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How to store conductivity sensors correctly?
All user manuals come with the necessary information about the short and long term storage of the respective sensor. Generally conductivity sensors should be stored dry for long term storage.
Why does the conductivity calibration work with just one point calibration?
Most customers measure conductivity in a quite narrow range, e.g. always the same beverage or always deionized water. With a 1-point calibration the range between 0 µS/cm and this calibration point is calibrated. Therefore, it is useful to choose a standard with higher conductivity than the expected value in the sample, e.g. 1413 µS/cm when expecting 1200 mS/cm. Performing a second calibration point in this example would not remarkably change the reading because the adjacent standards 500 µS/cm and 12.88 mS/cm are both quite far away.
A multi-point conductivity calibration is only useful when using the same sensor over a wide range, for example from 50 to 5000 µS/cm. In this case a suitable set of standards will be 84 µS/cm, 1413 µS/cm and 12.88 mS.
According to Method 2510B in Standard Methods for the Examination of Water and Wastewater and ASTM D1125, a one-point calibration of the cell constant at a representative conductivity is sufficient for accurate conductivity readings.
How is the temperature compensated in conductivity measurement?
There are several ways of temperature compensation.
Conductivity in aqueous solution is highly affected by temperature (~2 %/°C). That’s why it is conventional to link every measurement with a reference temperature. 20 °C or 25 °C are the commonly used reference temperatures in the world of conductivity measurement.
Different temperature correction methods have been developed to suit different users:
- Linear: for medium and highly conductive solutions
- Non-linear: natural waters such as ground water, surface water, drinking water, and waste water
- Pure water: ultrapure water, deionized water, distilled water
- None: some standards such as USP <645> prohibit any temperature compensation
The impact of temperature on different ions, and even varying concentrations of the same ion, can be a challenge. Hence, for each type of sample a compensation factor, which is called temperature coefficient (α), has to be determined. (This is also the case for the calibration standards. All Mettler-Toledo meters are able to automatically account for this compensation by the use of preset temperature tables.)
What is the expected lifetime of a conductivity sensor (esp. InLab® 741 and 742)?
Conductivity sensors have no expiration date. When the sensor is used within the specified temperature limits and neither severe mechanical force nor harsh chemical conditions are applied to the sensor and its cable, it can theoretically be used forever. Nevertheless, shifts of the cell constant may take place, due to deposits of fatty substances and precipitates. In most of these cases rinsing with ethanol, isopropyl alcohol or acetone can restore the sensor.
Which conductivity sensors have a nominal or certified cell constant?
InLab® 741, InLab® 742 and InLab® Trace come with a measured cell constant on their certificate. The cell constant of these sensors is precisely determined by the manufacturer right after production and under standardized conditions using a 100 μS/cm standard. The cell constant on the certificate can therefore be entered directly in the meter, thus making calibration with standard solutions redundant.
As these three sensors are particularly designed for use in low conducting media, such as pure water, ultrapure water, distilled water and deionized water, the measuring cell is very unlikely to be affected by contamination and hence the cell constant can be regarded as stable. Nevertheless, regular verification of the precision with a conductivity standard (e.g. 10 mS/cm) is crucial.
All other conductivity sensors from METTLER TOLEDO have nominal cell constants printed on the certificates. These sensors have to be calibrated prior to use with the appropriate calibration standard solutions.
In addition, InLab® 731-ISM and InLab® 738-ISM have the real cell constant stored on the ISM® chip which is used by the instruments the sensor is connected to.
How to avoid errors in conductivity measurement
The following set of tips and tricks should aid in reducing errors made in measuring conductivity:
In general, one must always make sure that the poles’ surfaces on the conductivity sensor are completely immersed in the sample solution.
Conductivity samples and standard solutions should never be diluted as the effect of dilution is not linear.
While dependent on the design of the conductivity sensor, the position of the conductivity sensor in the sample beaker can also greatly influence measuring results due to the occurrence of boundary effects outside the electrode surfaces. It is usually best to position the sensor in the middle of the beaker containing the solution.
A common source of error in conductivity measurements are air bubbles that may form on the surface of the poles. Bubbles are often not recognized by users as a source of error. They should be removed during measurement by briefly stirring the sample using a magnetic stirrer prior to measurement or, if necessary, through tapping the conductivity sensor. Successful removal of air bubbles often leads to a sudden jump in conductivity.
Since the accuracy of any measurement depends on proper calibration, a fresh standard must always be used. Ideally, sample beakers and sensor should be rinsed two to three times with the sample as the presence of contaminants can lead to additional errors in conductivity results.
Lastly, samples with low conductivities, such as pure or ultra-pure water samples, should be measured in a flow cell. Carbon dioxide dissolves in water, forming carbonic acid, which leads to higher than actual conductivity values. The flow cell ensures that atmospheric CO2 does not get into contact with low-conductivity samples and standards. This applies for both calibration and subsequent measurement. The flow cell and tubing must be thoroughly rinsed prior to use.