Conductivity Sensor Calibrations to Meet Water Industry Requirements

Transcription

1 Conductivity Sensor Calibrations to Meet Water Industry Requirements Victor M. Braga Technical Service/Training Manager Mettler-Toledo Thornton Inc. 36 Middlesex Turnpike Bedford, Massachusetts Phone (781) Phone (800) FAX (781) Web: Abstract As water purification technologies improve and the need for reliable, repeatable, and accurate conductivity measurements grows, instrument manufacturers are constantly challenged by water system manufacturers and end-users to continuously improve measurement accuracies and to provide dependable, accurate readings. The challenge to instrument manufacturers is not only to press forward with new more accurate technologies, but to also reexamine existing processes and identify new and reliable methods of improving them. By improving existing processes, instrument manufacturers can achieve higher levels of accuracy with existing technologies. One area that has recently been reevaluated is the calibration process, particularly the conductivity sensor s calibration. It is well documented that a significant percentage of the error associated with a conductivity-measurement-system is attributed to the conductivity sensor 1. It is therefore crucial that, to produce accurate conductivity measurements, the conductivity sensor undergo a well-defined calibration process that enhances or improves accuracy.

2 Conductivity Sensor Calibrations to Meet Industry Requirements 2 Introduction In response to the demand for better and more accurate conductivity measurements, instrument manufacturers have responded by not only evaluating new and improved technologies, but by also reevaluating existing calibration processes and examining ways to improve or minimize errors. One area that has recently experienced such a reexamination is the calibration process of the conductivity sensor. By reevaluating this process, instrument manufacturers have devised new and innovated methods to calibrated the sensor and achieve higher levels of accuracy without significant changes in technology. Each phase of the sensor s calibration process has been reevaluated and analyzed to improve overall accuracy. The conductivity sensor s calibration process produces two calibration factors, the conductivity multiplier (also know as the cell constant) and the temperature multiplier. These factors are determined by placing the sensor in known conductivity samples at a controlled and known temperature. This paper will examine ways of conducting and computing these factors to minimize overall system error. National Standards and Traceability In the United States, we have two national standards to which most conductivity and temperature calibrations are traceable, the American Society for Testing and Materials (ASTM) and the National Institution of Standards and Technology (NIST). The National Institute of Standards and Technology is an agency of the U.S. Department of Commerce s Technology Administration. It was established in 1901 to strengthen the U.S. economy and improve the quality of life by working with industry to develop and apply technology, measurements, and standards. It operates primarily in two locations; Gaithersburg, MD and Boulder, CO. NIST laboratories provide calibration services and calibrations standards to industries.

3 Conductivity Sensor Calibrations to Meet Industry Requirements 3 The American Society for Testing and Materials was organized in 1898; it is one of the largest voluntary standards development organizations in the world. It is a not-for-profit organization that provides a forum for the development and publication of voluntary consensus standards. It publishes more than 10,000 standards each year in the 73 volumes of the Annual Book of ASTM Standards. ASTM does not provide calibration services. It publishes standard calibration methods and procedures to which calibrations may be performed. These are standards that are used by laboratories and industries that could be responsible for forensics, environmental, density, chemical and other analytical analysis. Measuring Systems A complete measuring system consists of three basic components: measuring instrument (or analyzer), sensor or cell, and the cable linking the sensor and analyzer. Each of these components contributes to overall system accuracy. Analyzer Calibration Today s analyzers are highly sophisticated technological measuring instruments. The more advanced instruments have several measurement circuits which optimize overall accuracy (see Figure 1). The analyzer is capable of evaluating the input signal from the conductivity sensor and selecting the most accurate gain circuitry for optimal accuracy. This automated analysis and selection process happens instantly and the operator is never aware of its occurrence. It is therefore imperative that each circuit be fully calibrated each time the analyzer is recalibrated. In addition to measuring the conductivity of the solution, today s modern analyzers must also accurately measure the solution s temperature. This measurement is generally performed by yet another circuit and must also be fully calibrated during the analyzer s calibration.

