Error In Thickness Measurement Research Articles

Evaluation of several geometric models for estimation of left ventricular circumferential wall stress

Phasic left ventricular wall force and wall thickness were monitored with appropriate transducers to provide a direct measurement of circumferential wall stress in open-chest dogs. Left ventricular pressure and measurements of chamber geometry were used to estimate the wall stress using several geometric models. During the initial control period, peak and end-ejection measured wall stresses were 207 ± 19 and 104 ± 13 g/cm 2 , respectively. The best estimates of these values were 198 ± 18 and 117 ± 11 g/cm 2 obtained from a modified thin-wall ellipse formula in which the midwall rather than the endocardial radius was used. Wide variations in hemodynamic conditions were produced with intravenous infusions of nitroglycerin, phenylephrine, and isoproterenol. Comparison of directly measured and estimated values during all control periods and during the response to these interventions showed that both the modified thin-wall ellipse and a thick-wall ellipse were generally accurate predictors of the measured wall stress. All other models tended to underestimate measured stress. The sensitivity of the estimated wall stress computed by the models to geometric measurement errors was also evaluated. A thick-wall sphere was the most sensitive to both circumferential length and wall thickness measurement errors, and a thick-wall ellipse was the least sensitive. All models examined were relatively insensitive to base-to-apex length measurement errors.

Systematic and Random Errors in Dual Gamma Energy Soil Bulk Density and Water Content Measurements

AbstractSoil bulk density and water content may be obtained concurrently through measurement of the attenuation of gamma photons from two different gamma ray sources and simultaneous solution of the resulting attenuation equations. Errors in the soil bulk density and water content measurements result from random emission from the sources, random errors in attenuation coefficients and soil column thickness measurements, presence of a small, higher energy peak in the 241Am spectrum, and counting equipment dead time. Using gamma photons from 241Am at 0.060 Mev and from 137Cs at 0.662 Mev, attenuated in 10‐cm soil columns, the standard deviation in both water content and bulk density measurements is primarily due to random emission and is about 0.007 g/cm3 for 106 counts measured in air and about 0.005 g/cm3 for 2.5 × 106 counts. However, as larger counts are used the precision of measurement of column thickness and soil and water attenuation coefficients becomes limiting. If, by obtaining large counts, the variance due to random emission is reduced to the same magnitude as that associated with measurement of column thickness or attenuation coefficients, precision limit would be about 0.0035 g/cm3 at midrange values of water content and bulk density. At high count rates instrument dead time corrections are made to get accurate counts. Gamma rays scattered down from the small 0.103 Mev peak in the 241Am spectrum result in attenuation dependent attenuation coefficients for which appropriate corrections must be made. For narrow collimating slits the spatial resolution is only a little greater than slit thickness. Collimater and shielding requirements are shown.

Isotope Gauges: An Analysis of Some Errors in Hot Metal Thickness Measurement

Isotope gauges are particularly suitable as thickness measuring instruments for hot metal above 5 cm. thick and their application for this purpose is continuing to increase. The paper discusses some of the factors affecting the accuracy of these gauges and, in particular attempts a theoretical evaluation of the errors due to steel composition, temperature distribution and statistical variation. These errors are evaluated for three carbon steels. Errors in thickness measurement due to the different compositions of these steels are evaluated for the two isotope sources most commonly used for gauging purposes namely, Americium 241 and Caesium 137. These isotope sources are representative of those used for the entire practical range of thickness measurement. The paper considers the effect of temperature distribution across the depth of metal on the mass attenuating the γ-rays and the thickness measurements so evaluated are compared with those determined assuming a homogenous stock temperature. The principle of constant absorption is described and also some of its limitations where a wide range of measurements is required. Finally an artifice is discussed whereby some of the limitations may be overcome.

Dynamic and Statistical Errors in Thickness Measurement with Radiation Thickness Gauge

Dynamic and Statistical Errors in Thickness Measurement with Radiation Thickness Gauge

The measurement of thin films by interferometry

The measurement of thin films is becoming of increasing technical and scientific importance. In view of the high accuracy of comparison of path-differences attainable in polarimetry, experiments have been made to achieve some of this accuracy in thin film measurements. It is shown experimentally that setting accuracies are obtainable corresponding to an error of thickness measurement of ±1.