Conductivity, Temperature, Depth: know what I mean?

Published on 08.05.2010 - Catlin Arctic Survey - 2

Just the once won't hurt, so here's a little science to brighten up your humdrum day. Close-up on the first three vital seawater readings that Ann Daniels's group has been taking for the past three months.

Whereas the temporary base located 600 km from Resolute Bay closed on 1st May, the trio led by Ann Daniels still continues to make progress towards the North Pole, while taking the scientific readings required for their mission. So, what are these scientific readings? First and foremost they are the kind of readings that all oceanographers have to take if they want to understand about the physical composition of seawater. These readings are given a catchy name, 'CTD', which is the acronym for Conductivity, Temperature and Depth. The readings are taken using a CTD sensor or CTD Probe, which determine the physical properties of the seawater.

The CTD sensor or probe is able to provide a highly detailed description of the water tested. By measuring the conductivity of the seawater, which is done by analysing the way in which an electrical current passes through the water, researchers are able to obtain an accurate reading of the water's salt content. This is quite simple to do, because the current passes much more easily through water that has a high level of salinity.

The CTD sensor also measures the temperature of the water, as well as its depth -two elements that produce a reading of the water pressure (because the two elements are closely linked: the deeper the water, the colder it becomes). Scientists then use these two analyses to calculate the density of the water tested. Tests of the water analysed also include obtaining its level of dissolved oxygen, organic and inorganic carbon and various other nutrients.

The CTD sensor or probe enables samples of water to be taken at different depths. Ann and her two companions have been collecting water samples at 20 and 50 metres. As they do not have a winch for calculating the depth to which they are dropping the probe, they have developed the following highly technical method: the sensor device is attached to a rope, which in turn is tied to Charlie Paton's belt. He then walks away from the hole in the ice until there is no more slack in the rope. The sensor is placed on the ground above the hole through which it is due to be lowered. Charlie then strides 50 paces, each one measuring a metre, towards the hole and the sensor drops into the ocean by the same amount. After taking the required readings at that depth, Charlie then walks back 30 paces away from the hole, which means that the sensor is then at a depth of only 20 metres, which is where the other readings are taken. And so on...

Once the seawater samples have been taken, the water is stored in small containers and kept warm. They mustn't be allowed to freeze, otherwise their CO2 will disappear (don't ask me why...). The samples are then collected each time fresh supplies are brought to the expedition and taken off to the laboratory for analysis.

For those who would like to know more about CTD probves, here are three intgeresting sites : Ocean Instruments - Histoire des sondes CTD (in French) - NOAA (National Oceanic and Atmospheric Administration).

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