Accuracy of instrument

Pilot-plant design specifications should be established only after careful consideration of the experimental program because decisions on the accuracy of instruments, analyzers, and other equipment should be based on the requirements of the experiments planned for the unit. Flexibility and versatility are important but costly when provided unnecessarily or too profusely they can result in a unit that is difficult or impossible to operate successfully. 

The precision and accuracy of instrumental color measurements and the associated color tolerance can provide a system of control that equals that of an experienced color matcher. But to achieve this performance, the product standard and production test specimens must be of the highest quality. As was pointed out, more than two decades ago, the weak link in instrumental color control is the specimen. The human visual system can scan a specimen marred by many inconsistencies and the human mind will ignore those features that are not relevant to the job at hand. A analog or digital colorimeter cannot ignore those defects. A spectrocolorimeter is a tool for the color matcher but it is not a hammer. It needs to be treated like the precision instrument it is. A mold-maker would never use his or her tools in a dirty... 

There is at present no generally accepted criterion for the accuracy of instruments, although tolerances for the calibration of volumetric glassware and thermometers have been published. Manufacturers claims of accuracy within 1% are diflScult to assess without knowledge of the samples tested or method used to obtain the value. It should be possible for manufacturers to specify the accuracy of calibration of many instruments. As a general rule, it seems desirable that any inaccuracy in an instrument should not contribute significantly to the total inaccuracy of the result. 

In commercial systems the accuracy of instruments such as fuel and air flow meters is frequently poor due to the type of meter, placement of the meter, initial setup (mol.wt., temperature range, pressure, etc.), or even as the result of erroneous scaling factors. Attempts made to try to determine what instrument or if an instrument is in error by making mass and energy balance calculations are also usually not productive since all of the necessary parameters are usually not available to close a mass and energy balance around the system. For example, waste gas flow volumes are often not measured since they may contain aerosols, solids, or tar-like constituents that can plug or coat flow mefers causing them to fail or to have poor accuracy. 

Eisenhart, C., Realistic Evaluation of the Precision and Accuracy of Instrument Calibration Systems, /. Res. Natl. Bur. Stand. (U.S.) 67C,... 

Hall, L.S. (1931). Improving the accuracy of instruments. Civil Engineering 1(12) 1098-1101. Hall,L.S. (1940). Silting of reservoirs. Journal American Water Works Association25-42. Hall,L.S. (1943). Open channel flow at high velocities. Trans. ASCE 108 1394-1434 1494-1513. Hall, L.S. (1947). The influence of air entrainment on flow in steep chutes. Proc. Hydraulics Conference. 298-314, J.W. Howe, J.S. McNown, eds. State University of Iowa Iowa. 

Were there any differences between the accuracy of instrumentation readings in the control room compared with the requirements in the procedure ... 

Due to the limited accuracy of instruments for flow rates measurement, the accuracy of experimental data is limited. An error of about 10 % in the evaluation of pilot plant iso-amylenes conversion has to be considered. 

The accuracy of instruments needs to be defined with care. Additionally, because of the demands of Quality Assurance, manufacturers will probably need to justify the results produced by their instruments. It might be that certified data on the linearity and res nse of detectors etc will need to be provided with each instrument together with a standard test method (such as the reticle described above) and calibration check procedures. 

Secondly there is the safety in operation of the chosen process, the accuracy of instrumentation, the degree of human intervention, quality of water supply, etcetera. In general this adds up to asking how good is the general plant operation. 

During testing a depth resolution of 50-80 micron and a lateral resolution of 20-40 micron was achieved. The spatial resolution was limited not mainly hy source or camera properties, but by the accuracy of compensation of the instrumental errors in the object movements and misalignments. According to this results a mote precision object rotation system and mote stable specimen holding can do further improvements in the space resolution of microlaminography. 

Accuracy The accuracy of a controlled-current coulometric method of analysis is determined by the current efficiency, the accuracy with which current and time can be measured, and the accuracy of the end point. With modern instrumentation the maximum measurement error for current is about +0.01%, and that for time is approximately +0.1%. The maximum end point error for a coulometric titration is at least as good as that for conventional titrations and is often better when using small quantities of reagents. Taken together, these measurement errors suggest that accuracies of 0.1-0.3% are feasible. The limiting factor in many analyses, therefore, is current efficiency. Fortunately current efficiencies of greater than 99.5% are obtained routinely and often exceed 99.9%. 

Thus the m/z value for such ions is [M -i- n-l]/n, if the mass of hydrogen is taken to be one. As a particular example, suppose M = 10,000. Under straightforward Cl conditions, [M + H]+ ions will give an m/z value of 10,001/1 = 10,001, a mass that is difficult to measure with any accuracy. In electrospray, the sample substance can be associated with, for example, 20 hydrogens. Now the ion has a mass-to-change ratio of [M -t 20-H] and therefore m/z = 10,020/20 = 501. This mass is easy to measure accurately with a wide range of instruments. 

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. 

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 apphedat 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 cahbration 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 the traditional electronic measurement devices. 

As normally used in the process industries, the sensitivity and percentage of span accuracy of these thermometers are generally the equal of those of other temperature-measuring instruments. Sensitivity and absolute accuracy are not the equal of those of short-span electrical instruments used in connection with resistance-thermometer bulbs. Also, the maximum temperature is somewhat limited. 

The flow capacity of the transducer can be increased bv adding a booster relav like the one shown in Fig, 8-7.3/ , The flow capacity of the booster relav is nominally fiftv to one hundred times that of the nozzle amplifier shown in Fig, 8-7.3 3 and makes the combined trans-diicer/booster suitably responsive to operate pneumatic actuators. This type of transducer is stable into all sizes of load volumes and produces measured accuracy (see Instrument Society of America [ISA]-S5l, 1-1979, Process Instrumentation Terminology for the definition of measured accuracy) of 0,5 percent to 1,0 percent of span. 

The assessor should also find out whether an effective testing program is in place to help ensure the serviceability of process measurement equipment. The successful toller should have an established calibration program to address the accuracy of critical measurement equipment. Safety critical process parameters should be monitored and critical process equipment should automatically interlock when monitoring instrumentation detects safety critical deviations. Interlocks should either facilitate a remedy to the critical deviation or bring the process to the zero energy state. These instruments and interlocking devices should be routinely tested to ensure operational reliability. 

The external audit results are used to determine the accuracy of the measurements. Accuracy is calculated from percentage differences, dj, for the audit concentrations and the instrument response. 

One of the important advantages of ICPMS in problem solving is the ability to obtain a semiquantitative analysis of most elements in the periodic table in a few minutes. In addition, sub-ppb detection limits may be achieved using only a small amount of sample. This is possible because the response curve of the mass spectrometer over the relatively small mass range required for elemental analysis may be determined easily under a given set of matrix and instrument conditions. This curve can be used in conjunction with an internal or external standard to quantily within the sample. A recent study has found accuracies of 5—20% for this type of analysis. The shape of the response curve is affected by several factors. These include matrix (particularly organic components), voltages within the ion optics, and the temperature of the interffice. 

P. Mertens, S. DeGendt, K. Kenis Calibration accuracy of different Atomika TXRF instruments, IMEC, Leuven 1996. 

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