Conductivity Meters

Q: Is the conductivity meter measuring conductivity or conductance?

A: Conductivity is the measurement of the electrolytes in a solution. It is defined as the conductance in a given volume of sample. Conductance is the ability of the solution to conduct electric current.

Conductivity = Conductance x Probes cell constant (K)

OR Conductivity = Electrical Current/Voltage x Distance/Area

Q: How does temperature affect conductivity readings?

A: The effect of temperature on conductivity readings depends on the solution being measured. The effect is greatest in low ionic strength (low conductivity) solutions. A general rule to follow is there will be a 2% change (increase)/degree C. This rule can be followed for most aqueous solutions, however if you require a high degree of accuracy, you should consult a chart for the particular solution you are measuring.

Q: Can conductivity be measured in aqueous solutions only?

A: No, all substances possess some conductive properties. Generally organic compounds (such as benzene, alcohols, and petroleum products) have very low conductivities, while metals have very high conductivities. Measuring the conductivity of highly flammable liquids is very risky.

Q: How are conductivity and TDS related?

A: Salts, minerals, and even dissolved gases contribute uniformly to the conductivity of a solution. This means that the conductivity can be used as an indicator of the amount of dissolved materials in a solution. TDS can be used fairly accurately when determining the concentration of a single salt, such as NaCl, but error can arise when trying to compare two different types of solutions. It is necessary to calibrate the meter using the same dissolved materials that are in the test solution.

Q: What is the difference between conductivity and salinity?

A: The probe is the same for conductivity and salinity, but for salinity readings a correction factor is applied to the conductivity value. The correction factor takes the conductivity reading and converts it to ppm of NaCl.

Q: What is a cell constant K and why are there probes with different values of K?

A: The cell constant, K, is equal to the distance in cm between the probe's electrodes divided by the surface area of the electrodes in cm2. For solutions with low conductivities the electrodes can be placed closer together or made larger so that the cell constant is less than one. This has the effect of raising the conductance to produce a value more easily interpreted by the meter. The reverse also applies, in high conductivity solutions, the electrodes are placed farther apart or made smaller to reduce the conductance of the sample. By using the appropriate probe, K=0.1 for low conductivity solutions, K=1 for normal solutions and K=10 for high conductivity solutions, accurate measurements across the full range of conductivity values can be made.

Q: How do I find the correct temperature coefficient when not working with water?

A: For water, the correction factor is set at a default value of 1.91% per degree C. Check the conductivity of the sample at 25°C, then using the same sample, find the conductivity at another temperature to see what the percent change is. This will give you the temperature correction factor.

Q: How should conductivity probes be cleaned?

A: Clean cells with mild liquid detergent and/or dilute nitric acid (1% wt) by dipping or filling the cell with solution and agitating for 2 to 3 minutes. Dilute HCl (hydrochloric acid) or H2SO4 (sulfuric acid) may also be used. When stronger cleaning is needed, try concentrated HCl mixed into 50% isopropanol (rubbing alcohol). Rinse the cell several times with distilled or deionised water and recalibrate before use.

Q: How should conductivity probes be stored?

A: Rinse it in distilled/deionised water when you are finished using it. You can store your electrode either wet or dry. If it is stored dry you will need to recondition the electrode before use.

Q: How should a meter for TDS be calibrated if the dissolved solids are not the same as those in the calibration solution?

A: Making your own standard will yield the most accurate results. This is done by formulating a mixture of salts in relative proportions to those that simulate the solution being tested, then dissolving the mixture into distilled water. This should be done according to the following formula:

1 mg salt mixture/litre of distilled water = 1 ppm TDS,
X ppm TDS = X mg of salts + one litre of distilled water.

Remember that "X mg of salts" is the number of milligrams of the mixture of salts which simulate your test solution, not X milligrams of each salt in the mixture. An appropriate value for "X" is determined by the following rule:

Choose a ppm value for a calibration solution which is as close as possible to the expected ppm values of the test solutions. If the ppm content of the test solution is expected to vary a great deal, it is best to choose a ppm value for the calibration solution in the upper 1/3 of the expected TDS measurement range.