Conductivity is the measurement of the solution or substance’s ability to carry or conduct an electric current. Conductivity sensors are used to measure conductivity in aqueous solutions to determine the purity or impurity of a liquid.
An electric current will flow through liquids when a voltage difference exists between two points; the presence of ions or charged particles are necessary as they are the “carriers” of the current flowing through the solution. If there are no ions present in the liquid, such as with ultrapure water, then no current can flow and the solution is not conductive.
However, if the water is not ultrapure, then ions are present and electric current can flow through the solution (e.g. brine solution). The same is true for chemical solutions such as HCl.
There are two basic sensor styles used for measuring Conductivity: Contacting and Inductive (Toroidal, Electrodeless).
Contacting conductivity sensors are ideal for use in pure and ultrapure water applications. They are highly sensitive to ions present, providing the highest accuracy for low conductivity measurements.
Inductive conductivity sensors have a wide range capability and are better suited for measurements in dirty, corrosive, or high conductive solutions, requiring less maintenance than contacting sensors in the same environment.
|Pure Water (Low) Conductivity||High Pressure/High Temperature||High Conductivity||Percent Concentration||High Fouling/ Dirty||Portable Conductivity|
|SC42 Large Bore||x||x|
|SC42 Small Bore||x||x|
Intended for the low conductivity applications found in the semi-conductor, power, water and pharmaceutical industries, these sensors are in a convenient compact style.
Because of the sanitary process connection requirements, these sensors are suitable for applications that also experience steam sterilization or CIP cleaning.
The SC42 and SX42 conductivity sensors are ideal for high pressure, high temperature applications such as boiler blowdown and condensate leak detection applications.
The model ISC40 sensors are designed for use with the EXA ISC analyzers. This combination exceeds all expectations for conductivity measurement in terms of reliability, accuracy, rangeability and price performance.
There are numerous industrial applications where measurements and/or control of a specific chemical strength of the process is critical for optimizing the production of the end product. These specific concentrations are obtained by mixing a full strength solution with water to achieve the desired percent concentration.
Compact, lightweight, and dripproof, the SC72 is the ideal Conductivity meter for field use. Features wide-range auto-ranging, automatic temperature compensation, self-diagnostic functions, and a large, easy-to-read LCD display.
The measurement of specific conductivity in aqueous solutions is becoming increasingly important for the determination of impurities in water. Yokogawa has designed a full range of precision sensors and instruments to cope with these measurements, even under extreme conditions.
What is Conductivity?
Conductivity is the measure of a solution's ability to pass or carry an electric current. The term conductivity is derived from Ohm's Law, E=I•R; where Voltage (E) is the product of Current (I) and Resistance (R); Resistance is determined by Voltage/Current. When a voltage is connected across a conductor, a current will flow, which is dependent on the resistance of the conductor. Conductivity is simply defined as the reciprocal of the resistance of a solution between two electrodes.
How do we measure Conductivity?
There are two basic sensor styles used for measuring Conductivity: Contacting and Inductive (Toroidal, Electrodeless).
When Contacting Sensors are used, the conductivity is measured by applying an alternating electrical current to the sensor electrodes (that together make up the cell constant) immersed in a solution and measuring the resulting voltage. The solution acts as the electrical conductor between the sensor electrodes.
With Inductive (also called Toroidal or Electrodeless), the sensing elements (electrode coils) of an inductive sensor do not come in direct contact with the process. These two matched (identical coils) are encapsulated in PEEK (or Teflon) protecting them from the adverse effects of the process.
What makes a solution conductive?
Ions present in the liquid (Na, Ca, Cl, H, OH) are what is responsible for carrying the electric current.
Conductivity is only a quantitative measurement: it responds to all ionic content and cannot distinguish particular conductive materials in the presence of others. Only ionizable materials will contribute to conductivity; materials such as sugars or oils are not conductive.
Conductivity applications cover a wide range from pure water at less than 1x10-7 S/cm to concentrated solutions with values greater than 1 S/cm. Such application examples are WIFI, demineralizer water, RO water, percent concentration, boiler blowdown, and TDS.
In general, the measurement of conductivity is a rapid and inexpensive way of determining the ionic strength of a solution. Conductivity is used to measure the purity of water or the concentration of ionized chemicals in water. It is a nonspecific technique, unable to distinguish between different types of ions, giving instead a reading that is proportional to the combined effect of all the ions present.
