How to get the best conductivity measurement

Greg: Conductivity plays a significant role in processing applications offering, in some cases, an inferential measurement in a binary mixture. We’re fortunate to have Robert Sherman, ISA Life Fellow, offer his knowledge on how to make the most of this opportunity. He has deep proficiency gained through years of practice (continuous since January 1971) after his military service in the U.S. Army (1969-70). He’s authored and edited major texts on analytical instrumentation (ISA, 1996), sample conditioning (Wiley-Interscience, 2002), and instrumentation and process control (atp learning, 2019). He recently authored a self-paced, Internet-based analyzer technology course (ISA SP36, 2022).

Robert, what are the different types of conductivity measurements?

Robert: There are three types of conductivity sensors to make measurements:

• Magnetic field (two-pole, glass exterior shell);
• Toroidal inductive (electrodeless, EEK-machined exterior); and
• AC voltage system (four-pole, exteriors: 316SSt, PEEK and ceramic).

Greg: What affects conductivity measurement performance?

Robert: Two- and four-pole systems:

• Temperature excursions;
• Greases (viscous hydrocarbons dispersed in liquid being measured); and
• Gas bubbles.

Toroidal systems:

• Immune to all of the above; but
• Sensitive to mounting orientation and distance from pipe wall.

Greg: What are some conductivity measurement applications? 

Robert: Some examples are:

• Quality control, especially binary solutions that can detect ∆ of single PPM;
• Phase change situations, which interface between TDS and water (eluent);
• Ion concentrations in acid or a base;
• Use of “inflection point” in tabular data arrays or graphic representations;
• Steam boiler blowdown to lower TDS levels by dilution; and
• Efficiency of reverse osmosis operation by comparing inlet/outlet conductivity measurements.

Greg: Since there’s often a peak in conductivity versus concentration curves, changes from one side to the other reverse the sign of the process gain. How do you address this issue for inferential concentration measurements?

Robert: I recommend the following:

• Avoid the inflection point concentration range (keep conductivity well away from peak and always on one side of the peak in conductivity versus concentration plot);
• Utilize MemoSens licensed, signal-processing (digital) system; and
• Use MemoSens signal processing, which is compatible with HART and EtherNet/IP signal transmission protocols.

Greg: What guidance can you offer for measurement selection and installation?

Robert: Observe manufacturers’ recommendations regarding “wall effect” of metallic piping for toroidal and four-P/pole sensors. A common guideline is placement at least three-probe diameters from any pipe wall.

Toroidal (inductive) sensors require flow across sensor body (center-mount or wall-mount mixer in tanks). They also require flow to impinge on probe end in pipe tee installations.

For conductivity systems’ design, installation and maintenance Knick is an excellent firm with worldwide coverage.

Greg: How about calibration and testing?

Robert: Use MemoSens publicly licensed, inductive-connector, communication technology for probe-to-controller signals. Its major benefit is there’s no exposed wire-end or connector pin to corrode and degrade signal transmission quality. Properly installed conductivity probes produce very stable measurements.

All conductivity probe outputs are linear over their entire range of measurement. Four-pole and Toroidal conductivity probes, connected with MemoSens technology and using a MemoSens technology controller, will very rarely if ever require removal from process for validation or calibration. Two-pole conductivity probes, even when connected with MemoSens technology and using a MemoSens controller, will require manual calibration with a manufacturer’s certified solution, and adjustment of its certified cell constant at intervals that depend on the service in which the conductivity probe is installed.

Industrial Internet of Things (IIoT) data handling offers cloud storage of probe and system values, providing trend information, predictive maintenance data (cell constants for two-pole probes), and distribution of future IIoT enhancements.

Greg: What are some common problems and solutions?

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Robert: Ultra-low level conductivity measurements can be significantly affected by process fluid temperature variations. Depending on the manufacturer, some conductivity probes contain an RTD for feedback and correction of process fluid temperature variations. If there’s an RTD in the process fluid line, it can be connected to the conductivity probe controller through an independent feedback circuit.

Ultra-lO level ranging to very HI-level conductivity measurements in the same process may require the installation of a HI/LO sensor pair. The recommendations are:

• For measurements <500 microsiemens, a two-pole measurement probe is recommended;
• For measurements >500 millisiemens, a toroidal measurement probe is recommended; and
• The above specification constitutes a HI/LO sensor-pair application.

Greg: The following best practices are from my book, Process/Industrial Instruments and Controls Handbook, Sixth Edition (2019), McGraw-Hill, with comments from Robert.

• With contacting conductivity sensors, the choice of cell constant (determined by the distance between the electrodes installed inside of the conductivity probe) is the important attribute. Knowing what’s in your process is critical to selecting the right sensor. Typical choices of conductivity sensors are PEEK toroidal or ceramic four-pole.
• If there are any solids suspended in the process, high conductivity levels or corrosive liquids, contacting sensors (two-pole or four-pole) will not generally work, and so a toroidal sensor is usually a better choice.
• Conductivity sensors can’t tell what’s causing the ions, only the total concentration of ions. However, if the sample is pure, some analyzers have built-in conductivity curves that correlate conductivity levels with the concentration of a select number of common acids or bases
• Always install contacting and toroidal conductivity sensors, so they’re fully immersed in the process, preferably in a vertical pipe run with flow from bottom to top, to minimize the presence of air bubbles.
• A change in cell constant of 3% or less isn’t a concern. If the change in cell constant after a calibration is greater than 3%, redo the calibration, and if the reading is still in error, check if the sensor is dirty and needs to be cleaned, or if it’s damaged and needs to be replaced. Bracket the desired measurement value with manufacturer’s standard solutions. However, calibrate the conductivity probe with only one manufacturer’s standard solution. 
• Toroidal sensors need to be installed at least ¾ of the toroid’s (loop) diameter away from the process pipe’s wall to limit the effects of the wall on ionic current.
• Calibrate the RTD (use a stirred ice water bath) on the conductivity sensor regularly to maintain proper temperature correction.
• Use an analyzer with built-in temperature correction algorithms specific for the application. Many analyzers have standard linear temperature correction, high purity water, and cation conductivity (dilute HCl) temperature correction. These are typically included in the analyzer transmitter functionality.
• Regular cleaning is good for contacting (two-pole and four-pole) conductivity sensors, but be wary to use only a warm detergent (Dawn, manufactured by Procter & Gamble, is recommended) solution or isopropyl alcohol (rubbing alcohol), and not a strong acid, so the sensor isn’t degraded.