How to Accurately Measure the Conductivity of Ultrapure Water

Conductivity measurement is primarily used to determine the purity of water and is an effective, simple, and reliable method for detecting ionic impurities in water. Conductivity meters are categorized into several types, including field measurement, laboratory measurement, and online measurement. However, not every instrument is suitable for measuring the conductivity of ultrapure/high-purity water.

Theoretically, absolutely pure water contains only two types of ions: H+ and OH-, produced by water decomposition. A conductivity of 0.055 μS/cm corresponds to the conductivity of a water sample free of any impurities at 25°C. The conductivity of ultrapure water is very low, making it difficult to display. Therefore, resistivity (MΩ·cm) is often used to represent its purity. Resistivity is the reciprocal of conductivity.

The Influence of Temperature on Conductivity

Conductivity is significantly affected by the temperature of the water sample. Higher temperatures result in higher ionic activity and higher conductivity. We typically use the temperature coefficient α to characterize the effect of temperature on conductivity. Different media have different temperature coefficients; ultrapure water and syrups have relatively high temperature coefficients, while low-concentration acid and alkali solutions have relatively low temperature coefficients. In reality, the temperature coefficient α of a medium is not a constant but varies with temperature. It's important to note that the temperature coefficient at any given temperature is always based on a 25°C reference temperature; that is, α(T) is not the slope of the conductivity-temperature curve at temperature T. This point is easily confusing.

Furthermore, the temperature coefficient α depends not only on temperature but also on the concentration of the medium.

To compensate for the effect of temperature on conductivity measurements, conductivity transmitters must have an automatic temperature compensation circuit. This ensures that the conductivity displayed by the instrument is converted to the conductivity at the 25°C standard temperature. Temperature measurement is performed using a thermistor in the sensor. Higher the accuracy of the thermistor results in more accurate temperature measurements and more accurate conductivity after temperature compensation. For ultrapure water, a thermistor with an accuracy of ±0.1°C is strongly recommended. For different measured media, a matching temperature compensation mode must be used in the transmitter. For conductivity measurements of neutral salt solutions or acid/alkali solutions, transmitters generally use linear temperature compensation mode, where the temperature coefficient α is constant. However, for conductivity measurements of ultrapure water, a special algorithm must be used for temperature compensation. The main reasons are as follows:

(1) The temperature coefficient α of ultrapure water is very large (reaching 7.4%/K at 0℃) and varies greatly with temperature.

(2) The temperature characteristics of ultrapure water differ significantly from those of its impurities, so they must be considered separately.

Common conductivity instruments typically have linear temperature compensation for routine measurements, which generally meets the needs of everyday conductivity measurements. However, for ultrapure water conductivity measurements, since the temperature coefficient α varies greatly with temperature, using linear temperature compensation will result in significant measurement deviations. Therefore, nonlinear temperature compensation is required to obtain better measurements.

Professional instruments also offer temperature constant adjustment, facilitating accurate measurement data for professional users.