Day 2 Salinity labwork

The Guildline salinometer in the salt lab

My mascot pingu at the winch controls

The CTD control terminal

While the air-sea gas exchange team is measuring how the ocean breathes (see previous post) I was learning how to perform some basic tasks within the physical oceanography team. The physics of the ocean are driven primarily by the density of the water (which controls its buoyancy - dense water sinks and lighter water rise up) and the velocity (speed and direction of currents). To measure the density one needs to know the salinity, temperature and the sampling depth. None of these parameters are easy to come by when things need to be done accurately.

The depth can be derived from a pressure sensor, and the temperature can be measured electronically. How does one measure the salinity of the seawater? How do we know how much salt is in the water at a given depth? Those who have seen how sea salt is manufactured by evaporation might  suggest that simply boiling up a water sample would reveal the amount of  salts in the residue that's left over, but things are much much more complicated than this.

It turns out that the most practical way of measuring salinity of seawater is to measure its conductivity. Stick two wires into the water, measure how well it conducts electricity and you have an idea about its salinity. The better it conducts electricity the more saline it is. And instruments measuring this effect can be unbelievably precise. And because they are so precise (they really need to be, more on that later) they need to be constantly calibrated.

The device that measures the parameters required for the density of the water is called a CTD, short for Conductivity, Temperature, Depth. As salinity is derived from conductivity, those 3 values is all a physical oceanographer needs for the density (the other main interest of physical oceanography - current velocity - is measured using other means). The CTD probe is lowered into the ocean from a long cable (the winch controls are operated by Pingu, my mascot, as seen in the photo above) and it transmits its data to a computer on the ship (the work place with the blue chair in the photo above). The computer compiles a detailed profile of temperature and salinity and computes the density which is so crucial to the physics of how the ocean works.

Today my lab work involved learning how to operate the salinometer. The salinometer is the big box in one photo with a bucket at the lower end. One can easily grasp that "salinometer" is a device measuring salinity, but if the CTD probe already measures the conductivity, i.e. the salinity, why do we need to measure it again in the lab. Well, the reason is accuracy. The changes in salinity from the surface to the bottom are relatively big and the accuracy of the conductivity probe mounted on the CTD would be more than sufficient. When working in the deep ocean, however, the salinity readings may vary by only tiny amounts - several digits after the decimal point. Not only may the salinity near the bottom be virtually identical to the salinity several hundred metres further up, it may also only vary by a tiny amount from year to year. But even the smallest changes are significant for the processes that drive the ocean currents on a global scale.

To ensure that the conductivity probe on the CTD remains accurate it is constantly calibrated. To do this water samples are brought up (I'll show this process in a couple of posts time) and analysed in the lab. The analyser which never leaves a temperature-controlled laboratory is more accurate that the probe which gets hoisted over the side and dunked into the ocean every day. Using the accurately measured salinity from the water samples, the electronically recorded profile from the CTD can be calibrated to a given standard, which is used by all oceanographers regardless which ship they work on or which equipment they use. This ensures that all measurements are as accurately as is currently technologically feasible.

In the next few days I shall report more on the science experiments. Take a look at Helen Czerki's blog in the Guardian's science section, where she blogs about life on board and her experiments measuring bubbles in the ocean. The address for that blog is http://www.guardian.co.uk/science/series/scientific-log-southern-ocean