Low-level standardization of electrical conductivity cells

Electrical conductivity monitors are used either independently or in conjunction with other hardware to estimate the concentration of ionized chemicals in water. Until recently, this instrumentation was used primarily for trending. However, the need has developed for conductivity data that are accurate as well as precise. Interference from air makes it difficult to standardize electrical conductivity monitors in the low ranges. The traditional practice is to standardize at a conductivity that is up to 2700 times the lowest measured conductivity. In the current tests, five flow-through conductivity cells exhibited a bias (deviation from theoretical) of -9% to +9% at 146.93 μS/cm -- the usual standardization point for low-level conductivity measurements, and 1-3 orders of magnitude greater than the measured values of power plant samples. The bias was non-linear below 146.93 μS/cm, ranging from +3% to +21% at 0.055 μS/cm (the theoretical conductivity of pure water). Two on-line monitors showed similar trends. The upward logarithmic trend in bias at low conductivities may result from an increase in ion mobility at very dilute concentrations. The bias for glass flow-through cells or on-line monitors can be reduced to the range of +1% to +3% at 0.055 μcm or to the range of -5% to +3% at 0.055 - 1.0 μS/cm by standardization in the range of the expected measurements using a Standard Sample Synthesizer (SSS) which blends the lowest ASTM standard solution with pure water in various ratios to produce low-level standards