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"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.
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.
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 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.
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