmospheric pressure. In shallow coastal waters the pressure correction is usually small, but CTD processing normally includes pressure. Possible instrumental errors are reduced by using a conductivity ratio rather than absolute conductivity. In the formal PSS-78 definition, the reference is a KCl solution containing 32.4356 g KCl in a total mass of 1 kg solution. A seawater sample that has the same conductivity as this reference solution at 15°C and atmospheric pressure has Practical Salinity S=35.

Sensors

Platinum Electrode Conductivity Sensor

As Practical Salinity is not a physical parameter that can be measured directly, true “Salinity Sensors” do not exist. What is commonly referred to as a salinity sensor is in fact a conductivity sensor.

A Conductivity Sensor measures the ability of a solution to conduct an electric current between two electrodes. In seawater, the current is carried by ions; therefore, an increasing concentration of ions in the solution will result in higher conductivity values. Conductance is defined as the reciprocal of resistance. When resistance is measured in ohms, conductance is measured using the SI unit, siemens (formerly known as a mho). Seawater conductivity is commonly reported in mS/cm or S/m; freshwater instruments may use µS/cm.

Fig. 1. Electrode conductivity principle.

Fig. 2. Flow-through conductivity cell. Seawater is pumped through a narrow bore containing platinum electrodes. In this design, the electric field is confined mainly within the bore, so the calibration is little affected by nearby objects outside the cell.

The conductivity [math]C[/math] is related to conductance [math]G = I/U[/math] by

[math]C = G k_c[/math],

where [math]k_c = d/A[/math] for an ideal parallel-plate cell, [math]I[/math] is the electric current, [math]U[/math] the applied voltage, [math]d[/math] the electrode spacing, and [math]A[/math] the electrode area, see Fig. 1.

A potential difference is applied to the two probe electrodes in the salinity sensor. The resulting current is proportional to the conductivity of the solution. This current is converted into a voltage. A typical conductivity cell is shown in Fig. 2.

Electrodeless Conductivity Sensor (Inductive)

Fig. 3. Inductive conductivity sensor principle

Electrodeless conductivity sensors use inductive coils, see Fig. 3. The conductive loop is formed by the sample solution. The inductive conductivity sensor consists of two coils which are incorporated next to one another in a polymer or ceramic body. These coils form current transformers. The sensor is designed so part of the liquid media forms a closed conductive current path passing through the coils. Current is applied to the primary coil (generating coil), which induces an alternating voltage in the liquid loop. In liquids which conduct electricity, this causes a current flow captured by the second coil (receiving coil), which is proportional to the conductivity of the sample solution.

Comparing the sensors

Electrode Sensors

The main advantages of electrode sensors are:
- in well-designed enclosed conductivity cells, the electric field is confined, so nearby objects have little effect on the calibration
- easily calibrated in small baths
The main disadvantages are:
- any changes in the cell constant will be reflected in the conductivity, and the cell cannot be cleaned in the field
- electrodes are subject to corrosion or damage (however, most sensors used in oceanography are inside a cell that prevents this, as shown in the image above)
- electrodes inside a cell need seawater pumping which limits the measuring rate

Inductive Sensors

The main advantages of inductive sensors are:
- robust construction
- they are easier to clean mechanically
- there are no electrodes, so there is no possibility of their damage
The main disadvantages are:
- many inductive sensors have an external electromagnetic field. Nearby surfaces, metal parts, other instruments or biofouling can therefore affect the measurement. The required clearance depends on the sensor design and should be checked against the manufacturer’s specifications. It is particularly difficult to address this problem in companion mounts such as Argo floats or biofouling.
- mounting and calibration of the sensor to companion packages (Argo floats, current meters, AUVs, etc.) must be undertaken to avoid intrusion into the external area. Because final package geometry influences the results, the calibration must be performed on the fully assembled package.

Calibration

Calibration of salinity observations is in fact calibration of the conductivity, temperature and pressure measurements from which salinity is calculated. Conductivity sensors are calibrated against standard seawater or certified conductivity standards, usually in a temperature-controlled bath.

A zero-conductivity check in air or dry conditions can be useful for detecting electronic offset in some instruments, but it is not a salinity calibration point. The main calibration point is obtained by measuring standard seawater or a certified conductivity standard at a controlled temperature. For high-accuracy instruments, calibration is performed over the relevant conductivity and temperature range.

For high-quality monitoring, calibration before and after deployment should be combined with field checks using discrete water samples analyzed with a laboratory salinometer. Long deployments require attention to sensor drift, biofouling, air bubbles and contamination of the conductivity cell.^[1]

Operation issues

During operation, salinity errors can arise when the water measured by the conductivity cell is not the same water as measured by the temperature sensor. This can occur in sharp salinity or temperature gradients, especially if the sensor response times differ or if the flow through the cell is too slow. Air bubbles, intermittent pumping or incomplete flushing can also produce short salinity spikes. These artefacts are most common in strongly stratified estuaries, wave-exposed shallow waters and shipborne or profiling measurements where the instrument rapidly crosses water-mass boundaries.

More

Navigation

Tools