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Yokogawa Electric Corporation





Conductivity Sensors

Q1 Types and measurement ranges of conductivity sensors
A1 Conductivity sensors are roughly classified into two types: electrode contacting types and inductive types. The electrode contacting types are further classified into the 2-electrode contacting type and the 4-electrode contacting type. The following table lists the cell constants, minimum measurement ranges and maximum measurement ranges of the respective sensors.

The applications of conductive sensors are roughly classified as follows.
Types and measurement ranges of sensors
Types and measurement ranges of sensors
The applications of conductive sensors are roughly classified as follows.
Two-electrode method :
      This contacting conductivity sensor and is most suitable for low-conductivity applications for pure and ultra-pure water such as demineralizers and boiler water in Power Plants.
Four-electrode method :
  Because this measuring method is not significantly affected by dirt or the electrodes, it is suitable for measuring dirty objects such as industrial wastewater and sewage.
Conductive method :
  Because the electrode is covered with resin and does not directly contact the measurement solution, it is suitable for measuring corrosive solutions such as acid and alkaline and dirty solutions but is not suitable for measuring low-conductivity solutions such as pure water.

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Q2 About Cell constants
A2 A cell constant is a value which is determined by the shape of a sensor. The cell constant shown in the table in A1 is a nominal one only and varies slightly from sensor to sensor. Because the actual cell constant of each sensor is indicated on the nameplate of the sensor, set it in combination with the converter. The cell constant can be calculated from the following expression by measuring the resistance of the calibration solution whose conductivity is known.

     Cell constant (J) = Resistance (Rc) x Conductivity (K)

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Q3 Combinations of sensors and converters ( or transmitters)
A3 The sensor and converter (or transmitter) can be combined as follows.

Combinations of sensors and converters

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Q4 Cleaning of sensors
A4 When the 2-electrode contacting sensor becomes dirty, an insulating film forms on the surface of the electrode and the cell constant appears to increase, causing a measurement error. Cleaning the sensor will restore the measurement precision of the sensor.
In addition, the resistance owing to dirt and polarization does not greatly affect the measurement precision of the 4-electrode contacting sensor and inductive sensor.
Methods of cleaning the sensor are shown below.
· Dirt in standard applications can be effectively removed by using a small amount of household detergent mixed with warm water.
· Caustic lime and hydroxide compound can be effectively removed by a hydrochloric acid solution of 5 to 10% concentration.
· Organic dirt (fats and oils) can be easily removed by acetone.
· Algal bacteria and mold can be removed by a household detergent (hypochlorous salt solution).

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Q5 What is the maximum sensor cable length?
A5 The maximum cable length of a 2-electrode contacting sensor and a 4-electrode contacting sensor are 20 m and cannot be increased beyond this upper limit.
In the case of an inductive sensor, the ISC40GJ (sensor for general-purpose applications) can be extended up to 50 m including the sensor cable by using a relay terminal box.
The maximum cable length of the ISC40SJ (safety explosion-proof electromagnetic conductivity sensor) is 20 m and cannot be extended beyond this upper limit.

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Q6 Terminal shapes of cable terminals and applicable converters
A6 The following three types of terminals are available for the cable terminals of conductivity sensors.
Pin-shaped crimp : FLXA21, SC202G, SC202SJ, SC402G, ISC202G, ISC202SJ
Ring-shaped crimp : SC202G-A-□/TB, SC202SJ-1-□/TB, SC450G, ISC202G-A-□/TB, ISC450G
Fork-shaped crimp : SC100

For more information , refer to "Conductivity Detectors/Sensors"(GS 12D08B02-01E).

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