Recent advances in sensing technology have contributed to using conductivity for traditionally difficult measurements of aqueous concentrations of acids, bases and salts.

An understanding of how conductivity measurement works and today’s conductivity capabilities can prove invaluable for your next concentration measurement challenge.

Every binary solution, whether it is water, milk, or acid, is comprised of ions, most of which exhibit electrical conductivity. When the ion concentration increases or decreases, so can the conductivity. This usually provides a reliable means to measure the concentration by weight of the solution. For example, let’s look at water, the most abundant solution on earth. The typical ion in water is salt (NaCl). As the concentration of salt increases, so will the detectable conductivity. Because the same premise applies to other binary solutions, such as acids, bases, or other salts, conductivity measurement is ideal for a broad range of applications throughout all process industries. 

The real measure

In the broadest sense, conductivity is the ability of a material to carry electric current. The basic principle by which instruments measure conductivity is simple – two plates are placed in the solution, a potential is applied across the plates (normally a sine wave voltage), and the current is measured. Conductivity (G), the inverse of resistivity, is determined from the voltage and current values according to Ohm’s law. [G=I/R+1(amps)/E(volts)]. The higher the resistance the lower the conductivity.

A complete conductivity measurement loop consists of sensors that are exposed to the solution and an analyser that interprets and displays the conductivity measurement.

The basic unit of conductivity is the siemens, formerly called the mho. Conductivity is usually measured in microsiemens or micromhos. A millisiemen is equivalent to 1000microsiemens and a millimho is equivalent to 1000micromhos. Since there is no standard equation for determining the conductivity of all solutions, basic information regarding the electrolyte being measured, has to be entered into the analyser. Each electrolyte possesses a unique conductivity curve. Strong electrolytes, such as hydrochloric acid, dissociate fully in water, exhibiting a higher conductivity value than a weak electrolyte, such as acetic acid.

For most electrolytes, the conductivity curve reaches a maximum value (at a given temperature) and then reverses its slope, usually forming a bell-shaped curve. Conductivity can be measured on the increasing (front slope) or decreasing (back slope) part of the curve. However, it cannot measure concentration in the region where the curve changes slope (the top or flat portion of a curve), or on both sides of a curve, since two different concentration values can exhibit the same conductivity.

Temperature also has a significant effect on conductivity in that as the temperature increases so does conductivity. For instance, if you have a solution of five percent sodium hydroxide and you measure it at room temperature (25°C), you will get a certain conductivity value (223mS/cm). If you increase the temperature of that same solution (eg 50°C), the conductivity will increase (320mS/cm), though it remains five percent by weight concentration.

To account for the variables of chemical properties and effects of temperature, most conductivity systems are equipped with curve sets for specific chemical concentrations and temperature compensation relative to the specific binary solution. For instance, if you are working with sodium chloride (NaCl), you would need the chemical concentration curve for that specific ion and the corresponding temperature compensation curve for that ion species. Sever measurement inaccuracies can occur without these two working in tandem. Without the proper temperature compensation curves, temperature variations can cause inaccurate conductivity readings, such as a significant change in the displayed value of percent concentration, when in reality, the concentration has not changed at all.

For example, if a clean in place (CIP) application requires a 0to5percent concentration range, the end user may need to know that the process concentration does not drop below 2.5percent. If it did, they may not be getting the desired cleaning results because of an overly diluted solution. Conversely, if the concentration increases to 8 per cent, they may be wasting chemical, which increases production costs.

While chemical and temperature compensation curves are readily available for common solutions, custom curves can be developed for many non-standard binary solution applications. 

Contacting conductivity or not

Two methods of sensing conductivity to accommodate a broad range of process conditions are contacting, and electrodeless. A contacting conductivity sensor uses metal or graphite electrodes in direct contact with the fluid being measured. One electrode is excited by a high frequency ac signal, the resistance is then measured across the two electrodes and converted to conductivity. 

Electrodeless conductivity sensing uses multiple toroids that work together as sender-receiver. One set of toroids is energised by the analyser, creating an induced current in the liquid, and the other toroids sense the field created. As the conductivity varies, the size of the sensed signal varies proportionally.

Contacting conductivity (CC) sensors offer low detection limits and are usually used in applications with clean fluids, such as water. Where the measured value is typically less than 10microsiemens per centimetre, CC should be considered. These ‘pure water’, applications range from steam condensate and feed water for boilers and turbines, to ‘ultra pure’ water (less than 1microsiemen/cm resistivity – RS) for semiconductor processing. While CC/RS sensors are typically more accurate for precise and sensitive measurement (below 10microsiemens/cm), they do have an inherent problem; sensor coating. The electrode surfaces must be maintained in pristine condition to provide accurate measurements. Anything that coats, fouls, or otherwise contaminates them adds resistance, which lowers the displayed conductivity value. Contaminates can include mineralisation, oils, even bubbles.

To assure accurate measurement, the CC/RS sensors must be inspected, cleaned and calibrated routinely. The maintenance schedule depends on the application and how often and seriously the sensors become coated, or how critical the measurement. It may be every couple of days, every week, every two weeks, but the consequences of neglect could be grave. It’s a lot easier and less costly to plan frequent maintenance and/or replace a contacting conductivity sensor than a turbine or boiler.

