For years, resistivity/conductivity2 instrumentation has been used as a reliable indicator of ionic water quality, especially for ultrapure water (UPW). Traditionally, the instrumentation is procured, installed, powered up, and perhaps connected to a PLC, strip chart recorder, or a LAN. After that, the instrument and sensor receive no attention until a problem arises. Today, continuous calibration and verification are key elements in any QC and QA program. In recent years, users have been using standard solutions to calibrate sensors. However, a major complaint is that these solutions are not in the user’s operational range, and thus are not ideal standards. Numerous standard solutions have been proposed, but none possess all of the desirable requirements : ease-of-preparation, resistivity value in the range of operation, impervious to air and other contamination, and verifiable.
During the last several years, as a result of our investigations into the relationship between resistivity and temperature for UPW, we have developed a methodology for calibration that meet all of the criteria stated above. We will provide data to support 5 primary conclusions :
- The relationship between temperature and resistivity for UPW is well known.
- Resistivity sensors (for use in UPW) can be calibrated in high conductivity ASTM standard solutions under