Accuracy and Repeatability

Accuracy and Repeatability Definitions of terminology pertaining to process measurements can be obtained from standards available from the Instrumentation, Systems, and Automation Society 

Accuracy Accuracy refers to the difference between the measured value and the true value of the measured variable. Unfortunately, the true value is never known, so in practice accuracy refers to the difference between the measured value and an accepted standard value for the measured variable. 

For process measurements, accuracy as a percent of span is the most common. 

Manufacturers of measurement devices always state the accuracy of the instrument. However, these statements always specify specific or reference conditions at which the measurement device will perform with the stated accuracy, with temperature and pressure most often appearing in the reference conditions. When the measurement device is applied at other conditions, the accuracy is affected. Manufacturers usually also provide some statements on how accuracy is affected when the conditions of use deviate from the referenced conditions in the statement of accuracy. Although appropriate calibration procedures can minimize some of these effects, rarely can they be totally eliminated. It is easily possible for such effects to cause a measurement device with a stated accuracy of 0.25 percent of span at reference conditions to ultimately provide measured values with accuracies of 1 percent or less. Microprocessor-based measurement devices usually provide better accuracy than do the traditional electronic measurement devices. 

In practice, most attention is given to accuracy when the measured variable is the basis for billing, such as in custody transfer applications. However, whenever a measurement device provides data to any type of optimization strategy, accuracy is very important. 

The repeatability was determined by v, and Aj at 1417 cm and 1501 cm peaks of 10 parallel independent samples measured at 150 scans, Lorentzian to Gaussian peak function ratio at 50 50, factors between 0.00042 and 0.0004, and 2000 iterations. Three repeated measurements were obtained for each sample as well. The peak positions at 1501 cm 0.022 and 1416.97 O.O24 cm- were obtained. The reproducibility of the integral absorptions was determined by A,5oi/A,4,7 ratios, where the results indicated a 0.452 0.032g value. 

2 The Quantitative Ratio oe Liquid Crystals and Solid Samples 

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Accuracy and Repeatability Definitions of terminology pertaining to process measurements can be obtained from standard S5I.I from the International Society of Measurment and Control (ISA) and standard RC20-II from the Scientific Apparatus Manufac turers Association (SAMA), both of which are updated periodically. An appreciation of accuracy and repeatability is especially important. Some apphcations depend on the accuracy of the instrument, but other apphcations depend on repeatability. Excellent accuracy imphes excellent repeatabihty however, an instrument can have poor accuracy but excellent repeatability. In some apphcations, this is acceptable, as discussed below. 

Application of IP and NCS in conjunction with specification tolerance limits enables to substantiate acceptance criteria for linear regression metrological characteristics (residual standard deviation, correlation coefficient, y-intercept), accuracy and repeatability. Acceptance criteria for impurity influence (in spectrophotometric assay), solution stability and intermediate precision are substantiated as well. 

The best method of ensuring that the point of measurement, its orientation, and the compressive load are exactly the same each time is to permanently or hard mount the transducers, which is illustrated in Figure 43.26. This guarantees accuracy and repeatability of acquired data. However, it also increases the initial cost of the program. The average cost of installing a general-purpose accelerometer is about 300 per measurement point or 3000 for a typical machine-train. 

Another method used by some plants to acquire data is hand-held transducers. This approach is not recommended if it is possible to use any other method. Hand-held transducers do not provide the accuracy and repeatability required to gain maximum benefit from a predictive maintenance program. If this technique must be used, extreme care should be exercised to ensure that the same location, orientation, and compressive load are used for every measurement. Illustrates a hand-held device. 

Training is critical with any of the imaging systems. The variables that can destroy the accuracy and repeatability of thermal data must be compensated for each time infrared data is acquired. In addition, interpretation of infrared data requires extensive training and experience. 

Optical-alignment systems offer several advantages. Because laser fixtures eliminate the mechanical linkage and runout gages, there is no fixture sag. This greatly increases the accuracy and repeatability of the data obtained using this method. 

Accuracy and repeatability of temperature/time/velocity/pressure controls of injection unit, accuracy and repeatability of clamping force, flatness and parallelism of platens, even distribution of clamping on all tie rods, repeatability of controlling pressure and temperature of oil, oil temperature variation minimized, no oil contamination (by the time you see oil contamination damage to the hydraulic system could have already occurred), machine properly leveled. 

The voltammograms at the microhole-supported ITIES were analyzed using the Tomes criterion [34], which predicts ii3/4 — iii/4l = 56.4/n mV (where n is the number of electrons transferred and E- i and 1/4 refer to the three-quarter and one-quarter potentials, respectively) for a reversible ET reaction. An attempt was made to use the deviations from the reversible behavior to estimate kinetic parameters using the method previously developed for UMEs [21,27]. However, the shape of measured voltammograms was imperfect, and the slope of the semilogarithmic plot observed was much lower than expected from the theory. It was concluded that voltammetry at micro-ITIES is not suitable for ET kinetic measurements because of insufficient accuracy and repeatability [16]. Those experiments may have been affected by reactions involving the supporting electrolytes, ion transfers, and interfacial precipitation. It is also possible that the data was at variance with the Butler-Volmer model because the overall reaction rate was only weakly potential-dependent [35] and/or limited by the precursor complex formation at the interface [33b]. 

Two important, but often under emphasized, aspects of measuring c/w values and isotherms in foods are accuracy and repeatability. Variation in and isotherm values can be due to inherent variation in biological materials, as well as differences in measurement methods, protocols, and equipment employed. Wolf et al. (1985) presented the results of a COST 90 Project on the standardization of saturated salt solution isotherm measurement methodology and Lewicki and Pomaranska-Lazuka (2003) discussed errors in the static desiccator method. 

