Sample-container Buoyancy

Most adsorption data were collected by volumetric method until microbalance of high sensitivity appeared few years ago. It can hardly say which method is superior to the other, and both methods need the value of the skeleton volume of sample adsorbent. This volume has to be subtracted from the whole volume of the sample container to obtain the volume of void space, which is used for the calculation of the amount adsorbed. The skeleton volume of sample adsorbent was directly used in the calculation of buoyancy correction in gravimetric method. This volume was usually determined by helium assuming the amount of helium adsorbed was negligible. If, however, helium adsorption cannot be omitted, error would yield in the skeleton volume and, finally, in the calculated amount adsorbed. However, the effect of helium adsorption would be much less for volumetric method if the skeleton volume is considerably less than the volume of void space, but the volume of void space cannot affect buoyancy correction. In this respect, helium adsorption would result in less consequence on volumetric method especially when the skeleton volume was determined at room temperature and pressures less than IS MPa. The skeleton volume (or density) was taken for a parameter in modeling process in some gravimetric measurements. However, the true value of skeleton volume (or density) can hardly be more reliable basing on a fitted parameter than on a measured value. Therefore, one method of measurement cannot expel the other up to now, and the consequence of helium adsorption in the measured amount adsorbed should be estimated appropriately. 

Errors in thermogravimetry can lead to inaccuracy in temperature and weight data. Proper placement of the TGA instrument in the laboratory, away from sources of vibration and heat, is essential to minimize fluctuations in the balance mechanism. Older instruments suffered from an apparent gain in weight of a sample container when heated, known as the buoyancy effect. This effect, due to the decreased buoyancy of the atmosphere. 

Although more labor-intensive and less efficient, information on the densification kinetics and densification can also be obtained from density measurements on different individual samples as a function of time for otherwise identical sintering conditions. Bulk density measurements on <92% dense sintered samples containing open porosity can be determined from the measured mass and dimensions of the compact, while Archimedes method works well for closed pore, >92% dense bodies. The density of closed pore samples can also be determined by pycnome-try (e.g., helium pycnometry),, 37 mercury porosimetry, and by the sink-float method (i.e., whereby the buoyancy of the sample is assessed and compared in different density liquids). Density can also be estimated from micrographs using quantitative stereology. 

Sources of error in TG There are number of sources of error in TG and they can lead to inaccuracies in the recorded temperature and weight data. Some of these errors can be corrected and others are interrelated and can not be assessed separately. The list of main sources of errors are (i) buoyancy effect of sample container (ii) random fluctuations of balance mechanism (iii) electrostatic effects on balance mechanism (iv) condensation on balance suspension (v) measurement of weight by balance (vi) convection effects from furnace (vii) turbulence effects from gas flow (viii) induction effects from furnace (ix) measurement of temperature by thermocouple and (x) reaction between sample and container. 

Buoyancy effect This refers to apparent gain in weight that can occur when an empty and thermally inert crucible is heated. The effect is due to complex interaction between three factors (i) the decreased buoyancy of the atmosphere around the sample container at higher temperatures, (ii) the increased convection effect and (iii) the possible effect of heat from furnace on the balance itself In most modern thermobalances, attention to design factors has made the buoyancy effect negligible. However, if necessary a blank run with an empty crucible can be carried out over the appropriate temperature range. The resultant record can be used as a correction curve for subsequent experiments. 

With this relationship for all samples was calculated from ninh This M is used for evaluating the reaction data. The ultracen rifuge (u.c measurements were carried out in a Spinco model E analytical ultracentrifuge, with 0.4% solutions in 90% formic acid containing 2.3 M KCl. By means of the sedimenta- ion diffusion equilibrium method of Scholte (13) we determine M, M and M. The buoyancy factor (1- vd = -0.086) necessary for tSe calculation of these molecular weights from ultracentrifugation data was measured by means of a PEER DMA/50 digital density meter. 

Allowing and to be the volume and absolute temperature of the counterweight, and to be the same for the sample and its container, the buoyancy correction becomes... 

For the determination of the enthalpies of dissolution of solids in HF, a 5. 5 ml aliquot of 25% HF was placed in the sample cell and the acid was covered with a thin layer ( h) of paraffin oil ( Pro-labo. Rectapur) which is inert to HF. The Kel-F capsule (g) containing the solid was placed on the oil layer. Sufficient buoyancy was ensured by the latter any attack of the sample powder by HF vapor or by the solution was thus avoided. The reference cell contained the same volume of HF than the sample cell. 

This practice covers the procedure for obtaining representative samples of hollow microspheres of the type used for syntactic foam buoyancy materials. Hie procedure consists of procuring representative samples by the use of "spike" or "thief samplers which can be inserted all the way to the bottom of the container, thus sampling the entire vertical distance. 

To explain the observed peculiarities of the vertical structure of the ionic content, we recall that, as we substantiated in our previous publications, the water in the bottom part of the western trench usually contains a significant admixture of the water originating from the shallow eastern basin [2, 9,10]. Saltier, denser, and chemically altered to a larger extent, this eastern water penetrates into the western basin under favorable wind conditions and then sinks into the near-bottom layer, while gradually mixing with the ambient waters. In fact, this mechanism is likely to be the principal controller of the western basin stratification. As shown in [2], typically, 10-20% of the water mass in the western basin is associated with recent intrusions from the eastern basin. Because the advected eastern water transports negative buoyancy, its core must be located in the bottom layer and little or none of it is manifested at the surface. The deeper a sample is taken, the larger is the content of the eastern water in it. 

Actually, in most cases, a correction is not necessary because the error resulting from the buoyancy will cancel out in percent composition calculations. The same error will occur in the numerator (as the concentration of a standard solution or weight of a gravimetric precipitate) and in the denominator (as the weight of the sample). Of course, all weighings must be made with the materials in the same type of container (same density) to keep the error constant. 

The Sensys TG-DSC is based on the Calvet type DSC (Fig. 2.36) used in the vertical position [30]. On top of the DSC, is adjusted a symmetrical balance corresponding to the principle described on Fig. 2.29. The crucibles containing the sample and the inert material are hung on each side of the balance and introduced in the calorimetric zone of the DSC without touching the walls. In such a situation, the crucibles are fully surrounded by the fluxmeters, providing an accurate DSC determination. In the same time, the symmetrical balance allows a compensation of the buoyancy effect resulting on a very high sensitive TG determination. 

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