4 Conductivity Sensor Calibrations to Meet Industry Requirements 4 It could take an operator several hours to perform a multiple point calibration on all the measuring circuits. Some modern analyzers measure several parameters and require up to 72 calibration points. Fortunately, analyzer manufacturers have developed automated calibration systems that can perform a full calibration in minutes. G2 G1 Conductivity Cell Figure 1 Sensor Calibration Conductivity is measured by placing two electrodes of known area (a) in a solution at a fixed distance apart ( ). The ability of solution to conduct (conductivity) is measured by applying an alternating current (AC) to the electrodes and measuring how difficult (resistance) it is for the electrons to flow from one electrode to the other (current flow). The closer the electrodes are to each other, and the more surface area they have, the easier it is for current flow. Therefore, a fixed area of 1 square centimeter and a distance of 1 centimeter were established to standardize conductivity measurements worldwide (see figure 2). This makes conductivity measurements a volumetric measurement of 1 cubic centimeter and defines the cell constant as: cm K = = 2 cm a cm 1 It not imperative that the electrodes be exactly 1cm apart or have exactly 1 square cm of surface area as long as the exact ratio is known. As a matter of fact, instrument manufacturers routinely alter these dimensions to facilitate current flow from one electrode to the other. Cell constants of 0.1 cm -1 and even 0.01 cm -1 are frequently used

5 Conductivity Sensor Calibrations to Meet Industry Requirements 5 to measure ultrapure water (UPW) because they produce lower resistance to current flow in very high resistivity water. Figure 2 The electrode design illustrated in Figure 2 is impractical for general use due to mechanical instability; any small change in distance would compromise the cell constant. The concentric design (see Figure 3), which is far more robust, was developed to meet industry needs. Figure 3 The calibration process for a conductivity sensor not only computes cell constant, but also must calibrate the temperature sensor located inside the conductivity sensor. Most modern sensors use 1000 ohm platinum (Pt1000) resistance temperature devices (RTD) to accurately measure the temperature. Standard Calibrations Four parameters must be fully calibrated to produce a calibrated system. They are: 1. The analyzer s resistance circuit 2. The analyzer s temperature circuit

6 Conductivity Sensor Calibrations to Meet Industry Requirements 6 3. The sensor s cell constant 4. The sensor s RTD Standard calibrations consist of calibrating the analyzer and conductivity sensor individually. The analyzer and sensor will each have its own tolerance and accuracy limits, for both resistance and temperature. The analyzer resistance and temperature circuits are calibrated by placing precise resistance values on the analyzer s inputs and adjusting the analyzer s gain circuitry until the input value equals the displayed value. This can be performed with resistors traceable to national standards (NIST) or with automated fixtures whose internal resistors are also traceable to national standards. Since it would be very difficult and impractical to physically measure the area and distance of a concentric sensor s electrodes, the sensor is calibrated by placing it in a known conductivity solution, traceable to national standards (ASTM) and computing the cell constant. However, low level conductivity standards for UPW applications are not commercially available, nor can they be easily produced. Thus, the sensor is calibrated in a sealed, circulating ultrapure water loop against a standard sensor whose cell constant has been computed by placing it in ASTM D1125 solution D and in ultrapure water at various temperatures 1. The water loop circulates until the water quality reaches M-cm (0.055µS/cm) and the temperature is stabilized at 25 C. At this point, the unknown sensor s cell constant and temperature factors are computed. Calibration at Elevated Temperatures for Ultrapure Water Applications The need for accurate temperature measurements have been well documented 2. Accurate temperature compensated resistivity measurements depend not only on the measuring system s ability to read the raw uncompensated resistivity but also on the sensor s ability to deliver an accurate temperature measurement. Relatively small temperature errors at

7 Conductivity Sensor Calibrations to Meet Industry Requirements 7 or near ambient temperature can greatly impacted compensated resistivity measurements at elevated temperatures. Figure 4 illustrates how a small temperature error of just -0.2 C can have dramatic effects at elevated temperatures. Many factors can impact the sensors ability to measure an accurate temperate, such as location of the RTD, the ability of the sensor s material to conduct heat and temperature gradients. Temperature gradients occur when a portion of the sensor is at ambient temperature (outside the pipe) and another part of it is at an elevated temperature (inside the pipe). As the difference between the internal and external temperature increases, the temperature errors also increases and the greater the impact on compensated readings. To minimize the impact of temperature gradients, sensors can be calibrated at elevated temperatures. With knowledge of the end-user applications, especially temperature requirements, improved accuracy can be achieved by calibrating under similar conditions. This can significantly reduce the temperature error and optimize overall accuracy. Compensated Resistivity (MΩ-cm) No temperature or resistivity error -0.2 C temperature error Temperature ( C) Figure 4

