Conductivity Meter

Conductivity Meter

Calibration is akin to tuning, and just as a musical instrument must be tuned from time-to-time, a scientific instrument must be properly calibrated to achieve accurate test results.

Electrical conductivity or specific conductance is the reciprocal quantity, and measures a material's ability to conduct an electric current. It is commonly represented by the Greek letter σ (sigma), but κ (esp. in electrical engineering) or γ are also occasionally used. Its SI unit is siemens per metre (S·m⁻¹) and CGSE unit is reciprocal second (s⁻¹):

Operating Instructions

 Before use, soak the conductivity electrode in distilled or deionized water for 5 to 10 minutes. Connect the conductivity cell to the conductivity meter and follow the meter manual instructions for standardizing the cell for use at a given temperature. Rinse the conductivity cell sensing elements with distilled or deionized water between samples. Note: Each conductivity cell has a cell constant which is predetermined by the manufacturer and often indicated on the electrode upon shipment. The cell constant may change slightly during shipping and storage and should be remeasured on the user's conductivity meter before initial use. Measure the cell constant according to the meter instruction manual. Because temperature has a large effect on conductivity measurements allow probe to sit in solution until a stable temperature reading is obtained before taking measurements.

Reasons for varied readings include:

Ions:

The nature of charged positive ions (which is what the TDS meters are measuring) is that they are always moving. Therefore, there may always be variances in the conductivity, and thus a different reading.

Temperature:

Even with ATC, temperature changes by a tenth of a degree may increase or decrease the conductivity. Additionally, the temperature coefficient (what the reading is multiplied by to adjust for temperature differences) changes slightly depending upon the range of ppm.

Air bubbles:

Even a tiny air bubble that has adhered to one of the probes could potentially affect the conductivity, and thus the reading.

Lingering electrical charges:

Electrical charges off fingers, static eletricity off clothes, etc. on the meter and lingering electrical charges in the water will affect the conductivity of the water.

Beaker/cup material:

Plastic cups retain lingering electrical charges more than glass. If the meter touches the side of the glass or plastic, it could pick up a slight charge. If the plastic is retaining a charge, it could also affect the water.

Volume changes:

The amount of water in the sample may affect the conductivity. Different volumes of the same water may have different levels of conductivity. Displacement may affect the conductivity as well.

Probe positioning:

The depth and position of the probe in the water sample may also affect the conductivity. For example, if a meter is dipped into the water, removed and then dipped into the water again, but in a different spot, the reading may change.

How can I get the best possible readings?

Shake:

Always make sure to shake excess water off the meter before dipping it into a water sample, even if it's the same water.

Stir/tap:

After dipping the meter in the water, always lightly tap it against the side and stir the meter to remove any lingering air bubbles or electrical charges.

Positioning:

When taking the reading, always make sure to hold the meter straight up without it touching the sides or bottom of the glass/beaker/cup. The probes should be suspended as close to the center of the water sample as possible.

Time:

The longer the meter is in the water, the more accurate the reading will be.

Temperature:

25 degrees Celsius is the ideal temperature for conductivity readings, even if the meter has ATC.

Rinse:

If switching between very low and very high ppm water, always rinse the probes with distilled water to avoid any build-up.

KCl:

Potassium Chloride is the international standard to calibrate instruments that measure conductivity. The COM-100 is factory calibrated with a 1413 microsiemens solution is the default mode is EC-KCl.  The KCl conversion factor is 0.5-0.57.

Is "EC" a parameter or a scale?

“EC” is a parameter. It stands for Electrical Conductivity. There are a number of scales used in EC, most commonly micro-Siemens (µS) or milli-Siemens (mS). For example, if a particular application calls for water with “2.0 EC,” this is an incorrect determination. Most likely, the application is calling for an EC level of 2.0 mS. 2.0 mS = 2000 µS.

What's that little µ symbol on my EC meter?

The symbol 'µ' is not a lowercase U, but the Greek letter Mu.  It is the abbreviation for micro, and when used with an S (µS) it stands for mirco-Siemens, which is a scale used for measuring EC.