The accuracy of the measurement is strongly influenced by temperature variations, polarization effects at the surface of the contacting electrodes, cable capacitances, etc.
Yokogawa has designed a full range of precision sensors and instruments to cope with these measurements, even under extreme conditions.
How do I select the right sensor?
When selecting a conductivity sensor for an application we need to consider the following:
What is a cell constant, and why do we need to be concerned about it?
The cell constant is mathematical value for a "multiplying factor" that is used to determine the measuring range of the sensor. This mathematical value is determined by the geometric design of the cell. It is calculated by dividing the distance (length) between the two measuring plates by the area of the plates (area of the plates is determined by the area of the outside-the area of the inside = area between the electrodes).
The raw conductivity value is then multiplied by the cell constant which is why we see the unit µS (micro siemens)/cm.
Yokogawa offers four cell constants: 0.01, 0.1, 1.0, and 10.0, which provide accurate of the entire Measurement range of 0-2,000,000 µS. These values are known as the nominal cell constant, whereas the printed cell constant on the sensor can vary slightly (you will see 0.0198 instead of 0.02) is the specific cell constant for that sensor.
One of the problems that occur when an incorrect cell constant is used is Polarization.
The first example shows a solution with the correct cell constant where the ions are free to travel from one plate to the other.
The second example shows the same cell constant being used in a highly conductive solution. When the voltage alternates (switches polarity), the ions cannot freely move to the other plate because the ion density is too high. This results in fewer ions contacting the correct plate which will result in a false low reading.
However, for the ISC40 Inductive Sensor, there is only one cell factor (constant). It covers the entire conductivity measurement range 0-2,000 S/cm. But on only the low end (below 50µS) does the accuracy of the sensor suffer.
To defray energy costs, many industrial plants have their own boilers to generate steam to produce a portion of their energy needs. In addition to generating power, the steam may also be used directly in plant processes or indirectly via heat exchangers or steam jacketed vessels.
The term "cooling tower" is used to describe both direct (open circuit) and indirect (closed circuit) heat rejection equipment. Cooling towers are heat-transfer units, used to remove heat from any water-cooled system. The cooled water is then re-circulated (and thus, recycled) back into the system. Since the process water is re-circulated, the mineral concentration increases as a result of the evaporation.
Industry:Refining, Food and Beverage, Power, Oil and Gas, Pulp and Paper, Chemical
Control of sodium chloride (NaCl) concentration at a salt dissolver where solid salt is dissolved in water, is highly important because of the electrolysis efficiency. A conventional way of measuring the concentration of supersaturated NaCl solution had been performed by using non-contact type sensors (e.g., γ-ray density meter) since NaCl, impurities, and precipitates are in the solution.
In a semiconductor plant, a variety of chemicals are used in various manufacturing processes. The chemicals used for specific purposes are produced by diluting raw liquid with demineralized water using in diluting equipment, and the control of the concentration at this point is performed by conductivity measurement.
In the manufacturing process of Pharmaceutical, Chemical and Food & Beverage industries, the cleaning and sterilization of tanks and piping are done with various cleaning solutions, fresh or hot water and steam after manufacturing products. Clean-In-Place (CIP) is the system designed for automatic cleaning and disinfecting.
Reverse osmosis (RO) is a separation process that uses pressure to force a solution through a membrane that retains the solute on one side and allows the pure solvent to pass to the other side. More formally, it is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure.
The kraft process (also known as kraft pulping or sulfate process) describes a technology for conversion of wood into wood pulp consisting of almost pure cellulose fibers. Wood chips are by harsh chemicals (white liquor) to produce pulp and spent liquor (black liquor).
The United States Pharmacopoeia (USP) and European Pharmacopoeia (EP) regulations require total organic carbon (TOC) to be monitored in pharmaceutical waters. Implementing these guidelines is required for companies that bring drugs to the US market. These guidelines are contained in the USP monograph (article).