8 Conductivity Sensor Calibrations to Meet Industry Requirements 8 Calibration to Meet and Exceed USP Requirements Water quality standards for pharmaceutical and biotech industries in the United States, or for any manufacturer who wishes to sell pharmaceuticals in the United States, are set by the United States Pharmacopeia (USP) and enforced by the Food and Drug Administration (FDA). These standards require that Water for Injection (WFI) and Purified Water (PW) meet certain conductivity limits before it can be used to manufacture product. The water quality limits range from 0.6µS/cm at 0.0 C to 3.1µS/cm at 100 C, as illustrated in Table 1. Temperature ( C) Stage 1 USP Conductivity Limits as a Function of Temperature Conductivity Limit (µs/cm) Temperature ( C) Conductivity Limit (µs/cm) Table 1 Standard sensor calibrations compute the cell constant at a single conductivity level. Although this is adequate, and meets USP requirements, many pharmaceutical and biotech company s Standard Operating Procedures (SOP) require that the sensor be verified at a different conductivity level and demand before calibration or as found readings be provide to assure that product produced with the sensor met all the requirements. To meet the water system manufacturers and end-users needs, instrument manufacturers are providing calibration options that calibrate or compute the cell constant in ultrapure

9 Conductivity Sensor Calibrations to Meet Industry Requirements 9 water ( µs/cm) and verify it in ASTM D1125 Solution D ( µs/cm), as illustrated in figure 5. By calibrating at a point below the requirement and verifying at a point above the requirement, the user is assured of accurate and linear performance throughout the dynamic range. Additionally, as found readings with the sensor s previous or original cell constant are provide for historical data and to assure that all product produced with the sensor meets USP requirements. µs/cm 1, µs/cm Solution D Figure 5 USP waters 25 C UPW System Calibration As the need for more accurate and precise conductivity measurements increases, instrument and sensor manufacturers have looked for new innovative ways to reduce the overall system accuracy. One method of reducing overall system accuracy is to calibrate the entire measuring system (analyzer and sensor) as a single unit. As discussed earlier, a measuring system consists of four basic measurements: 1. The analyzer s resistance circuit 2. The analyzer s temperature circuit 3. The sensor s cell constant 4. The sensor s RTD Table 2 shows typical inaccuracies associated with each measurement. If all the inaccuracies are added in a negative or positive direction, errors greater than 3% could be expected. However, it is highly unlikely that all errors add in the same direction. A more

10 Conductivity Sensor Calibrations to Meet Industry Requirements 10 typical analytical method is to take the square root of the sum of all errors. Using this method, a typical system accuracy of about 2% is more reasonable. Still, more precise accuracies are desirable. Instrument manufacturers have improved system accuracy by first calibrating the analyzer s resistance and temperature circuits with known traceable standards. Then, the sensor is placed in known and traceable conductivity standards at a known and fixed temperature. Using the previously calibrated analyzer, the sensor s cell and temperature constant are computed. By computing the sensor s constants with its own analyzer, the only unknown inaccuracy is the conductivity solution. Thus, only the sensor s inaccuracies, and the conductivity standard solution, contribute to the overall system accuracy. In most cases, even the cable which connects the sensor to analyzer is used during the calibration process to further reduce inaccuracies. Using this method, system inaccuracy can be reduced to about 1% over the dynamic range of sensor and analyzer, and to less than 0.5% at the calibration point, usually ultrapure water. Conclusion TYPICAL MEASURING SYSTEM ERRORS Analyzer s Resistivity Error 0.5% Analyzer s Temperature Error 1.0% Sensor s Cell Constant 1.0% Sensor s RTD 0.8% Worst Case Error 3.3% Square root of the sum of the squares 1.7% Table 2 The need for more accurate conductivity measurements is well documented. By using existing technologies and unique innovative calibration methods, the technology offered

11 Conductivity Sensor Calibrations to Meet Industry Requirements 11 in today s instrumentation provides more precise and accurate conductivity measurements. Biography Mettler Toledo Thornton Inc. has been a leading innovator and manufacturer of sensors and instrumentation to monitor water purity and other fluid-based parameters since 1964, specializing in Ultrapure Water for the semiconductor, pharmaceutical, and power generation industries. Thornton instrumentation includes measurements for Resistivity, Conductivity, TOC, Temperature, % Acid/Base, ph, ORP, Flow, Pressure, Level and more. Thornton is a leader and technological innovator in the design and development of multi-parameter instrumentation, accurate temperature compensation algorithms, accurate UPW and hot UPW resistivity measurements, patented Smart Sensor calibration technology, rapid TOC measurements for UPW and reclaim/recycle, high resistivity applications such as ethylene glycol coolant and isopropyl alcohol cleaning, and high conductivity applications such as regenerant acid/caustic and wastewater. References 1. A.C. Bevilacqua, "Ultrapure Water The Standard for Resistivity Measurements of Ultrapure Water, 1998 Semiconductor Pure Water and Chemicals Conference, March 2-5, K.R. Morash, R.D. Thornton, C.H. Saunders, A.C. Bevilacqua, and T.S. Light, "Measurement of the Resistivity of High-Purity Water at Elevated Temperatures", Ultrapure Water, 11(9), pp , December, 1994.