How do I convert from EC (µS) to TDS (ppm)?

The best thing to do is use a TDS meter, which will automatically do the conversion.  EC meters do not use conversion factors because there is no conversion.  To convert to TDS, if you do not wish to use a TDS meter, you will need to determine which conversion factor you want to use (NaCl, 442 or KCl) and do the math.

Units of Conductivity

The units of measurement used to describe conductivity and resistivity are quite fundamental and are frequently misused. Once the units are known, various waters can be quantitatively described.

The basic unit of resistance is the familiar ohm. Conductance is the reciprocal of resistance, and its basic unit is the siemens, formerly called mho. In discussions of bulk material, it is convenient to talk of its specific conductance, now commonly called its conductivity. This is the conductance as measured between the opposite faces of a 1-cm cube of the material. This measurement has units of siemens/cm. The units microsiemens/cm (μS/cm) and millisiemens/cm (mS/cm) are most commonly used to describe the conductivity of aqueous solutions. The corresponding terms for specific resistance (or resistivity) are ohm-cm (Ω-cm), megaohm-cm (MΩ -cm) and kilohm-cm (kΩ -cm).

Users of ultra-pure water prefer to use resistivity units of Ω-cm, because measurement in these units tends to spread the scale out in the range of interest. These same users frequently use k -cm when dealing with less pure water such as tap water. Others, however, use the units of μS/cm and mS/cm when dealing with any stream from quite pure to very concentrated chemical solutions. In these applications, the use of conductivity has the advantage of an almost direct relationship with impurities, especially at low concentration. Hence, a rising conductivity reading shows increasing impurities, or a generally increasing concentration in the case of a chemical stream (with some exceptions in concentrated solutions). See Table 1 for a comparison of resistance and conductivity.

TABLE 1
 

SPECIFIC CONDUCTANCE MICROMHO/CM*	SPECIFIC RESISTANCE MEGOHM-CM*	PARTS PER MILLION As ION As CaCO3 As NaCl**			GR. / GAL. As CaCO3
.055 .056 .063 .071 .083 .100 .500 1.000 10.000 80.000 625.000 10,000.000	18.240 18.000 16.000 14.000 12.000 10.000 2.000 1.000 .100 .0125 .0016 .0001	NONE .036 .041 .046 .054 .065 .325 .650 6.500 52.000 406.250 6,500.000	NONE .028 .031 .036 .042 .050 .250 .500 5.000 40.000 312.500 5,000.000	NONE .022 .025 .029 .033 .040 .200 .400 4.000 32.000 250.000 4,000.000	NONE .002 .002 .002 .002 .003 .015 .029 .292 2.340 18.273 292.398

*	At 25oC
**	At 25oC given specific conductance values included in this table.

TABLE 2
CONDUCTIVITY / RESISTIVITY / TDS CONVERSIONS

CONDUCTIVITY (MICROMHOS-CM)	RESISTIVITY (OHMS-CM)	DISSOLVED SOLIDS (PPM)
.056	18,000,000	.0277
.084	12,000,000	.0417
.167	6,000,000	.0833
1.00	1,000,000	.500
2.50	400,000	1.25
20.0	50,000	10.0
200	5,000	100
2000	500	1,000
20,000	50	10,000

Note: ppm x 2 = conductivity

Table 3 below lists the increasing conductivity of different types of solutions.

