Measuring techniques

An impedance spectrum is the result of a sweep about the selected frequency band. Narrow band measuring (filters or lock-in amplifiers) is able to improve the accuracy and sensitivity but it is time consuming at low frequencies. For precise measurements (suppressing of transients), a signal averaging at least five periods is recommended. 

For electrochemical impedance spectroscopy a number of excellent commercial measuring systems exist. In some cases it is favourable to use precision impedance analysers designed specially for accurate impedance measurements of electronic components and materials in a broad frequency scale. 

The input signal can be a single frequency, a discrete number of frequencies (e.g. computer generated) or a theoretical unlimited spectrum of frequencies (white noise). Primarily, it seems of advantage to apply a large number of frequencies and measure the response simultaneously. However, the electrochemical system generates noise and, because of the linearity condition, the signal amplitudes are very 

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The measurement techniques most frequently used are derived from Raoult s and Van t Hoff s laws applied to cryometry, ebulliometry, osmometry, etc. They are not very accurate with errors on the order of ten per cent. Consequently, the molecular weight is often replaced by correlated properties. The mean average temperature or viscosity can thus replace molecular weight in methods derived from ndM. 

Speckle shearing interferometry, or shearography, is a full field optical inspection teclmique that may be used for the nondestructive detection of surface and, sometimes, subsurface defects. Whilst being more sensitive in the detection of surface defects, it may also be considered for pipe inspection and the monitoring of internal conoslon. In contrast, laser ultrasound and other forms of ultrasound, are point by point measurement techniques, so that scanning facilities and significant data processing is required before information on local defects is extracted from any examination of extensive areas [1 - 3]. 

Table 1. Three different ultrasonic measuring techniques for thickness measurement.

Development of a photothermal measuring technique for the determination of hardness profile in steel ... 

Craig V S J 1997 An historical review of surface force measurement techniques Colloids Surf. A Physicochem. Eng. Aspects 129-30 75... 

The definition of polymer thermal stabiUty is not simple owing to the number of measurement techniques, desired properties, and factors that affect each (time, heating rate, atmosphere, etc). The easiest evaluation of thermal stabiUty is by the temperature at which a certain weight loss occurs as observed by thermogravimetric analysis (tga). Early work assigned a 7% loss as the point of stabiUty more recentiy a 10% value or the extrapolated break in the tga curve has been used. A more reaUstic view is to compare weight loss vs time at constant temperature, and better yet is to evaluate property retention time at temperature one set of criteria has been 177°C for 30,000 h, or 240°C for 1000 h, or 538°C for 1 h, or 816°C for 5 min (1). 

Detailed reaction dynamics not only require that reagents be simple but also that these remain isolated from random external perturbations. Theory can accommodate that condition easily. Experiments have used one of three strategies. (/) Molecules ia a gas at low pressure can be taken to be isolated for the short time between coUisions. Unimolecular reactions such as photodissociation or isomerization iaduced by photon absorption can sometimes be studied between coUisions. (2) Molecular beams can be produced so that motion is not random. Molecules have a nonzero velocity ia one direction and almost zero velocity ia perpendicular directions. Not only does this reduce coUisions, it also aUows bimolecular iateractions to be studied ia intersecting beams and iacreases the detail with which unimolecular processes that can be studied, because beams facUitate dozens of refined measurement techniques. (J) Means have been found to trap molecules, isolate them, and keep them motionless at a predetermined position ia space (11). Thus far, effort has been directed toward just manipulating the molecules, but the future is bright for exploiting the isolated molecules for kinetic and dynamic studies. 

A final example of laser measurement techniques is measurement of the velocity of fluid flow (79) (see Flow measurement). If a laser beam is directed into a transparent fluid, some of the light may be scattered with a frequency shift zy given by... 

Metrologia International Committee of Weights and Measures (CIPM) Pavilion de Breteuil Parc de St. Cloud, Prance Includes articles on scientific metrology worldwide, improvements in measuring techniques and standards, definitions of units, and the activities of various bodies created by the International Metric Convention. 

It is common to employ microwave power monitoring by means of a dual-directional coupler in the waveguide transmission system between the power tube and the useful load. Part of the coupled signals may be used for examination with spectmm analy2ers, frequency meters, and other microwave instmmentation for special purposes. Generally, this is not necessary in a practical appHcation. Many microwave measurement techniques have been described (59,60). AvailabiHty of components, plumbing, and instmmentation is weU described in trade journals. 

