Isokinetic

If the coiiplin g parameter (the Bath relaxation constan t in IlyperChem), t, is loo Tight" (<0.1 ps), an isokinetic energy ensemble results rather than an isothermal (microcan on leal) ensemble. The trajectory is then neither canonical or microcan on-ical. You cannot calculate true time-dependent properties or ensemble averages for this trajectory. You can use small values of T for Ih CSC sim ii lalion s ... 

Source sampling of particulates requites isokinetic removal of a composite sample from the stack or vent effluent to determine representative emission rates. Samples are coUected either extractively or using an in-stack filter EPA Method 5 is representative of extractive sampling, EPA Method 17 of in-stack filtration. Other means of source sampling have been used, but they have been largely supplanted by EPA methods. Continuous in-stack monitors of opacity utilize attenuation of radiation across the effluent. Opacity measurements are affected by the particle size, shape, size distribution, refractive index, and the wavelength of the radiation (25,26). 

The stack sample is withdrawn isokinetically from the source, with... 

For sampling particulate matter, one is dealing with pollutants that have very different inertial and other characteristics from the carrying gas stream. It becomes important, therefore, to sample so that the same velocity is maintained in the probe tip as exists in the adjacent gas stream. Such sampling is called isokinetic. Isokinetic sampling, as well as anisokinetic sampling, is illustrated in Fig. 13-3. 

Modern transducers and microprocessors have been used successfully to automate particulate sampling trains in order to eliminate the operating curves and manual adjustments (7). The automated samplers adjust continuously to maintain isokinetic conditions. In addition, the microprocessor continuously calculates and displays both instantaneous sampling conditions and the total sample volume collected at any given moment. The use of the automated system with the microprocessor, therefore, eliminates both operator and calculation errors. 

Air-dilute with necessary blowers, flow measurement, and control systems Use isokinetic sampling, refrigerated sample transport, and careful handling to minimize physical or chemical changes... 

With particles, the contaminant concentration in the duct is determined by isokinetic sampling with subsequent laboratory analysis use of a calibrated direct reading instrument. If the concentration distribution in the duct is uneven, a complete survey of the concentration distribution with the corresponding duct velocities and cross-sectional area is required. National and ISO standards provide information on isokinetic sampling and velocity measurements. In the case of particles, the airborne emission differs from the total emission, for example in the case of granular particulate. The contaminant settling on surfaces depends on particle distribution, airflow rates, direction in the space, electrical properties of the surfaces and the material, and the amount of moisture or grease in the environment. 

Particulate emission is sampled at a temperature of 70 °C by an isokinetic sampling nozzle, i.e., the sampling velocity is the same as the gas velocity around the nozzle in the duct. 

Adjust the sample gas flow extracted (isokinetic sampling). 

Procedure A sharp-edged nozzle is positioned in the duct, facing into the moving gas stream, and a gas sample is extracted isokinetically (see later) for a measured period of time. To allow for nonuniformity of particulate concentration in the duct, samples are taken at preselected positions in the duct cross-section. The particulate concentration is calculated from the weighed particulate mass and the gas sample volume. Figure 13.39 shows the measuring arrangement. 

Isokinetic Sampling I he sample gas partial volume flow must be extracted isokinetically to avoid aerodynamic separation effects and to ensure correct particle size distribution. Isokinetics means that the velocity and direction of the sample gas partial flow at the sample nozzle are the same as at the main gas stream. ... 

For isokinetic sampling, the doer sampling-point gas velocity has to be measured, and the corresponding sample gas flow calculated and adjusted. " Normally, a Pitot static tube is used for the measurement of duct gas velocity. 

In the measurement of emission gas mass concentration at sources, a gas sample is extracted via an automatic isokinetic particulate-sampling instrument. The monitoring system generates a direct, real-time emission particulate mass... 

Anisokinetic Not isokinetic a sample collected at a velocity different from that of the airflow in the ventilation system. 

Isokinetic A process in which the velocity at the entrance to the sample probe in a gas stream is the same as the velocity at a given point in the duct or stack at a given time. 

Isokinetic sampling The sampling of a gas such that the motion of the gas entering the sampling device is identical to that of the gas being sampled. 

The second use of activation parameters is as criteria for mechanistic interpretation. In this application the activation parameters of a single reaction are, by themselves, of little use such quantities acquire meaning primarily by comparison with other values. Thus, the trend of activation parameters in a reaction series may be suggestive. For example, many linear correlations have been reported between AT/ and A5 within a reaction series such behavior is called an isokinetic relationship, and its significance is discussed in Chapter 7. In Section 5.3 we commented on the use of AS to determine the molecularity of a reaction. Carpenter has described examples of mechanistic deductions from activation parameters of organic reactions. 

The proportionality constant p has the dimension of absolute temperature, and it is called the isokinetic temperature. It has the significance that when T = P,... 

AG = 0 that is, all substituent (or medium) effects on the free energy change vanish at the isokinetic temperature. At this temperature the AH and TAS terms exactly offset each other, giving rise to the term compensation effect for isokinetic behavior. 

Most of the isokinetic relationships in the literature have been established from plots of AH against AS collections of these have been published and several authors have discussed the mechanistic implications.9 6. csap. 12 ... 

It was not until the 1970s that the statistics of the isokinetic relationship was satisfactorily worked out.Exner first took this approach Let k, and 2 be the rate constants for a member of a reaction series at temperatures T, and T2, with T2 > T, and let k° and k° be the corresponding values for the reference member of the series. Then Eqs. (7-76) and (7-77) are easily derived for the reaction series. 

Equation (7-76) shows that, in general, a plot of log 2 against log k, is not expected to be linear. Linearity in such a plot can be assured, however, if an isokinetic relationship, 8rAW = P8rA5, is followed by the system. By incorporating this relationship into Eqs. (7-76) and (7-77), we obtain... 

Thus, a linear plot of log 2 against log ki for a reaction series implies an isokinetic relationship for the series. The reason that this plot is a reliable test for such a relationship is that the errors in and 2 are independent (unlike the errors in A// and AS ). From the slope b of the straight line the isokinetic temperature p can be found ... 

Obviously for this method to work the ratio T1IT2 must be appreciably smaller than unity. Provided this condition is met, this method is a simple and reliable way to test for an isokinetic relationship or to detect deviations from such a relationship. Exner shows examples of systems plotted both as log 2 vs. log and as AH vs. A5, demonstrating the inadequacy of the latter plot. Exner has also developed a statistical analysis of the Petersen method this analysis yields p and an uncertainty estimate of p. Exner has applied his statistical methods to 100 reaction series, finding that 78 of them follow approximately valid isokinetic relationships. 

Thus, y is the slope of the plot of A// against AG at the harmonic mean temperature, and from y the isokinetic temperature P is calculated. Tomlinson has shown many examples of this type of analysis. 

Earlier analyses making use of AH vs. AS plots generated many p values in the experimentally accessible range, and at least some of these are probably artifacts resulting from the error correlation in this type of plot. Exner s treatment yields p values that may be positive or negative and that are often experimentally inaccessible. Some authors have associated isokinetic relationships and p values with specific chemical phenomena, particularly solvation effects and solvent structure, but skepticism seems justified in view of the treatments of Exner and Krug et al. At the present time an isokinetic relationship should not be claimed solely on the basis of a plot of AH vs. A5, but should be examined by the Exner or Krug methods. 

Suppose it is known that an isokinetic relationship holds for a reaction series. Then give the slope of a plot of log k against for the series. 

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