TABLE 3
CONDUCTIVITY OF VARIOUS AQUEOUS SOLUTIONS AT 25^oC

Application	Conductivity	Resistivity
Pure water	0.05 μS/cm	18 MΩ-cm
Power plant boiler water	0.05-1 μS/cm	1-18 MΩ-cm
Distilled water	0.5 μS/cm	2 MΩ-cm
Deionized water	0.1-10 μS/cm	0.1-10 MΩ-cm
Demineralized water	1-80 μS/cm	0.01-1 MΩ-cm
Mountain water	10 μS/cm	0.1 MΩ-cm
Drinking water	0.5-1 mS/cm	1-2 kΩ-cm
Wastewater	0.9-9 mS/cm	0.1-1 kΩ-cm
KCl solution (0.01 M)	1.4 mS/cm	0.7 kΩ-cm
Potable water maximum	1.5 mS/cm	0.7 kΩ-cm
Brackish water	1-80 mS/cm	0.01-1 kΩ-cm
Industrial process water	7-140 mS/cm	rarely stated
Ocean water	53 mS/cm	rarely stated
10% NaOH	355 mS/cm	rarely stated
10% H2SO4	432 mS/cm	rarely stated
31% HNO3	865 mS/cm	rarely stated

Temperature effects

Conductivity has a substantial dependence on temperature. This dependence is usually expressed as percent / ^oC at 25^oC. Ultrapure water has the largest dependence on temperature, at 5.2% / ^oC. Ionic salts run about 2% / ^oC, with acids, alkalis, and concentrated salts solutions are around 1.5% / ^oC. Temperature variation causes frequent problems with conductivity measurements when the solution under testing has a rapid varying temperature. The change in conductivity is instantaneous, since it is an electrical measurement. The thermistor, however, has a response time of 15 seconds to several minutes. A good rule of thumb is to allow 5 times the time it takes for the thermistor to respond to allow the reading to stabilize. Any sudden dips or peaks should be ignored during this time.

Conductivity Electrodes (Cells)

Simple conductivity sensors are constructed of an insulating material imbedded with platinum, graphite, stainless steel or other metallic pieces. These metal contacts serve as sensing elements and are placed at a fixed distance apart to make contact with a solution whose conductivity is to be determined. The length between the sensing elements, as well as the surface area of the metallic piece, determine the electrode cell constant, defined as length/area. The cell constant is a critical parameter affecting the conductance value produced by the cell and handled by the electronic circuitry.

A cell constant of 1.0 will produce a conductance reading approximately equal to the solution conductivity. For solutions of low conductivity, the sensing electrodes can be placed closer together, reducing the length between them and producing cell constants of 0.1 or 0.01. This will raise the conductance reading by a factor of 10 to 100 to offset the low solution conductivity and give a better signal to the conductivity meter. On the other hand, the sensing electrodes can be placed farther apart to create cell constants of 10 or 100 for use in highly conductive solutions. This also produces a conductance acceptable to the meter by reducing the conductance reading by a factor of 10 to 100.

In order to produce a measuring signal acceptable to the conductivity meter, it is highly important that the user choose a conductivity electrode with a cell constant appropriate for his sample. The table below lists the optimum conductivity range for cells with different cell constants.

Cell Constant	Optimum Conductivity Range
0.01	0.055 - 20 μS/cm
0.1	0.5 - 200 μS/cm
1.0	0.01 - 2 mS/cm
10.0	1 - 200 mS/cm

 Effects of polarization

 When a DC voltage is applied across the electrodes of a conductivity cell, the ions present in solution will be discharged onto the electrodes and by surrendering or accepting electrons, be changed into molecular form. The flow of ions will then virtually cease within a very short time, and consequently, the current will decrease to virtually zero. Therefore, an AC voltage is used for conductivity measurements. Polarization, however, can still actually take place during a half cycle of one polarity, causing a space charge buildup around the electrodes, resulting in a loss of current flow. In addition to polarization effects, conductivity cells with higher cell constants require long, narrow passages to obtain these constants, which make the electrode contacts more susceptible to coating by oils, slurries, or sludges commonly found in streams of high conductivity.

 Platinization

Platinization, or depositing a layer of black platinum on the electrode cells, results in a decreased polarization resistance. The platinum black catalyzes the electrochemical reaction rate, reducing the current density on the electrode cells and the reduction overvoltage for H⁺ ions.