The value of the glass-transition temperature, T, is dependent on the stereoregularity of the polymer, its molecular weight, and the measurement techniques used. Transition temperatures from —13 to 0°C ate reported for isotactic polypropylene, and —18 to 5°C for atactic (39,40). 

Some particle size measuring techniques ate more particle shape sensitive than others. Data obtained by different methods can be significantly different, and whenever a particle size is reported, the measuring technique and conditions should always be mentioned. Even using the same equipment, the extremes of the distributions (low and high 10%) are usually not readily reproducible. 

Statistical Control. Statistical quahty control (SQC) is the apphcation of statistical techniques to analytical data. Statistical process control (SPC) is the real-time apphcation of statistics to process or equipment performance. Apphed to QC lab instmmentation or methods, SPC can demonstrate the stabihty and precision of the measurement technique. The SQC of lot data can be used to show the stabihty of the production process. Without such evidence of statistical control, the quahty of the lab data is unknown and can result in production challenging adverse test results. Also, without control, measurement bias cannot be determined and the results derived from different labs cannot be compared (27). 

Deformation is the relative displacement of points of a body. It can be divided into two types flow and elasticity. Flow is irreversible deformation when the stress is removed, the material does not revert to its original form. This means that work is converted to heat. Elasticity is reversible deformation the deformed body recovers its original shape, and the appHed work is largely recoverable. Viscoelastic materials show both flow and elasticity. A good example is SiEy Putty, which bounces like a mbber ball when dropped, but slowly flows when allowed to stand. Viscoelastic materials provide special challenges in terms of modeling behavior and devising measurement techniques. 

M. Roland, Uiscoelasticity—Material Behavior and Measurement Technique, at the CL Meeting of the 147th Rubber Division, Philadelphia, Pa., May 2—5, 1995, American Chemical Society, Washington, D.C. 

Existing droplet measurement techniques may be classified into three broad categories (/) optical nonimaging techniques (2) imaging techniques and (3) nonoptical methods. A comprehensive review of these techniques are available (35—39). 

V. G. McDoneU and G. S. Samuelsen, Eiquid Particle Si Measurement Techniques, Vol. 2, ASTM STP1083, American Society for Testing and Materials, Philadelphia, Pa., 1990, pp. 170—189. 

Procedures for determining the ash content and moisture level, solution preparation, and viscosity measurement techniques are given in the manufacturer s hterature (50). 

Measurement technique Value, MPa Reference Value, MPa " Reference... 

In continuous dyeing there are many variables and the rapidity of the dyeing process requites many adjustments during the period in which several thousand meters of textile are dyed. Instmmental science has continued to advance rapidly so that continuous ranges are available which are entirely computer-controUed except for the makeup of the dye mix. These units feature computer control and closed-cincuit television and continuous color measurement techniques. 

More recent data is often preferred to older data, since end effects are better understood, the new correlation often builds on earlier data and analysis, and better measurement techniques are often available. 

The gravimetric method is accepted as the most accurate humidity-measuring technique. In this method a known quantity of gas is passed over a moisture-absorbing chemical such as phosphorus pent-oxide, and the increase in weight is determined. 

It turns out that in low-viscosity blending the acdual result does depend upon the measuring technique used to measure blend time. Two common techniques, wliich do not exhaust the possibilities in reported studies, are to use an acid-base indicator and inject an acid or base into the system that will result in a color change. One can also put a dye into the tank and measure the time for color to arrive at uniformity. Another system is to put in a conductivity probe and injecl a salt or other electrolyte into the system. With any given impeller type at constant power, the circulation time will increase with the D/T ratio of the impeller. Figure 18-18 shows that both circulation time and blend time decrease as D/T increases. The same is true for impeller speed. As impeller speed is increased with any impeller, blend time and circulation time are decreased (Fig. 18-19). 

The X-ray disc centrifuge is a centrifugal version of the gravitational instruments and extends the measuring technique well into the sub- Im-size range. 

Nellis, W.J., Properties of Condensed Matter at Ultrahigh Dynamic Pressures, in High Pressure Measurement Techniques (edited by Peggs, G.N.), Applied Science, London, 1983, pp. 69-89. 

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