Among other functions, the USP states qualifications for sterility and packaging methods that delineate between the various specific types of water. However, there are two basic types of water preparation, Water for Injection and Purified Water. The analytical standards for these two types of water are very similar, differing in the fact that Water for Injection has stricter bacterial count standards and must also pass the bacterial endotoxin test. Preparation methods are very similar to a point, however, Water for Injection preparation must incorporate distillation or double pass reverse osmosis. Discussion of the various methodologies used in preparation of USP water applies equally to Purified Water (PW) and Water for Injection (WFI).
There are two main types of water, purified water (PW) and water for injection (WFI). Purified Water is water obtained by distillation, ion-exchange treatment, reverse osmosis, or other suitable process. It is prepared from water complying with the regulations of the U.S. Environmental Protection Agency (EPA) with respect to drinking water. It contains no added substances. And, Water for Injection (WFI) is water purified by distillation or reverse osmosis.
These waters are used as ingredients in either dose form or bulk pharmaceuticals, so purity is critical. WFI is the purest grade of bulk water monographed by the USP, and is found in the manufacture of parenteral (injected), ophthalmic (eye drops), and inhalation products. The Fundamental objectives of the USP are:
I. To maintain or improve the existing water quality
II. Improvement of the reliability of the measured values by means of modern analytic instruments
III. Reduction of the number of samples
IV. Authorization of the in-line measuring method
Before 1996, the quality of these waters was determined by a number of off-line, “antiquated” laboratory tests. The USP monograph 23 (and currently 24) replaced these tests with an on-line conductivity measurement as the initial marker. While PW only needs to meet a TOC limit, the WFI has to meet bacterial tests in addition to the TOC and conductivity limits. In this application note we will concentrate on the USP requirements for conductivity only. This change to an on-line conductivity measurement was precipitated by many desires including improving the reliability of the testing by using modern instrumentation, providing immediate alarms and options for quality control, eliminating sample collection and handling errors, and reducing the cost of testing.
The conductivity requirements mandated by USP are tiered in three stages:
Stage One: Use In-line or grab sample methods to measure the conductivity and water temperature. This conductivity reading must not be temperature compensated. Compare these readings to the Stage I graph shown, or the values in table I. If the conductivity is below the limit stated for that temperature, the water meets the requirements. If the conductivity is above the limit, proceed to Stage Two. Advantages to In-Line measurement are:
I. Real-time information for conductivity and temperature
II. Immediate limit value alarm
III. Data output for recording and documentation of the water quality
IV. Simple and reasonably-priced measurement
V. Avoiding errors due to sampling, handling and transport
Stage Two: Take a grab sample and measure the conductivity after equalization with atmosphere and temperature normalization to 25ºC. If the water conductivity is below 2.1 μS then Stage Three is needed.
Stage Three: If Stage Two is exceeded, measure the pH of the grab sample and check conductivity against the results in table I of conductivity vs. pH. If the sample is within the limits, it passes. If it does not, the water is deemed unacceptable for PW or WFI use.
Yokogawa’s conductivity transmitters and converters possess USP functions that make this seemingly complex and troublesome requirement pain free, and automatic. The FLEXA two-wire conductivity transmitter has the USP23/24 Stage One table pre-programmed in its software. When enabled, the transmitter will send a FAIL signal when the water exceeds the USP limit. It also can display and transmit the uncompensated conductivity that USP mandates for compliance recording.
The SC450 and DC402 four-wire conductivity converters have additional USP features. These units have the ability to display and transmit the uncompensated conductivity for USP compliance, as well as the NaCltemperature compensated measurement, valuable for process control. The USP23/24 Stage One table is pre-programmed into these instruments, and a FAIL alarm will be given if the conductivity limits are exceeded. Alarms on these units can be dedicated as USP “warning” alarms with user defined safety margins. These “warning” alarms will inform the operator that his/ or her water is trending towards the USP limit, and will allow him/or her to take preemptive corrective action.
Installing the sanitary fitting on an ISC40 sensor. While we strive to provide the most safe and accurate information possible, we are not responsible for any loss or damages resulting from attempting to replicate the acts conducted in this video. Furthermore, we shall not be held liable for use or misuse of information contained in this video.
In this 40 min session you will learn the fundamental requirements for aqueous conductivity measurements; the differences between "Contacting" and "Inductive" measurement techniques and which one to use for a particular application. As well as learn the importance of online diagnostics. The goal is to provide participants with simple techniques they can implement to improve their day to day operations and to identify causes of errors and how to correct them
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