Components of a Complete Conductivity Measurement System
12 Nov,2021
A complete conductivity measurement system comprises an inductive or conductive measuring electrode, a cable, and a measuring instrument. Unlike pH electrodes, conductive electrodes are made from materials with well-defined mechanical properties, so they do not require special protective components. When selecting a conductive electrode, the following considerations should be taken into account:
1. Measurement range of the electrode
2. Measurement method: conductive or inductive. When using a conductive electrode, select an appropriate electrode constant “K”.
3. Temperature
4. Pressure Related to Mechanical Structure
5. Chemical Corrosion Resistance
6. Installation Configurations (Threaded, Flanged, Three-Clamp Connections, etc.) All measurement cables must be shielded. In particular, for conductive electrodes—where the electrode is in direct contact with the solution being measured—when the conductivity exceeds 100 mS, the effects of cable length and cable cross-sectional area must be taken into account, since the cable resistance is also part of the measurement circuit.
For example: if the measured value is 100 mS and the electrode constant K is 1, the resistance is 10 ohms.
|
When the electrode constant K = 1 The resistance value is 10 ohms. |
Resistance (ohm) | 1 |
|
Electrical conductance (S) |
|
For a 100-meter-long cable The cable cross-section is 0.75 mm. 2 At the time, The resistance of the cable (two-wire) is 4.76 ohms. |
Resistance (ohm) | Cable length (m) |
|
K (copper = 56) • F (mm 2 ) |
The measured electrode resistance of 10 ohms must be augmented by the cable resistance of 4.76 ohms, resulting in a final instrument reading of 67.75 mS. In such cases, it is advisable to use a shorter measurement cable or one with a larger cross-sectional area. For inductive electrodes, however, the cable resistance does not affect the measurement value.
Measurement instruments, on the other hand, must convert the measured resistance value into siemens units and also eliminate various interfering factors mentioned earlier—such as temperature and the α coefficient—in order to provide an accurate conductivity value.
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