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FAQ on Conductivity Electrodes
Calibration
Recalibrate at least once a week, or daily if in doubt of results being accurate or reproducible. Conductivity meters and electrodes should be calibrated using a calibration standard solution before using. When selecting a standard solution, choose one that has the closest approximate conductivity value of the sample to be measured.
Calibration Standards
Calibration solutions should remain stable or unchanged at respective standard value if unopened throughout their shelf-life provided they are stored in dry, cool indoor environment. DO NOT use expired calibration standards or any calibration solution left uncapped for sometime, especially very low Conductivity solutions as it gets oxidised quickly and actual value differs. Always pour out from bottle into dry clean container (rinse with de-ionised water several times) and NEVER pour back used standards into bottle after calibration. Discard used calibration standards appropriately. Typically calibration solutions are good for up to a year while low value standards remains stable for only few days to a month once opened. For freshness and adherence to strict laboratory regulations, air-tight calibration sachets or pouches are preferred over bottle types as they are highly accurate and used as singles. These hassle-free pouches also serve as containers for user to dip electrode directly while calibrating his meter.
It is important that user must choose and calibrate using same type of calibration standards as the specific dissolved salt type of his test sample. These calibration standards must not be transferred from one type of dissolved solids solution to the next type as if they are co-related; doing this results in serious measurement errors. For example user cannot use TDS 442 standards to calibrate meter while measuring Conductivity KCl samples.
Some calibration standards offer 2 to 3 different types of Conductivity, TDS NaCl and TDS 442 formulation values. Depending on what user is specifically measuring, he is referencing his sample solution to that calibration standard. If it is Conductivity (KCl), use the standards value with the units of microsiemens (µS/cm) or .micromhos (µΩ/cm). If it is TDS, use the one labelled as NaCl ppm (salinity) when measuring TDS based on Sodium Chloride solution. If measuring natural water, use the 442 formulation (442 is 40% sodium sulfate, 40% sodium bicarbonate, and 20% sodium chloride).
Rinsing
Always rinse thoroughly with de-ionised water 2 to 3 times before and after each calibration as well as each measurement taken. Blot dry electrode housing and gently shake electrode to dislodge any droplets on its sensor.
Storage
It is recommended to store electrodes by immersing in de-ionized water water. If any electrode that has been stored dry, it should be soaked in distilled water for 5 minutes before use to assure complete wetting of the electrode sensor.
Temperature Effects
Conductivity measurement is highly dependent on temperature. This dependence or commonly known as Temperature Coefficient is often expressed as percent per ºC at 25 ºC. Ultrapure water has the largest temperature coefficient temperature at 5.2 %/ºC while most ionic salts have about 2 %/ºC. Acids, alkalis, and concentrated solutions are around 1.5 %/ºC. Organics also have very different temperature curves. Temperature variation causes frequent problems with conductivity measurements when the test solution has a rapid varying temperature condition. Hence in most cases Conductivity electrode have incorporated Automatic Temperature Compensation (ATC) with Conductivity cells to handle temperature variations for better results.
2-Cell vs 4-Cell Electrodes
2-cell electrodes offer reduced cost of purchase and easy maintenance. They allow minimum depth during measurement. However its main shortcoming is limited Conductivity measurement ranges typically not more than 50 mS/cm.
4-cell electrodes offer wider measurement ranges suitable for low to very high values with improved accuracy. Its improved circuity eliminates possible error due to polarisation effect. However most 4-cell electrodes requires minimum immersion depth of 3 to 4 cm.
Cell Constant
It is important to choose a suitable cell constant based on expected measuring range. If a cell constant other than 1.0 is required, a meter with other selectable cell constants like 0.1 and 10.0 is necessary.
Cell constant of 0.01, for range below 1 µS/cm
Cell constant of 0.1, for range of 0.5 to 200 µS/cm
Cell constant of 1.0, for range of 10 µS/cm to 20.0 mS/cm
Cell constant of 10.0, for range of 1 mS/cm to 1000 mS/cm
Cleaning
A dirty cell not only contaminates the test solution, it can cause the measured conductivity value to change. Grease, oil, fingerprints, and other contaminants on the sensing elements can cause erroneous measurements and sporadic responses.
For most applications, hot water with domestic hand cleaning detergent can be used for cleaning.
For lime and other hydroxide containing solutions, clean with a 5-10% solution of Hydrochloric Acid.
For solutions containing organic fouling agents (fats, oils, etc.), clean probe with mild acetone.
For algae and bacteria containing solutions, clean probe with a bleach containing liquid.
Clean electrode by dipping or gently stir with cleaning solution and agitating for 2 to 3 minutes. Then rinse the electrode several times with distilled or de-ionized water and recalibrate meter before use.
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