Measurement Techniques under High Pressure

In the research field, exploiting the parameter pressure is synonymous with recurring scientific breakthroughs, such as the recent discovery of metallic hydrogen, as well as with the frequent removal of technological barriers to the development of new materials.

In the industrial sector, high pressures (typically in excess of 50 MPa) are also synonymous with discoveries, even if until recently their replacement by other processes was often quickly sought. For example, the growth in the use of high pressure by the chemical industry in the 20th century led in the 1930s, in parallel with the invention of nylon, the first totally synthetic fiber, to the discovery of polyethylene, the best-known plastic now produced at 100 Mt/year. In the following decade, the pressures required for the large-scale production of low-density polyethylene necessitated a number of improvements, such as those aimed at increasing the fatigue strength of the thick-walled cylindrical vessels used, but the search for milder temperature and pressure conditions for ethylene polymerization then led, with the help of transition metals, to the development of catalytic systems in the 1950s. However, high-pressure applications continue to develop, such as the drastic reduction in the number of particles emitted by high-pressure petrol injection, the pascalisation A cold pasteurization method that preserves the nutritional and taste qualities of food, or high-pressure pyrolysis for recycling plastics.

Common to all high-pressure applications, the prerequisite for their implementation is the use of suitable equipment, or even its design when necessary, as mentioned in the first part of this article because of the constraints to be aware of depending on the level of pressure sought, and the ability to carry out the appropriate measurements. To this end, the second part of this article presents the means of acquiring data under high pressure. Specific methods are described for measuring state variables (e.g. temperature) and transport properties (e.g. thermal conductivity), Raman and infrared spectrometry, and microstructural characterization by X-ray and neutron diffraction.

This article details the recommendations necessary for carrying out measurements at high and very high pressures. It does not, however, cover in any depth the points covered exhaustively or briefly in other articles in Techniques de l'Ingénieur, to which the reader is invited to refer, i.e.: high pressure measurement; pressure-generating devices (compressors, separators, multipliers and cylinder systems); measurement techniques specific to the field of dynamic pressure.