What is salinity?The salinity of sea water is the measure of its saltiness,

usually determined by measuring the electrical

conductivity of a water sample. Expressed scientifically,

‘The Practical Salinity Scale 1978 (PSS 78) defines the

practical salinity of a sample of seawater in terms of the

conductivity ratio (K15) of the conductivity of the sample

at a temperature of 15°C and pressure of one standard

atmosphere to that of a potassium chloride (KCl) solution

containing 32.4356 g of KCl in a mass of 1kg of solution’.

The measure of salinity is a ratio and has no units, but is

usually labelled as practical salinity units (PSU) for clarity.

History of salinity analysis at FRSSalinity analysis at Fisheries Research Services (FRS)

began in the early 1960s with the introduction of the

Autolab electronic salinometer. Prior to this, analysis had

been carried out at the Laboratory of the Government

Chemist by a silver nitrate titration method. Since the early

1980s, Guildline salinometers have been used, starting

with the Autosal, and latterly the Portasal (Fig. 1). The

Portasal as its name suggests, is smaller and easily

transportable. Designed for use on research vessels, it

maintains a high level of accuracy.

Why take salinity samples?The majority of the salinity samples collected by FRS are

used to calibrate conductivity, temperature and depth

profiling instruments (CTDs), and thermosalinograph

instruments used to measure sea surface temperature

and salinity (Fig. 2). Other salinity samples are collected

at shallow water positions, such as sea lochs and at the

FRS Coastal Long Term Monitoring sites. Salinity data are

used, (usually in conjunction with temperature values)

to characterise the marine environment for studies of

marine biology, water circulation and climate change.

Salinity Analysis

Figure 1. Portasal salinometerFigure 2.CTD conductivity calibration graph. The results of this linearcalibration are applied to the data collected by the CTDinstruments.

The Oceanographic Services Group at FRS presently

analyse approximately 2,500 salinities per year; almost

half of these are analysed at sea and the remainder at

FRS’ Marine Laboratory.

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CTD Conductivity Calibration 911+

CTD Conductivity

y = 1.000210x - 0.004419 (y = mx + c)

Sam

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cond

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vity

Sample stationsFRS has an extensive water sampling programme in

support of fisheries and environmental research.

On-going programmes include the recently established

Long Term Coastal Monitoring project, long-term data

collection east of Orkney, and in the Faroe-Shetland

Channel. The latter has been in operation for over 100

years and has yielded a unique and valuable data-set.

Sample collectionSea water is collected at specified depths by a set of

specially designed bottles called Niskens. Twelve Niskens

are attached to a carousel, which also contains a CTD.

The salinity sample is drawn from the Niskin into a

200 ml glass bottle which is filled to the neck (see picture

above). Samples are stored in a temperature controlled

room until analysis.

Sample analysisOnce collected, the sample is pumped into the measure-

ment cell four times to ensure adequate flushing out of

the previous sample before the conductivity is measured

and the result displayed. The result can be displayed as

either conductivity ratio or Practical Salinity Units (PSU).

The range of conductivity measurement is wide, from

0.004 milli Siemens/centimetre (mS/cm)-76 mS/cm with

a resolution of 0.0003 mS/cm. This means salinity can

be measured to an accuracy of better than 0.003 Practical

Salinity Units.

CalibrationThe salinometer is regularly calibrated in-house

to a reference sample of precisely known salinity. The

reference is IAPSO Standard Sea Water*. The salinometer

is also returned to the supplier for an electronic alignment

and linearity check every second year.

For further information contact oceanography staff at the

address below.

* ‘The IAPSO Standard Sea Water Service is the only internationally recognised standard for the calibration of salinity measurement devices. Its widespreaduse over 100 years of IAPSO history has been of great importance to the quality and comparability of salinity data worldwide’.

Figure 3.Long-term coastal monitoring sites (white circles), Faroe-ShetlandChannel (Fair Isle – Munken and Nolso – Flugga Lines) and eastof Orkney (Jonsis Line) sampling stations.

Cromarty

Scalloway

YthanFindon

Stonehaven

Scapa

Loch Maddy

Jonsis Line

Fair Isle – Munken Line

Nolso – Flugga Line

Loch Ewe

Millport

Fair Isle

Mallaig

FM23|05|07

Automatic sampling carousel with salinity sample bottle.