Sample diameter

Gas velocity is measured over an aperture in the heated zone of the sampling train, at a temperature of 110 °C, to remove the moisture by heating. Determine the gas velocity at the sampling nozzle if the measured velocity is 28 m s for the sampling diameter used. The water concentration determined from the condensate is 75 g m" (n). 

Solution The gas volume under the same pressure is greater by 383/343 at the higher temperature. When the sampling diameter remains the same, the gas velocity at the higher temperature is greater by the same factor. Water is assumed to be in vapor form, behaving as a gas, as the concentration is less than that in saturated air. 

This information together with l(o,v) which represents the number of particles in the detector cell at retention volume v, leads to the evaluation of overall sample diameter averages. 

For Equation 6.7 to be valid it is assumed that all other experimental conditions are equal for the two samples. If this is not true, additional corrections may be required for differences in modulation amplitude (M), microwave power attenuation in IdBI OP), magnetic field scan width (W) (or equivalently, the step width in gauss between two subsequent digitization points), electronic gain (G), sample diameter Of), and absolute temperature (I) ... 

Fm = 7r/4PcrD i = (3-4.8V/4auD (5) where the minimum sample diameter, Dm, must satisfy the conditions required for propagating detonation in the particular thick explosive sample. The obvious condition is that... 

Sample Diameter, fx Slope Bound,a fi Bound, fi Bound, p... 

The appropriateness of the value of e0 can be checked by comparison with the counting efficiency values obtained for very thin samples - where f approaches 1.0 - in Experiment 2. Interpolation is needed for the K maximum beta-particle energy of 1.311 MeV, and the lesser value of e0 at the larger sample diameter in the present experiment must be taken into account. An estimate based on calculating the detector-sample geometry underestimates the counting efficiency because additional beta particles are back-scattered into the detector from the planchet and its support. 

The major factors that affect thermal analytical data are related to the shape, size, and weight of the sample. Samples are preferably in a powder or particle form, smaller than 0.1mm in diameter. If water or volatile solvents are present, the sample diameter should be smaller than 0.05 mm. However, an exceedingly small sample size may create a buoyancy effect and should be avoided. 

Increasing the sample diameter (D) increases the ratio of volumetric heat generation to surface heat loss The dependence of U on D, for different systems, is shown in Fig. 54. By increasing the diameter above a critical value, the combustion temperature approaches the adiabatic value, and U becomes constant. This behavior has been observed for the solid flame Ta-C system (curve 1), the Ti-C system (curve 2) where one reactant melts, and in Ni-Al (curve 3) where both reactants melt. The maximum measured temperature as a function of radial position in a cylindrical pellet with a diameter of 2 cm for the Mo+2Si system is presented in Fig. 55. The data show significant heat losses from the specimen. For this reason, incomplete combustion often occurs for samples with small diameter and may lead to an undesirable product phase composition (Martynenko and Borovinskaya, 1975 Bratchikov et al., 1980). 

Fig. 56. Final conversion as a function of initial sample diameter for the B-N2 system (Adapted from Mukasyan, 1986).

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