INDICATORS OF WATER QUALITY:

temperature, depth and conductivity

Water can transmit heat, sound and electricity. And Keller’s CTD multiparameter probe measures all three variables.

The acronym ‘CTD’ stands for conductivity, temperature and depth and is determined by the following sensor elements:

• conductivity sensor (conductivity)

• temperature sensor PT1000 (temperature)

• pressure sensor (depth). Since these parameters are directly dependent on each other, Keller incorporates three sensors into one device to provide compensated and thus highly accurate measured values.

Conductivity

The more dissolved ions (chlorides, sulfates or carbonates) there are in a substance, the higher its electrical conductivity. Thanks to the high number of dissolved ions, sea- or salt water has a conductivity many times higher than conventional drinking or fresh water.

Carefully distilled water contains very few to zero dissolved ions, which prevents the transportation of the electrical current in the water. Pure water has an extremely low electrical conductivity because of the lack of impurities within it.

Conductivity is determined by six titanium electrodes. The electrical current flow between the electrodes immersed in a medium is measured.

The greater the current flow, the higher the conductivity. Conductivity sensors for water bodies measure from a few micro-siemens per cm (µS/cm) to about 200 millisiemens per cm (mS/cm).

Some example values:

• distilled water – 0.05 µS/cm to 1 µS/cm

• drinking water – 300 µS/cm to 800 µS/cm

• seawater – approximately 50 mS/cm.

Temperature

Temperature changes in a substance also mean changes in conductivity. At higher temperatures, the number of dissolved ions and their mobility increases significantly and the conductivity therefore also increases.

The temperature can be read out as an independent measuring value, but Keller uses it to compensate for the other parameters. The PT1000 temperature sensor is located directly next to the titanium electrodes, so the temperature and conductivity are measured at the same point.

Depth

Depth can be used to help calculate flow volumes and thereby convert pollutant concentration values into environmental loads.

The water depth is determined by the difference between the surface and the immersion depth. In measurement technology, one refers to the so-called water column. For example, a water column of 10 m corresponds to a pressure difference of approximately 1 bar. Factors influencing depth measurement in water include barometric pressure, density and temperature. It is important to consider that the pH of water can vary with depth in the water column.

The application dictates which of two different measuring methods is used:

• absolute pressure (related to vacuum)

• relative pressure (related to ambient pressure).

Keller’s multiparameter probe series 36XiW-CTD and the multiparameter loggers’ series DCX-22-CTD/DCX-22AACTD combine all the above-mentioned measurements in one device.

Instrotech is the local supplier of Keller, and the market leader in producing isolated pressure transducers and transmitters. It has a CTD multiparameter probe and multiparameter logger that measures and monitors the quality and quantity of drinking water.

By Ralf Christoph, GM, Hanna Instruments South Africa