Proof pressure method

The proof pressure method is permitted where DOT or TC regulations do not require the determination of total and permanent volumetric expansion of the cylinder. It consists of visually examining the cylinder under specified test pressure for leaks or deformation of localized thinned areas. 

Fig. 10-13. Typical schematic diagram of proof pressure method test apparatus, from CGA C-1-1996.

Several points need to be considered when applying this pressure method of sample collection. All of the extraction system parts must be explosion proof and the distance from the ceramic cup to the sample bottle must be as short as possible. The sample collection bottle should be at the same level as the ceramic cup. If it is higher, additional vacuum will need to be applied to move the sample water into the sample bottle. Sample storage, once the water is collected, is determined by the analyte of interest. 

Explosion-proof containers can be enhanced with pressurization or purge and pressurization methods. 

Sulphur and Iodine.—Various methods have been described for the preparation of compounds of iodine and sulphur,3 but to-day the products are regarded merely as mixtures of the elements.1 In solution in carbon disulphide, iodine and sulphur exist side by side permanently uncombined. The freezing-point curves for mixtures of the two elements,5 as also the vapour pressure curve of the fusion products,6 likewise give no indication whatever of chemical combination, although sulphur forms a solid solution in iodine. A further proof of the absence of combination is the fact that when dissolved in iodine, sulphur has a normal molecular weight, determined eryoseopically, only a little below that required for S8.7 The present condition of our knowledge, therefore, may be summed up in the statement that no definite compound of sulphur and iodine has yet been obtained.8... 

The phase-behaviour depends on the composition, temperature and pressure. The composition of the reaction mixture can, in some cases, become rather complex and therefore difficult to measure with traditional methods. One simpler method of determining singlephase conditions is to observe the reaction rate. The difference in reaction rate between two-phase and single-phase conditions is dramatic. The reaction times go from hours to seconds [28 - 30], For a given system, the reaction rate is proof of the presence or absence of a liquid phase. When comparing different systems, the reaction rate is a strong indication of the presence or absence of a liquid phase. 

A more formal proof of this simple method of determining the relative amounts of the phases from the concentrations of the phases and the system is treated in Problem 7. Tie lines are not provided on phase diagrams, but are drawn by the user of the diagram at the pressure of interest. 

The pressure—proof capability of a sealed cell has limitations, and the pressure generated in a sealed cell depends on the amount of gas produced by the substance being tested. The amount of sample for typical self—reactive substances is thus confined to 1 to 3 mg. This is an advantage of the SC-DSC method in that safety is ensured and that a trace amount of sample suffices. In contrast, the difficulty of accurately weighing a sample and the occurrence of relative errors are problems. Moreover, no correct exothermic peaks may be obtained if the amount of sample is too small. 

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