 How 4 cell conductivity probe eliminates polarization and contact coating effects

 The 4 cell conductivity probe consists of 4 bands along a measuring column or sets of concentric rings opposite each other. An AC voltage is applied across the two outermost bands, which causes a current flow through the measuring cell. Located between this pair of electrodes is a second pair of bands. These bands measure the voltage generated across the liquid . The measured voltage across the outer bands is compared with the voltage measured by the inner bands. Any difference between the measured voltages of the two pairs of bands (whether the conductivity of the solution changes, or changes due to polarization or coating effects) initiates correcting action for the voltage across the outer bands. The correcting action remains until the current through the cell will generate a voltage across the outer bands which equals the voltage between the inner bands. Therefore, the four-band conductivity cell can correct for any fouling or polarization which may occur.

 Choice of Conductivity Cell Design: 2-Cell or 4-Cell?

 How 2-Cell and 4-Cell Electrodes Compare

2-Cell Electrodes Offer:	4-Cell Electrodes Offer:
Reduced cost of purchase and easy maintenance Direct access to cell plates facilitates cleaning.	Improved accuracy over a wide range The improved circuitry eliminates error due to polarization effect. Analysts achieve accurate calibration with just one standard.
Compatibility with sample changers Applications requiring a sampler changer will do better with a 2-electrode cell, which requires minimal insertion depth - allowing quick readings.	Flexibility for high or low range measurement One cell and one calibration provides testing capability over several decades of conductivity.
Limitation of 2-Cell Electrodes:	Limitation of 4-Cell Electrodes:
Measurement Range Accurate conductivity measurement range tops out at about 50 mS/cm.	Critical Minimum Immersion Depth Minimum immersion depth is 3 to 4 cm.

Cleaning

The single most important requirement of accurate and reproducible results in conductivity measurement is a clean cell. A dirty cell will contaminate the solution and cause the conductivity to change. Grease, oil, fingerprints, and other contaminants on the sensing elements can cause erroneous measurements and sporadic responses.

 Cleaning Methods

 For most applications, hot water with domestic cleaning detergent can be used for cleaning.

1.   For lime and other hydroxide containing solutions, clean with a 5-10% solution of hydrochloric acid.

2.   For solutions containing organic fouling agents (fats, oils, etc.), clean probe with acetone.

3.   For algae and bacteria containing solutions, clean probe with a bleach containing liquid.

Clean cells by dipping or filling the cell with cleaning solution and agitating for two or three minutes. When a stronger cleaning solution is required, try concentrated hydrochloric acid mixed into 50% isopropanol. Rinse the cell several times with distilled or deionized water and remeasure the cell constant before use.

 Storage

 It is best to store cells so that the electrodes are immersed in deionized water. Any cell that has been stored dry should be soaked in distilled water for 5 to 10 minutes before use to assure complete wetting of the electrodes.

Some platinum conductivity cells are coated with platinum black before calibration. This coating is extremely important to cell operation, especially in solutions of high conductivity. Electrodes are platinized to avoid errors due to polarization. Cells should be inspected periodically and after each cleaning. If the black coating appears to be wearing or flaking off the electrodes or if the cell constant has changed by 50%, the cell should be cleaned and the electrodes replatinized.

Replatinizing

The platinum electrode should first be cleaned thoroughly in aqua regia being careful not to dissolve the platinum. If the cell remains too long in aqua regia the platinum elements will dissolve completely. Prepare a solution of 0.025 N HCl with 3% chloroplatinic acid (H₂PtCl₆) and 0.025% lead acetate. Connect the cell to a rheostat or 3-4 V battery to which a variable resistor has been connected. Immerse cell in the chloroplatinic acid solution and electrolyze at 10 mA/cm for 10-15 minutes. Reverse the polarity to the cell every 30 seconds until both electrodes are covered with a thin black layer. Disconnect the cell and save the platinizing solution. It may be reused many times and should not be discarded as it is expensive to make. Rinse the electrode with tap water for 1 to 2 minutes, followed by distilled or deionized water. Store in distilled or deionized water until ready for use.

 Dr.A.N.Giri

("Many Species: One Planet ,One Future)
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