CS>CS and PPM-levels From: Matthew McCann wrote: Date: Tue, 10 May 2005 04:35:36
-------------------------------------------------------------------------------- This may be pedantic, but is not conductivity measured in units with dimension of conductance per length? For EIS the convenient unit of conductance is microsiemens, uS, and the convenient unit of conductivity is microsiemens per centimeter, uS/cm. Best regards, Matthew *************************************************** Conductivity is measured in Mhos, the reciprocal of Ohms. Mhos was recently changed to Siemens. In a liquid, you have a special 3D situation and have to define your cell constant which is distance between the electrodes over electrode area. From: http://www.sensorex.com/support/education/conductivity_education.html (The text also has some illustrative pictures and tables) 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 know 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 increases 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 2 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, TEFLON, 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 increases the surface area which alters the cell constant and provides a retention area for old samples which can cause 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 2-pin conductivity cell is all we have discussed to this point. There is 4-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. -- The Silver List is a moderated forum for discussing Colloidal Silver. 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