How to get pure water on ships
We studied about fresh water generators used on board various types of ships to generate pure water from sea water, which by itself is unfit for human consumption. We studied in the article relating to Salinometer as to how the presence of salt was detected using conductive properties of water in order to find out whether the water is fit for human consumption or not (pH meter is used to check for pH value). But purity is not only about reduced NaCl levels but also has to do with several other dissolved impurities which need to be removed. Of course there needs to be a method which can measure the level of purity in quantitative terms for human beings to see. This is the purpose of the Ionic water purity meter as we shall see below.
Ionic Water Purity Meter
The basic principle behind an ionic water purity meter is same as that of the Salinometer, i.e. the conductivity of water which gives an idea about its purity. As you must know the specific conductivity of water refers to the conductance of a cubic unit of water which is also known as Dionic unit and is measured in Ohm/cm3 or equivalent units.
Such a column of water is artificially contained in a setup which consists of plates of gun-metal and platinum collars and conductivity of water is measured using the arrangement shown below.
As you can see in the diagram above the entire setup consists of a gunmetal casing which has platinum wire ring electrodes as well as insulating tubes and insulating plunger. Since we know that the conductivity also gets affected by temperature there is a provision for temperature compensation as well which can cater to a change of 20 degrees Celsius. This change is achieved by varying the water flow rate in proportion to the change in temperature of water.
This is necessary in the case of ships which keep moving around the world in different waters and where temperature could vary from sub zero to 30 degrees. The picture below gives an indication of the variation of sea water temperature in the areas of the Atlantic Ocean. As you can notice the temperatures towards the top right hand corner is nearly two degrees below zero and that in bottom left hand corner is nearly 30 degrees, speaking in terms of Celsius scale. Just in case you are not able to related this image to the actual political map of the world, the region shown is a portion of the Atlantic ocean between the continents of North America and Europe.
One precaution that needs to be taken is that if any carbondioxide is present in the water in the dissolved state, it needs to be removed before measuring the conductivity as its presence could lead to unexpected errors; hence this is done with the help of de-gasifiers. Also since any impurities on the electrodes can also lead to errors, they should be kept sparkling clean and all the electrical connections should be tightly adjusted.