Differentiation of conductive and inductive technology

The classic set up of conductive conductivity sensors, whose electrodes are in galvanic contact to the medium to be measured, is limited by the point at which high ion concentrations may result in the so-called polarization effect. This can constitute an additional resistance, distorting the measurement result. Similarly, deposits (e.g. sodium hydroxide) can create insulating layers that make conductive conductivity measurement impossible.

For applications with high ion concentrations, such as leaches and acids with conductivity values in the range of 100 mS/cm and when there is a risk of deposits, inductive technology offers the only reliable conductivity measurement and thus reliable control of processes. However, the inductive measurement principle is not suited for measuring very small conductivity values. The smallest measurement range is 500 µS/cm (0.5 mS/cm); in this range precise measurements of around 50 µS/cm can still be carried out.

Design of the inductive conductivity sensor AFI

The sensor element with a seamless PEEK housing contains two ring core coils that act as two virtual transformers arranged in series. The first primary winding is fed by an oscillator in the kilohertz range. The liquid loop that is created by the current measuring passage through the interior of the two ring core coils and the surrounding area connects the secondary side of the first transformer to the primary side of the second transformer. This connection can be considered a common winding of both transformers. The secondary winding of the second transformer is connected to a galvanometer. The volume of the liquid resistance R_M results in an associated current measurement value. Using a simple calculation rule (Ohm’s law), this value together with the known oscillator voltage U_G is transformed into the conductance value G_M to be output.

The sensitivity of the galvanometer must be adjusted for different conductance value ranges. The user can do this by setting freely defined measurement ranges that can be selected during operation by activating control inputs.

The inductive measurement principle is not reliant on the movement of liquids in the current measuring passage. Nevertheless, it is recommend to align the channel in the direction of the flow to achieve better cleaning results. The symmetrical design of the channel accommodates both flow directions without the risk of clogging by media components.

A precise and highly responsive Pt100 temperature sensor is integrated into the tip of the sensor element. The temperature of the medium measured by this is available as a measuring signal and is used in addition to the physical temperature compensation of the raw conductivity value.
 

液體的電導率一般與溫度密切相關。溫度每升高1K，許多水溶液的電導率會增加2%。為了測量結果的可比性，測量設備會將直接測得的電導率換算成參考溫度下對應的電導率（一般為25 °C）。此外，AFI變送器還允許溫度系數以%/K為單位直接輸入。如無需進行溫度補償，溫度系數必須定為0%/K。非線性溫度系數可額外使用一個二次項來加以定義。