Short Guide to Electrical Conductivity

22 March 2010

Short Guide to Electrical Conductivity

Electrical Conductivity, the same as  resistivity, is the ability of a fluid to conduct electrical current. Conductivity is simply the reciprocal of resistivity: conductivity = 1/resistivity. This is useful in agriculture because when measuring liquid from a soil sample or perhaps a liquid fertilizer mixture, the conductivity will measure the total  quantity and mobility of the ions in the sample. In the soil or fertilizer solution this is a measure of the total amount of elements or food available for the plants to take in (such as Nitrogen, Phosphorus and Potassium). Electrical Conductivity is also a great measurment of how pure water is with the lower the number, the purer the water.

The unit of conductivity is the Microsiemen per centimeter (µS/cm) or sometimes known as simply as a Sieman (S).  1000 Microsiemen's is equal to 1 Millisiemen (mS/cm). Conductivity is also sometimes refered to as a micromhos/cm.  "mho" is ohm spelled backwards to indicate that it is the reciprical of resistivity as in Ohm's law.  A "mho" is equivalent to a Siemen and are used interchangeably. Soil is generally expected to be above 200 microsiemens and below 1000 microsiemens.

Conductivity is greatly influenced by temperature.  Most ionic solutions will increase about 2% for each 1°C increase. Unfortunately, this temperature coefficient (TC) is not linear and high resistance water can be closer to 5% or so per °C. A good conductivity meter will include temperature compensation. Often it is a fixed TC of 2.0% per °C so a 1413 µS standard at 25°C (77°F) when warmed to 30°C (86°F) will apply a correction of 5 degrees so it corrects from actual conductivity of 1548 µS. When the sample cools to 25°C, it will again read 1413 µS as no correction is applied. Although conductivity cell response is immediate, results may fluctuate as temperature measurement stabilize.

A feature of advanced meters is a selectable normalization temperature. This allows temperature compensated readings to be adjusted either to 25°C (77°F) or another value, usually 20°C (68°F). The advantage here is that 20°C (68°F) is often closer to the actual sample temperatures than 25°C (77°F). When using a normalization temperature other than 25°C it is important to calibrate to the appropriate value of the conductivity standard at the specified normalization temperature. For example, a 1413 µS standard at 25°C should be calibrated to its value at 20°C which is 1278 µS.

Where possible use a meter that measures conductivity rather than TDS (total dissolved (ionic) solids). TDS meters measure conductivity and multiply the reading by a fixed or adjustable "TDS factor" to determine TDS. There are many limitations when using TDS. First, the TDS factor used is salt specific so if there are multiple or unknown salts in solution, its nearly impossible to determine the correct factor to use. Second, since ionic concentrations are not linear, the TDS factor changes with concentration. TDS values are generally not considered with low conductivity values. TDS values are usually expressed as parts per million (ppm) or ppt (parts per thousand).

Below is some tips to getting more accurate EC results.

• Take readings and calibrate at 25C whenever possible. Even with temperature compensating meters, the sensors/thermistors can go bad with little warning.
• It is necessary to choose between a 2-cell or a 4-cell battery units. Units with 4 batteries are more expensive but considered better as they resist polarization effects and fouling, however for clean water applications, this advantage may be small.
• Do not store samples or calibration solutions in the refrigerator as this will attract CO2 to the solution which raises the conductivity of the standard.
• Use an EC meter rather than a TDS meter as TDS is the same meter with a conversion built into it that may be innaccurate depending on the sample. 
• Always store platinum probes submerged in clean deionized water. You will get quicker, more stable conductivity readings when calibrating.
• Opt for the lower frequency settings on your meter if given a choice and if in the desired conductivity range.
• Store samples in high density polyethylene containers, and avoid soft plastics which contain phthalate plasticizers such as DEP or DEHP
• Pre rinse beakers/probes with the conductivity calibration solution after a DI water rinse before calibrating your meter.
• Conductivity calibration solutions of less than 100 µS are usually not stable for over 3-6 months with respect to a 1% tolerance.  This is due to absorption of atmospheric CO2, this results in an increase of the ions dissolved in the solution. Also the lower the conductivity value of the calibration solutions, the higher the proportion of the added conductivity contributed by the contaminant CO2.  So replace low conductivity samples after at least 3-6 months.
• CO2 absorption into conductivity standards higher than 5000 or 10,000µS has a minimal effect under normal conditions as the conductivity which the absorbed CO2 contributes to the conductivity is much less than the 1% error tolerance stated on the label. Above a conductivity standard of about 50,000 µS the effect is insignificant, and if you get a suspicion that something is very wrong with your standard, you should look toward other sources of contamination.

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