Conductivity Measurement
All aqueous solutions have the ability to pass an electric current and have a measurable conductance. This measurement can tell us about the quality of the water or the make-up of a solution.

In most water solutions as the concentration of dissolved salts increases there are more ions and the conductivity increases. This continues until the solution saturates, restricting the freedom of the ions to move and the conductivity can then decrease with increasing concentration.

A conductivity measurement system consists of a sensor or cell, a connection cable and an instrument or transmitter and offers a fast, reliable, on-line means of measuring the ionic content of a solution.

Historically the unit for conductivity measurement was the mho, (ohm backwards) however the SI unit of measurement for conductivity is the Siemen (S), with a unit of length added, e.g. S/cm.

If you take two small electrodes and place them a long distance apart, the current that flows will be different if you make the electrodes larger and move them closer together. For this reason it is important that the cell isolates a precise volume of solution in order to make accurate repeatable measurements.

The usable range for conductivity measurement is from 0.000,000,055 S/cm (ultra-pure water) to 1 S/cm (a strong acid). Obviously it is difficult working with all these zeros, so a smaller unit is required. A micro-Siemen (µS) is 1 millionth of a Siemen and you will also come across Milli-Siemens which are 1 thousandths of a Siemen.

From this it can be determined that 1,000 µS (micro-Siemens) = 1 mS (Milli-Siemen).

The table below shows approximate conductivities for common solutions:

Solution Conductivity at 25˚C
Ultra-pure water 0.055 µS/cm
De-ionised water 0.1 µS/cm
Distilled water 1.0 µS/cm
Boiler feed water 10.0 µS/cm
Tap water 100-800 µS/cm
Electro plating rinses 1000 µS/cm
Seawater 53.0 mS/cm
10% H2SO4 432.0 mS/cm
31% HNO3 865.0 mS/cm

There are two methods of conductivity measurement:

Conventional or contacting method.

The conventional electrode method uses a sensor usually referred to as a cell, which consists of two or more electrodes, typically 316 stainless steel or graphite, mounted in an insulated body. A small alternating current is passed between the electrodes and measured by the instrument. The current is directly proportional to the conductivity of the solution.

Any configuration of two or more electrodes will have a cell constant. The cell constant (K) is related to the physical characteristics of the measuring cell. K is defined for two flat, parallel measuring electrodes as the electrode separation distance (d) divided by the electrode area (A). Thus if a very simple conductivity sensor were constructed with two metal electrodes 1 cm2 held between an insulating material 1 cm apart this is determined to have a cell constant of K=1.0

The construction of the contacting conductivity cell provides a sensor with an accurate, repeatable and stable cell constant which is unaffected by surrounding pipe work. A cell constant of 1.0 will generally be used over the conductivity range of 10 uS/cm to 100 mS/cm, however measurement beyond this range can be limited by the electronics in the instrument.

By adjusting the electrode area and the space between them it is possible to change the cell constant. The lower the cell constant the lower the conductivity reading that the instrument will be able to detect and vice versa. The contacting method is ideal for conductivity measurements up to 1,000µS/cm where sensor fouling has little effect.

Electrodeless or inductive method.

With the LTH Electrodeless method two toroidally wound coils are encapsulated with screening to form the sensor. When it is immersed in the solution to be measured a conductive loop is created through the sensor. A high frequency alternating current is applied to one of the coils which induces a current in the conductive loop. The second coil senses the magnitude of the induced current which is directly proportional to the conductivity of the solution.

The Electrodeless or inductive method has been developed to overcome the greatest problem with the contacting method, which is electrode blinding or fouling. Any coating on the electrode surface impedes the current flow and will give a low reading. By inducing a current into the solution this problem is eradicated. The only effect will be due to the geometry of the sensor being changed, which will change the cell constant.

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