Conductivity Frequently Asked Questions

METTLER TOLEDO does supply standard conductivity solutions to recalibrate the cell constant of sensors. However, Thornton ISO-9001, NIST and ASTM-traceable factory calibration is performed under carefully controlled conditions of water purity and 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 METTLER TOLEDO, and we will re-certify them for you and issue a calibration certificate for 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 values.
• Third, you can calibrate in a standard solution. The standard must be accurate and unaffected by contamination. In addition for pure water sensors, the ideal standard is recirculating ultrapure water at 18.2 Megohm-cm (0.055 uS/cm), sealed from air. If such a system is unavailable, 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 and then calibrate the sensor's temperature factor to that value.

Conductivity or resistivity depends on the composition or purity of the process media 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 and 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. The 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 relationship between the surface areas of the inner and outer electrodes and the space, or amount of sample between them. It directly affects the sensitivity and accuracy of measurement. Lower cell constants are best suited to low conductivity, high resistivity samples like semiconductor UPW or Water for Injection. 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 METTLER TOLEDO Thornton UniCond conductivity/resistivity sensors or METTLER TOLEDO InPro7100i ISM contictivity/resistivity sensors, the cell constant is stored on the sensor and automatically recognized by the transmitter. For analog sensors, the cell constant must be entered into the transmitter manually.