Performance and Calibration

Since most electrochemical measurements relating to corrosion of metals are satisfied with a sensitivity of 1 jiV or 1 xA, modern instrumentation usually employs electronic operational amplifiers where the noise limits control the range of measurements. The function of the operational amplifier is to amplify the potential (Vg) applied at the input so that it can be displayed on a low impedance analogue or digital meter (V ) as shown in Fig. 1.1. The output potential of the operational amplifier is proportional to the source potential and is required to have sufficient input impedance to avoid polarisation of the potential source. 

The basic corrosion instrumentation requirement involves the measurement of potential difference. Currents are measured as the potential across a resistor (R ) as shown in Fig. 1.2, where the potential difference is again determined with an operational amplifier. More sophisticated measurements such as polarisation characteristics and zero resistance ammetry involve the use of potentiostats which again use operational amplifiers in a differential mode. The potentiostat is an instrument for maintaining the potential of an electrode under test at a fixed potential compared with a reference cell, and the basic circuit is similar to that for potential measurement with the earth return circuit broken to an auxiliary electrode in the electrochemical cell. Such a circuit would maintain the potential of the test electrode at the reference cell potential. This potential may be varied by inserting a variable potential source (V ) in the input circuit as shown in Fig. 1.3. The actual cell potential (V ) and the current required to polarise the test electrode to this potential may be measured using the basic circuits shown in Figs. 1.1 and 1.2 respectively. 

A further modification of the basic voltage measurement, given in Fig. 1.1, is the zero resistance ammeter (ZRA) shown in Fig. 1.4. The measurement of current as the potential drop across a resistor (R ) shown in Fig. 1.2 involves an error due to the value of resistor A. This may be overcome with the ZRA in which the current is determined as the potential measured across a feedback resistor (Rp of the high gain operational amplifier. Thus, the current required to maintain electrodes A and B shown in Fig. 1.4 at the same potential can be determined and displayed on a high impedance voltmeter for which most commercial digital multimeters are suitable. 

The performance characteristics of commercial instrumentation should be supplied by the manufacturers. There are, however, measurement limitations which are controlled by the electrochemical cell under investigation. The major limitations and requirements are listed under their separate headings. 

The internal resistance of an ammeter (R ) in Fig. 1.2 presents a potential drop in the circuit. This may be overcome using the zero resistance ammeter shown in Fig. 1.4. 

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PHSS performance characteristics are described below and compiled in Table 8.1. PHSS performance and calibration were evaluated in the temperature-controlled... 

Note At the conclusion of the experimental work with refractive index, maintain the logbook for the equipment used, recording the date, your name(s), the experiment name and number (the parts you performed), and calibration information, if applicable. 

Analytical Methods. Iron (II) and Iron (III). Methanolic solutions of varying concentrations of iron (II) and iron (III) were made. Spectro-photometric analyses were performed and calibrated according to the reported procedure (20, 37). They both obey Beer s Law. In a kinetic... 

Measuring surface crack depth is performed by calibration samples made of the same material like the object being tested. Calibration samples are the plates having narrow grooves like slits of various depth 0.2 mm, 0.5 mm, 1.0 mm, 2.0 mm, 3.0 mm, 4.0 mm, 5.0 mm and made by electric erosion method. The samples have dimensions 50 mm X 150 mm x 6 mm and 25 mmx 150 mm x 6 mm and are made of magnetic... 

Checking of NDT devices is mandatory and is strictly regulated. Checking is performed and supervised by National metrological service. The term calibration is rather new in our practice. The works on calibration of NDT equipment are presented in the block-scheme 2. 

In situ control and calibration of flare and other gas metering systems is performed by gaseous tracers using the transit time method without affecting the normal production. Details about methodology are given in / /. 

The next step is to show that the response for the analysis of any target compound is linear. This step is known as the initial calibration and is achieved by the analysis of standards for a series of specified concentrations to produce a five-point calibration curve (Figure 41.2a, b). On subsequent days, a continuing calibration must be performed on calibration check compounds to evaluate the calibration precision of the GC/MS system. 

Details of any adjustments, servicing, repairs, and modifications carried out The name of the person performing the calibration Details of any limitation on its use... 

If three consecutive samples show a trend of being on either the high or the low side of the average, a fourth sample is run immediately. If this sample shows the same trend, a new calibration is performed and a new run chart is created. In this case the average is created using only 15 injections and the previous standard deviations are used to compute the new upper and lower control limits. 

Detectors will require frequent maintenance and calibration to perform accurately. 

We perform CLS on A6 to produce 2 calibrations. K6 and K6, are the matrices holding the pure component spectra and calibration coefficients, respectively, for CLS with zero intercepts. K6a and K6aMl are the corresponding matrices for CLS with nonzero intercepts. 

