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.