CTD – The Three Parameters

November 15, 2022  290  290   

The supply of clean drinking water and guaranteeing its quality and quantity is becoming more acute, essential and costly. To record parameters from a body of water, in addition to the most measured parameters such as determining the water level and temperature, other measured variables must also be defined, for example, conductivity. Conductivity is a good indicator of water cleanliness and an important parameter in the supply of drinking water.

A CTD-level probe can be used to measure and monitor the quality and quantity of drinking water.

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

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 transportation 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:

• 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, for example, 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)
• Relative pressure (related to ambient pressure).