Conductivity Sensor Calibrations to Meet Industry Requirement Braga 2

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Conductivity Sensor Calibrations to Meet Water

Industry Requirements

Victor M. Braga

Technical Service/Training Manager

Mettler-Toledo Thornton Inc.

36 Middlesex Turnpike

Bedford, Massachusetts 01730Phone (781) 301-8600

Phone (800) 642-4418

FAX (781) 271-0675

Web: www.mt.com/thornton

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 sensors calibration. It is well documented that a significant percentage of

the error associated with a conductivity-measurement-system is attributed to the

conductivity sensor1. It is therefore crucial that, to produce accurate conductivity

measurements, the conductivity sensor undergo a well-defined calibration process that

enhances or improves accuracy.


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Conductivity Sensor Calibrations to Meet Industry Requirements 2

ULTRAPURE WATER Asia 2004 March 10-11, 2004

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 waysto 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 sensors

calibration process has been reevaluated and analyzed to improve overall accuracy.

The conductivity sensors 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 Commerces 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.


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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 theAnnual 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

Todays 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, todays modern analyzers must

also accurately measure the solutions temperature. This measurement is generally

performed by yet another circuit and must also be fully calibrated during the analyzers

calibration.


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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.

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:

1

2

=

= cm

cm

cm

aK

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

G1

G2

ConductivityCell

Figure 1


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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 analyzers resistance circuit2. The analyzers temperature circuit


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3. The sensors cell constant4. The sensors 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 analyzers inputs and adjusting the analyzers 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 sensors 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 temperatures1. The water loop circulates until the water quality reaches 18.18

M-cm (0.055S/cm) and the temperature is stabilized at 25C. At this point, the

unknown sensors cell constant and temperature factors are computed.

Calibration at Elevated Temperatures for Ultrapure Water

Applications

The need for accurate temperature measurements have been well documented2. Accurate

temperature compensated resistivity measurements depend not only on the measuring

systems ability to read the raw uncompensated resistivity but also on the sensors ability

to deliver an accurate temperature measurement. Relatively small temperature errors at


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or near ambient temperature can greatly impacted compensated resistivity measurements

at elevated temperatures. Figure 4 illustrates how a small temperature error of just -0.2C

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 sensors 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.

17.4

17.5

17.6

17.7

17.8

17.9

18.0

18.1

18.2

18.3

18.4

0 10 20 30 40 50 60 70 80 90 100

Temperature (C)

CompensatedResistivity(M-cm)

No temperature or resistivity error

-0.2C temperature error

Figure 4


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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 DrugAdministration (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 0.6S/cm at 0.0C to 3.1S/cm

at 100C, as illustrated in Table 1.

Stage 1 USP Conductivity Limits

as a Function of Temperature

Temperature

(C)

Conductivity

Limit (S/cm)

Temperature

(C)

Conductivity

Limit (S/cm)

0 0.6 50 1.9

5 0.8 55 2.1

10 0.9 60 2.2

15 1.0 65 2.4

20 1.1 70 2.5

25 1.3 75 2.7

30 1.4 80 2.7

35 1.5 90 2.7

40 1.7 95 2.9

45 1.8 100 3.1

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 companys Standard Operating Procedures (SOP) require that the sensor be

verified at a different conductivity level and demand before calibration or as foundreadings be provide to assure that product produced with the sensor met all the

requirements.

To meet the water system manufacturers and end-usersneeds, instrument manufacturers

are providing calibration options that calibrate or compute the cell constant in ultrapure


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water (0.0550 S/cm) and verify it in ASTM D1125 Solution D (146.93 S/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 sensors

previous or original cell constant are provide for historical data and to assure that all

product produced with the sensor meets USP requirements.

Figure 5

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 analyzers resistance circuit2. The analyzers temperature circuit3. The sensors cell constant4. The sensors 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

0.011.0100.01,000.0

S/cm S/cm10.0 0.1

25C

UPW

Solution DUSP waters


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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

analyzers 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 sensors cell and temperature

constant are computed. By computing the sensors constants with its own analyzer, the

only unknown inaccuracy is the conductivity solution. Thus, only the sensors

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.

TYPICAL MEASURING SYSTEM ERRORSAnalyzers Resistivity Error 0.5%

Analyzers Temperature Error 1.0%

Sensors Cell Constant 1.0%

Sensors RTD 0.8%

Worst Case Error 3.3%

Square root of the sum of the squares 1.7%

Table 2

Conclusion

The need for more accurate conductivity measurements is well documented. By using

existing technologies and unique innovative calibration methods, the technology offered


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in todays 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 ofUltrapure Water, 1998 Semiconductor Pure Water and Chemicals Conference,

March 2-5, 1998.

2. 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. 18-26, December, 1994.

Documents

Conductivity Sensor Calibrations to Meet Industry Requirement Braga 2