Test & Measurement

Applying and Maintaining Analytical Sensors

June 3, 2016 By: Jay Mershon, Endress+Hauser

Sensors Insights by Jay Mershon

Process control decisions are often driven by one or more critical parameters, such as conductivity, pH, turbidity, total suspended solids, dissolved oxygen and chlorine. Analytical sensors for these parameters are widely used in the water and wastewater, chemical, food and beverage, pharmaceutical and other industries. This article provides for a basic understanding of the application, theory, installation and maintenance requirements for these common analytical sensors.

Conductivity Measurements

Conductivity is the ability of a liquid to conduct electricity based on the ionic concentration of a solution. In food and beverage, for example, a conductivity measurement is frequently used to confirm clean in place (CIP) solution concentration and to optimize the CIP cycle. The conductivity measurement monitors the concentration of solutions in the return line, and measures the temperature with an integral temperature sensor. Together, the conductivity sensor and its integral temperature element provide critical data used to detect, control and monitor the CIP process including product, rinse water and CIP solution transitions.

In the pharmaceutical and food and beverage industries, requirements for hygiene and cleanliness are critical. Water for injection (WFI) and other water purification systems use conductivity as a means to detect contamination.

Three types of conductivity analyzers are most commonly used:

Two Electrode: Contacting, two electrode sensors (figure 1) use alternating voltage applied to the medium. The resulting current flow through the medium is then used to calculate the process conductivity value according to Ohm's law. Two electrode sensors must be selected with the appropriate cell constant to avoid linearity issues associated with polarization. Lower cell constants are used in low conductivity applications and higher cell constants in high conductivity processes. The dynamic range of contacting sensors and their resistance to fouling are very limited.

Fig. 1: Two-electrode conductivity sensor.
Fig. 1: Two-electrode conductivity sensor.

Generally, two-electrode sensors are used in processes where fouling is unlikely and the operational conductivity range is limited. When properly applied, operated and maintained, the two-electrode sensor is the most accurate sensor type in low conductivity processes.

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