Conductivity Sensor Technical Education

What is Conductivity?

Conductivity is a measurement of the ability of a solution to conduct an electric current. An instrument measures conductivity by placing two plates of conductive material with known area and distance apart in a sample. Then a voltage potential is applied and the resulting current is measured.

Using Ohms Law , V= iR and knowing conductivity G= 1/R

then G can be determined as

G= 1/R = i/V
The number of ions that are conductive, metals, salts, etc, provides the conductive path between two electrodes of the conductivity cell. Higher ionic concentration yields higher conductivity. Typically an AC signal is used to prevent ionization of the electrodes.

Temperature effects and compensation:

Increase temperatures can make the ions in the water move faster
Conductivity levels falsely increase approximately 2% per °C --- more for resistive waters (up to 4 or 5% per °C )

Terminology

The Terminology used to express a unit of electrical conductance is a microSiemen (Formerly a micromho). High conductivity values can be expressed as milliSiemens.

Below 1 microSiemen, we express units of measure as ohms of resistance rather than fractions or decimals of conductance.

• 1 micromho = 1 microSiemen
• 1,000,000 ohms = 1 megaohm.
• 1/1,000,000 ohms is = microSiemen
• 1000 micromhos = 1000 microSiemens = 1 milliSiemen.

Many years ago, the water treatment industry adopted a nomenclature of PPM. Correlating PPM to microSiemens can be difficult, as water can be make up of different salt concentrations and dissolved metals, which can alter the conversion factor. It is preferable to use microSiemens as a unit of measure; however, if you need to convert to PPM, you can use the following formula:

• 1 ppm = 1.5 microSiemen.
• 1 ppm (sodium chloride) ˜ 2 micro siemens (<30,000 uS).
• 1ppm (mixed salts) ˜ 1.5 micro siemens (<1,000 uS).

A more exact conversion factor is:

ppm = 0.64 x conductivity

Cell Constant (K) Values

Cell constants define the volume between the electrodes. Cell constant k is directly proportional to the distance separating the two conductive plates and inversely proportional to their surface area. K = L/a, where a(area) = A x B.

Materials of Construction

The basic conductivity probe is comprised of two conductive surfaces separated by a given distance in a body. The body material can be anything from PVC, CPVC, PVDF, PTFE, PEEK, or even stainless steel. The measuring surfaces (usually pin configuration) are typically constructed of graphite, stainless steel, titanium, or platinum. The basic criteria for determining which is best are based on cost and performance requirements.

Cleaning and Maintenance

Some care should be taken when cleaning conductivity probes. Scratches and abrasions on the surface of the pins increase the surface area which alters the cell constant and provides a retention area for old samples, causing calibration and measurement difficulties. Graphite being a soft material is most susceptible. Cleaning should be done with chemicals and soft non-abrasive cloths. Sanding is not recommended. HCL is an excellent material to dissolve many coatings.

Alternative Technologies

The basic two-pin conductivity cell is all we have discussed to this point. There is four-pin technology that tries to better control the field surrounding the conductivity sensor to improve stability. These are known as contacting type conductivity cells.

Another type of technology is the non-contacting (Toroidal) cell, which uses a magnetic field to sense conductivity. A transmitting coil generates a magnetic alternating field that induces an electric voltage in a liquid. The ions present in the liquid enable a current flow that increases with increasing ion concentration. The ionic concentration is then proportional to the conductivity. The current in the liquid generates a magnetic alternating field in the receiving coil. The resulting current induced in the receiving coil is measured and used to determine the conductivity value of the solution. Advantages to this type of cell are:

• No polarization
• Reduced maintenance and resistance to chemical attack
• Complete galvanic separation of measurement from medium (eliminates ground loss)

Conductivity Applications

Chemicals:

• Sulfuric acid and oleum
• Chlorine-alkali plants
• Sodium chloride, sodium hydroxide
• Hydrochloric acid
• Superphosphate
• Phosphoric acid
• Nitric acid
• Glycerine
• Fertilizer
• Detergents
• Waste water
• Moisture detection in HF
• Scrubbers.

Steam:

• Boiler blow down.

Generation:

• Flue gas scrubbers.
• Foods:
• Brines – concentration
• Sugar: First carbonation
• Clean-in-place (CIP) applications
• Saturation control
• Cooker control
• Desalting of food products
• Cheese souring
• Evaporation control – dried milk, etc.
• Glucose
• Lye peeling of fruits and vegetables
• Rinsing water
• Waste water; and
• Pickle making.

Hydrocarbon:

• Oil well drilling (mud and sediment).

Processing:

• Interface monitoring and control
• Leak detection
• HF alkylation; and
• Scrubbers.

Metals and Mining:

• Caustic/Alumina ration control
• Continuous steel pickling
• Plating solution monitoring/control
• Alkaline/caustic metal cleaning process
• Copper flotation; and
• Heavy metal recovery.

Streams and Lake Water:

• Water pollution; and
• Salt intrusion.

Textiles:

• Water quality surveys
• Scouring baths
• Rinsing water
• Carbonizing baths
• Mercerizing baths
• Boiler water systems; and
• Acid washing.

Pulp and Paper:

• White liquor
• Cooking liquor
• Black liquor
• Green liquor
• Weak wash liquor
• Brown stock washing
• Steam generation
• Heat exchangers
• Waste water; and
• Cl2/ ClO2 scrubbers.

Water Treatment:

• Ion exchangers
• Regeneration monitors
• Reverse osmosis
• Reverse osmosis
• Scrubbing towers (HCl gas in water)
• Softener regeneration.