Once initial analyses are completed, random samples are sent from SGS to ActLabs for check assays, to establish precision (repeatability) and analytical bias. Additionally, coarse sample rejects are chosen at random and sent to ActLabs for preparation and analysis, to check the accuracy and repeatability of the original sample preparation. A further check on SGS Lab precision is conducted by renumbering pulps and re-submission from ActLab to SGS for analysis. Tournigan monitors quality assurance by plotting and analyzing the data, as received, and activates re-assaying of sample batches that do not meet predetermined standards. 

Table 26.3 Numerical Requirements for Accuracy and Repeatability of Analytical Methods in the European Union...

Virtually all new pilot plants will be computer controlled and heavily automated due to the high cost of operating labor, need for high accuracy and repeatability, and ease of data gathering and work-up. Stand-alone computer and programmable logic controller systems will continue to dominate the... 

Because of its fundamental nature and superior accuracy and repeatability. this kind of instrument is widely used as a secondary standard (National Bureau of Standards lor calibrating other lower level humidity instruments... 

For the control/monitoring instrumentation, regulating devices, and any associated electrical equipment, predelivery testing and calibration is normally the responsibility of the instrument/equipment manufacturer and should be carried out to approved written procedures using calibration test equipment that is traceable back to agreed-upon national standards. The test equipment must have precision, accuracy, and repeatability that are higher than that of the instrument being calibrated. 

Instruments which can monitor the important process variables during plant operation must be specified. These instruments must be capable of measuring the variables and should have an acceptable accuracy and repeatability of measurement, usually the latter attribute is more important than the former on chemical plant measurements. The instruments may be used for manual measurements or included in automatic control loops. Automatic alarms may also be required to indicate deviations outside acceptable limits. If possible, direct measurement of the process variable should be made, however it is often easier to measure a dependent variable, e.g. temperature measured as an indication of composition for distillation column top product. 

Some outcomes of the metrological evaluation of instrumental accuracy and repeatability of ten different types or model of turbidimeters frequently used for water quality analysis are discussed here. Considering eight points on the turbidity measurement range, uniformly distributed within 0.5-200 FNU, the result for each instrument was obtained in the form of five single values, from which the corresponding mean and standard deviation was calculated. 

Research various particle-size distribution and chemical concentration measurement technologies and related instrumentation. Create a list of their capabilities and limitations, measurement ranges, and accuracy and repeatability. Evaluate the applicability of these in the CMP slurry measurements. Discuss the possibilities of zeta-potential changes as a result... 

This apparatus is based on the principle of the detection by flame ionisation. It is especially dedicated to the study of gaseous organic compounds (except formaldehyde, formic acid and carbon suliur). It allows the measurement of the VOC molar ratio in the helium flow (y). The gas analyser is able to perform VOC molar ratio measurements up to 10000 ppm THC. The measuring range may be selected (minimum end of scale value 4 ppm THC). Accuracy and repeatability on each range are better than 1% FSD. 

Many modern rotary tablet presses use off-the-shelf load cells for force measurement. These load cells are highly accurate, durable, and easily replaced and calibrated. However, the final accuracy and repeatability of force measurement in the machine not only depend on the quality of the load cell, but also on the design of the compression assembly and the placement of the load cell within the assembly. 

Any attempts to obtain the parameters of the chromatograms and the physicochemical constants which are measurable in theory, by FFF, will be affected by the sample mass injected into the FFF channel. All of the concentration effects on the chromatograms discussed in the previous sections will be transferred, in turn, to those measured parameters and the physicochemical constants, such as the mass selectivity (S ), the common diffusion coefficient (D), the thermal diffusion coefficient (Dj-), and so forth. The increased retention of large polymers will result in enhanced mass selectivity in ThFFF. For a long time, this enhanced selectivity, in turn, the enhanced ThFFF universal calibration constant n, has led to confusion concerning the accuracy and repeatability of FFF, because different research groups have reported different data for selectivity and physicochemical constants measured by FFF for a given polymer-solvent combination [2,11]. Recent studies show that the enhanced selectivity and the different values of the physicochemical constants reported by different laboratories, measured by ThFFF, may be caused by different concentrations (sample mass) used by different laboratories. 

For physical and chemical measurements it is essential that measurements are referenced to standards accepted by all the laboratories undertaking a particular type of measurement. In particular, the use of chemical analytical procedures validated through their application to certified reference materials (CRM) (26) is highly recommended. It is to be noted that a number of CRMs prepared with Antarctic matrices are already available, i.e., marine sediment and krill, or in preparation (26-29). These and other CRMs should be also used routinely by the participating laboratories to assure a periodical assessment of accuracy and repeatability of measurements. These laboratories should also undertake regular intercalibration studies. It is possible that in the first application of these exercises systematic errors will be found, but better results are expected in subsequent rounds together with a general improvement in the performance of laboratories and data quality. 

In many countries undertaking the most technically demanding analyses, it is expected that the laboratories will be registered under quality assurance schemes. Accuracy and repeatability should be covered by quality assurance requirements, but resolution and detection limits are critically dependent on technique and instrumentation. Details on the general subjects of quality assurance/quality control, quantification of uncertainty in analytical measurements, accreditation of laboratories, and on the general concepts and strategy for ensuring that analytical chemical measurements are comparable - in one word traceability - can be found elsewhere in the chemical literature (26,32-36). 

Abstract There are a wide variety of spectrophotometric devices nowadays used in health services with various qualities of manufacture methods of measurement and metrological characteristics for performing the necessary measurements. Therefore, to meet the accuracy and repeatability requirements needed in medical diagnosis and treatment, the validation of the performance of such systems by clinical chemistry laboratories is essential. However, the validation of a spectrophotometric system for clinical analyses requires several... 

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