The supply of clean drinking water, and guaranteeing its quality and quantity, is becoming a more acute, important and costly issue. To record parameters from a body of water, in addition to the most commonly measured parameters such as determining the water level and temperature, other measured variables must also be determined, for example, conductivity. Conductivity is a good indicator of water cleanliness and an important parameter in the supply of drinking water.
Instrotech, the local supplier of Keller, a market leader in producing isolated pressure transducers and transmitters, has a CTD multiparameter probe and multiparameter logger that measures and monitors the quality and quantity of drinking water.
The acronym ‘CTD’ stands for conductivity, temperature and depth. These parameters are determined by the probe through a conductivity sensor, a PT1000 temperature sensor, and a pressure sensor. 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
Some media or substances, such as water, can transmit heat, sound or electricity. The more dissolved ions (chlorides, sulphates 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, on the other hand, contains very few to zero dissolved ions, which prevents the flow of electrical current in the water.
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 milli-siemens per cm (mS/cm).
Some example values are:
• 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 a change in conductivity. This means that at higher temperatures, the number of dissolved ions and their mobility increases significantly, hence the conductivity 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
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.
The application dictates which of two different measuring methods is used: absolute pressure (related to vacuum) or relative pressure (related to ambient pressure).
Keller’s 36XiW-CTD multiparameter probe series and DCX-22-CTD / DCX-22AA-CTD multiparameter loggers combine all the aforementioned measurements in one device.
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