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You can learn more from our asked questions

We believe that learning never stops and therefore we bring you our FAQ’s as we expect you to learn more from here. Should your question not surface here do not hesitate to contact us.
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Thornton conductivity cells can be left dry between uses without affecting performance. If used in dirty water, they should be cleaned first. pH and ORP sensors, on the other hand, must be kept wet at all times and may be stored with their original shipping cap on, filled with water.

You could send the sensor back to us for re-calibration and re-certification, however, since the cell constant of the concentric cell design is reliable and stable, we have added a statement at the bottom of our Certificate of Accuracy which should help eliminate the need for this. The statement is as follows:

"Conductivity cell constant and temperature constant calibrations are generally valid for one year from date of installation. However, rough handling or use in samples containing suspended solids which accumulate on cell surfaces can degrade sensor constant accuracy and require more frequent calibration". Therefore, if you can verify that the sensor has not been installed, then the original calibration is still valid.

Thornton does not supply standard conductivity solutions to recalibrate the cell constant of sensors. Thornton ISO-9001, NIST and ASTM-traceable factory calibration is performed under carefully controlled conditions of water purity & temperature. Sensors for pure water are actually calibrated in a closed, ultrapure water loop under conditions identical to their use. In most installations, this calibration is valid for at least a year and is more accurate than what can be achieved at users locations. For subsequent re-calibration, our recommendation is that cells be re-certified in one of three ways:

First, you can return the cells to Thornton and we will re-certify them for you and issue a certificate of calibration with each cell.
Second, you can purchase a certified cell from us that can be used as a standard. You can then use this cell to compare against the readings of your other cells if both can measure identical water.
Third, you can calibrate in a standard solution. The standard must be accurate and unaffected by contamination. For pure water sensors, the ideal standard is recirculating ultrapure water at 18.2 Megohm-cm (0.055 uS/cm), sealed from air. If a such a system is not available, a standard solution used in open air should have a conductivity of 100 uS/cm or higher to reduce the effect of variable contamination from carbon dioxide in the air. ASTM D1125 Solution D at 147 uS/cm is well-suited for this. DO NOT USE commercially available conductivity standards below 100 uS/cm. The uncertainty of air contamination creates larger errors than the non-linearity of Thornton instruments over the range from 150 uS/cm to pure water.
To calibrate the temperature, you must control the water to a known temperature then calibrate the sensor's temperature factor to that value.

Conductivity or resistivity depends on the composition or purity of the water and is basically independent of the flowrate past the sensor. However, several secondary effects can influence the composition and measurement, especially in high purity water.

Low Flowrates

Trace impurities dissolving from the surfaces of a new or altered piping system are more likely to accumulate & reduce resistivity at low flows and especially in any dead legs.
Any large air leaks producing bubbles in the sensor will cause unstable, high resistivity readings. Low flowrate will allow these bubbles to cling to sensor surfaces which changes the effective cell constant. Orientation of the sensor should allow bubbles to rise and escape through the outlet.
Dissolved air in cold water will become less soluble when it reaches a warmer, lower pressure part of a treatment system and may produce bubbles within a sensor and cause the same problems noted above. The same effect can occur when carbon dioxide is released following a cation exchanger.
Any trace air leaks, though insufficient to produce bubbles, can still contain enough conductive carbon dioxide that they can lower the resistivity of ultrapure water. Changes in flowrate may reduce or dilute these leaks and yield an apparent flow sensitivity.
Where conductivity/resistivity is measured in a side stream or sample line, low flows will cause delays in sensing the actual process. They are also subject to the same problems of air leaks.
High Flowrates

High flowrates are usually better conditions for measurement, however, extremely high flow can cause a very large pressure drop when hitting the conductivity cell and cause cavitation in the sensor - the product of water vapor bubbles due to a partial vacuum. This will produce wildly varying readings and could damage the sensor.
As a rule of thumb, flow velocities between 1 and 10 feet/second (0.3 to 3 m/s) usually produce good results, however, the above considerations should be taken into account as they apply to a particular installation.

A conductivity sensor cell constant describes the precise geometry of the two electrodes of the sensor. It is the ratio of the length between electrodes divided by the cross-sectional area of sample between them. It directly affects the sensitivity and accuracy of measurement. Lower cell constants are needed to provide good signals to the measuring instrument for low conductivity (high resistivity) samples. Higher cell constants are needed to measure high conductivity samples. The measuring instrument must "know" the precise cell constant of the sensor connected and normalize the readout accordingly. With Thornton Smart Sensors for 770-Series instruments, this is communicated automatically when the sensor is connected. For 200-Series instruments and sensors, the value is provided on the sensor label and certificate of calibration and should be entered into the instrument manually at startup.
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Plot 25 Valley Road, Ministers' Village Ntinda, Kampala

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