Alternatively, electrodeless conductivity (EC) requires far less maintenance and is typically unaffected by coatings, such as chemical films and algae growth. The two most common EC configurations are insertion (invasive), which is immersed in the liquid flow, and flow-through, which is installed in, and becomes a section of, the process pipeline. The surface or material of an EC sensor is not critical to making the measurement. Instead, a field develops around the sensor head, and that field is what detects the resistance of the solution passing through. EC sensors can be used in applications with conductivity ranging from approximately 5 microsiemens to 2millionmicrosiemens (2000millisiemens). Newer, large bore insertion and flow-through EC sensors can operate at a minimum full scale range of 0to50microsiemens/cm with demonstrated measurement capability at or below approximately 5microsiemens/cm. Electrodeless conductivity technology is predominant for demanding applications, such as acids and other aggressive materials and process conditions with temperatures up to 411°F and pressures to 300psi+.

While conductivity measurement is a mature and established technology, recent advances in system capabilities and materials have improved and expanded applications in process industries. For instance, the Measurements & Instruments Division of Invensys Process Systems introduced Foxboro sensors made from PEEK, a thermo-plastic material proven to be compatible with the widest array of process solutions. Invensys- Foxboro also pioneered the used of virgin PEEK (P-oxyphenylenep-oxyphenylenep-carbooxyphenylene), which is FDA compliant and 3A approved for its sanitary sensors. For standard industrial sensors, the company uses glass-filled PEEK (polyetheretherketone), along with other alternate sensor materials for non-standard applications. These include PVDF (polyvinylidenedifluoride), PCTFE (polychlorotrifluoroethylene), Noryl, as well as borosilicate glass, glass-filled Teflon, virgin polypropylene, and several other thermoplastic materials.

For sanitary applications, Invensys has developed a Foxboro electrodeless conductivity flow-through sensor that is appropriate for the full gamut of applications and allows the process to be automated.

Conductive solution solution

Following are two applications where conductivity sensing proved to be the cost effective solution.

Sulphuric acid is one of the most important industrial chemicals worldwide. The highly corrosive properties of this clear and colourless fluid are critical in applications ranging from the production of fertilisers to removing oxides from iron and steel. But there is a subtler, more delicate side of sulphuric acid, and that is precise production. A variation of 0.1 per cent in acid strength could corrode profits by increasing production and material costs, compromising quality and violating emissions standards.

Producing sulphuric acid involves precisely combining sulphur, air and water. A major international  chemical processing company accomplishes this by passing air through an oleum tower where it combines with sulphur trioxide (SO3) and moisture to form sulphuric acid mist particles at 99+percent strength. While the typical strength is in the 99.6to99.2 percent range, the company needs the ability to measure up to 99.9percent.

A Foxboro conductivity measurement loop proved to be the solution. The system includes electrodless sensors and an intelligent analyser that together measure acid strength by sensing changes in electrical conductivity. The sensor is installed in-line in a sample loop of the sulphuric acid towers. The analyser/sensor determines the conductivity and converts that to concentration by weight, then the analyser sends data back to the plant control systems. Based on these measurements compared to preset conductivity limits, the control systems bring in water or acid to reduce or maintain strength. To meet the specific materials compatibility demands of this application, Invensys provided a PFA Teflon-coated toroid head, combined with a Carpenter20 alloy wetted metal housing, and Viton O-rings. However, for even more demanding applications, such as high purity acid where no wetted metal is permitted, other sensor selections are required … and available.

To take full advantage of the sensor’s resistant material and innovative design, the company worked closely with Foxboro engineers to develop a custom curve set. While the analyser came programmed from the factory to handle the 99.5percent to 93percent range, the custom curve set allowed accurate measurement of 99.85-pluspercent.

With sulphuric acid, the higher the strength, the greater the profitability. This company has found that the reliability and robustness of both the sensor and analyser have allowed them to consistently produce high quality, and highly profitable, sulphuric acid.

The second example involves automating production of a high-value organic solvent. As part of the process, the high-value solvent separates from a salt concentrated by product. A critical step in the process is draining the heavier aqueous salt solution layer while leaving the lighter solvent. This was traditionally done by qualified technicians who would observe the fluids emptying the tank, relying on sight and individual experience to try to stop the flow at the right moment, to avoid losing the revenue rich chemical. However, even seconds of delay could result in the loss of thousands of dollars worth of profits quickly down the drain. 

The solution to this balancing act proved to be the Foxboro 871FT flow through sensor. The saltwater byproduct has a high ionic concentration, which exhibits a strong conductivity compared with the organic compound that exhibits negligible conductivity. The non-invasive sensors measure the ionic content of the liquids, and instantly signals when a sudden and significant change in conductivity occurs. This triggers a signal that shuts the drain. The result is greater process efficiency, reduced maintenance costs and reduced worker interaction with chemicals. 

J Kevin Quackenbush is Senior Conductivity Measurement Specialist, Invensys Process Systems, Measurements & Instruments Division, Foxboro, MA, USA. www.invensys.com