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  1. Analytical sensors
  2. Conductivity sensor LDL
  3. Technology

Conductivity sensor technology

Conductivity measures how well a substance conducts an electric current. It is influenced by the quantity of free ions (salts, acids, alkalis) in the medium and by the temperature of the medium: the more free ions, the higher the conductivity. A conductivity sensor typically consists of two metal plates in contact with the medium. If two electrodes are immersed in a conductive liquid and a voltage is applied to these two electrodes, a current will flow.

The positively charged ions (cations) move to the negatively charged electrode, and the negatively charged ions (anions) move to the positively charged electrode. The more free ions are in the medium and the higher the electrical conductivity of the medium, the higher is the current.

The technology used in conductivity sensors differs depending on the design. A distinction is made between conductive and inductive conductivity sensors.

Conductive conductivity sensors

The LDL100 conductive sensor

The LDL100, like other directly measuring conductivity sensors, has two metallic electrodes. The difference in our design is that the sensor housing and metal tube serve as the first electrode and the metal tip of the sensor serves as the second electrode.

Voltage is applied between the sensor tip and the housing screw connection and the current flow is measured.

Note: Given the design of its electrodes, the LDL is not recommended for use in plastic pipes.

The LDL101 conductive sensor

Unlike the LDL100, the LDL101 does not use its housing as electrode, but has two ring-shaped electrodes set into one another. The voltage is applied between the inner and outer electrode and the current flow is measured there.

It is important to note that in contrast to the LDL100, the LDL101 has a fixed cell constant. With the internally used software, different cell constants can be mapped in order to achieve the best resolution over the entire measuring range at all times. So the LDL101 offers functions in one device other sensors require different versions for.

Inductive conductivity sensors

An inductive conductivity sensor consists of two metal coils wound with wire and enclosed in a plastic body (ifm uses PEEK or polypropylene for this purpose). The first coil (transmitter coil) generates an electrical voltage in the liquid. Depending on the conductivity of the medium, an alternating current is generated. The latter generates an alternating magnetic field in the second coil (receiving coil) that is proportional to the conductivity of the medium.

Inductive conductivity measurement has several advantages:

  • High resistance to corrosion thanks to the plastic tip.
  • Insensitivity to solids in the medium as long as the measuring channel is not clogged.

Did you know? (LDL2)

A common problem with injection-moulded, long PEEK tips is that they tend to break off. This is due to the stress caused by temperature and pressure fluctuations that occur especially in CIP applications.
Turned from one piece, the tip allows the PEEK material to expand evenly with temperature changes, distributing pressure more evenly across the shaft and preventing potential stress points. General machine availability is maintained.

The influence of temperature on LDL sensors

The conductivity of a material is particularly dependent on temperature – approximately 1...5% per °C. All conductivity sensors have a built-in temperature measurement to compensate for temperature changes in the medium.

The graph is intended to show the difference between compensated and uncompensated conductivity. Without compensation (blue line), the conductivity increases or decreases based on the temperature, i.e. the conductivity no longer remains constant although the medium is still the same. When using compensation (orange line), a constant and repeatable measurement is provided. This renders the measured values comparable at different times. More information on temperature compensation and how to adjust it can be found in the section on calibration.

Calibration of LDL sensors

A free factory certificate is available for each ifm conductivity sensor. It is generated directly in production and assigned to the serial number. The sensor passes through different calibration stations, each with different temperatures and conductivities. During final calibration, the sensor is compared with a reference sensor. All this information can be taken from the factory certificate.
Download the factory certificate free of charge from our website. Make sure that you have the serial number of the sensor at hand to enter it.

Field calibration

ifm sensors arrive at your premises ready for use. However, you can still adjust the sensor to specific media or reference temperatures on site. For this purpose, the two parameters "Calibration gain – CGA" and "Temperature compensation – T.cmp" can be set so that the sensor is adjusted to a known reference medium.

Calibration gain [CGA]: aligns the measurement curve of the sensor to the known value of the reference medium. It is possible to set a value between 80 to 120 %. For the calculation, the known value is divided by the measured value.

Temperature compensation [T.cmp]: extent to which a temperature deviation from the reference temperature (usually 25 °C) causes a change in conductivity.

  • The compensation can be set freely between 0 and 5 %/K.
  • The temperature compensation is either provided in the data sheet of the medium (for water-based media the standard is 2%) or determined via a straight line equation by measuring the same medium at 2 temperatures.

Adjusting CGA and T.cmp can lead to higher accuracy, but in most cases is not necessary.

ISO calibration and recalibration

For long-term reliable measurement results, ifm offers the calibration and recalibration of conductivity sensors. The comparative measurement of conductivity sensors is carried out with reference solutions that have known conductance values. In the context of comparative measurement, the device under test is immersed in the reference solution and the deviation between the actual and target value is documented. Based on this, measures can be taken to correct the deviations and ensure precise measurement.

FAQ on the LDL sensor technology