In summary, to obtain 7% from a platinum resistance thermometer, one selects the range of interest, calibrates the thermometer at the fixed points specified for those ranges, and uses the appropriate function to calculate AW(Tw) to be used in equation (A2.5). Companies are available that perform these calibrations and provide tables of W T<)0) versus 790 that can be interpolated to give 7% for a measured W T90). 

Quality systems require that facilities and equipment should be appropriate to the activities undertaken. Surfaces that are easy to clean and maintain in hygienic condition are a requirement in many situations. For example, cloth-backed chairs would not be acceptable in a laboratory that handled potentially biohazardous materials. Equipment should be checked at installation to demonstrate that it can perform its desired function. This is frequently done using an Installation Qualification, Operational Qualification and Performance Qualification (IQ /OQ/PQ) commissioning process. Routine maintenance and calibration programmes are then required to ensure that equipment continues to deliver the specified performance. 

Operational qualification involves performing a series of tests to check that all elements of the system are functional across the specified operating range. This usually involves performing challenges at the worst case extreme operating conditions. The process should allow confirmation of final operation, maintenance and calibration procedures. 

Example 33 Assume that a simple measurement costs 20 currency units n measurements are performed for calibration and m for replicates of each of five unknown samples. Furthermore, the calibration series of n measurements must be paid for by the unknowns to be analyzed. The slope of the calibration line is > = 1.00 and the residual standard deviation is Sres = 3, cf. Refs. 75, 95. The n calibration concentrations will be evenly spaced between 50 and 150% of nominal, that is for n = 4 x, 50, 83, 117, 150. For an unknown corresponding to 130% of nominal, should be below 3.3 units, respectively < 3.3 = 10.89. What combination of n and m will provide the most economical solution Use Eq. (2.4) for S x and Eq. (2.18) for Vx-Solution since Sxx is a function of the x-values, and thus a function of n (e.g. n = 4 Sxx = 5578), solve the three equations in the given order for various combinations of n and m and tabulate the costs per result, c/5 then select the... 

Given their radio-frequency electrical properties and nuclear magnetic resonance chemical shift components, solutions of reversed micelles constituted of water, AOT, and decane have been proposed as suitable systems to test and calibrate the performance of magnetic resonance imagers [68]. 

The present state of the art in blood pH measurements allows for rapid (1 minute) determination of pH between 6.4 and 8.0 to within at least 0.005 units for whole blood sample volumes < 100 microliters. The temperature of the electrodes and sample is generally controlled to within 0.1 °C for this level of precision and frequent calibration is carried out (in some cases a one point calibration for each sample). The electrodes require (both the glass and external reference) some maintenance due to protein fouling, however this procedure is largely automated. The useful life of an electrode is one year or less and the cost is well over 100 (U.S.) each. New technologies, both electrochemical and non-electrochemical, must compete with this attractive performance and provide for lower operating costs in order to be successful. 

The effectiveness of a number of crude oil dispersants, measured using a variety of evaluation procedures, indicates that temperature effects result from changing viscosity, dispersants are most effective at a salinity of approximately 40 ppt (parts per thousand), and concentration of dispersant is critical to effectiveness. The mixing time has little effect on performance, and a calibration procedure for laboratory dispersant effectiveness must include contact with water in a manner analogous to the extraction procedure otherwise, effectiveness may be inflated [587]. Compensation for the coloration produced by the dispersant alone is important only for some dispersants. 

Defined maintenance procedures should include model or manufacturer specifics and a list of parts to be inspected, cleaned, lubricated, replaced, and/or calibrated. The replacement part numbers, cleaning solutions and lubricants, and calibration standards, along with the manufacturer s maintenance procedures to be followed, should be specified. Documentation is easily managed by creating a one-page checklist of instructions or performance parameters that can be checked off as each task is completed. Any issues or comments can be captured directly on the checklist. Provisions for failure or out-of-tolerance notification need to be clearly defined, as the equipment cannot be put back into service until the performance has been verified by conducting a performance qualification. 

An organized document filing system must be maintained. This could be a paper file, an electronic document file, or a mixture of both. The equipment inventory system contains key information on the components of each system, their performance criteria and maintenance and calibration status. All documents including installation and performance documentation, as with other documents necessary to demonstrate the quality of the data, include SOPs for the qualification procedures, calibration, maintenance, personnel training, etc. If the manufacturer s operating, service, or maintenance manuals are used or cited in the operating procedures, copies of these manuals should be maintained. To facilitate retrieval, documentation should be stored in a central location and be indexed for easy retrieval. 

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