ABSTRACT
The characteristic of these types of cells is that they use a liquid or gaseous pressure transmitting medium which is injected by a compressor through a high pressure hose, usually a capillary. The cell is hence permanently (i.e. at least for the length of the measurement) connected to the compressor. The pressure can be changed in situ, and measured easily by standard manometers attached to the compressor-side of the equipment. This is hence the ideal tool when fine-tuning of pressure is required. The pressure conditions are per se hydrostatic, guaranteed by the choice of the adequate pressure transmitting medium, since otherwise the equipment does not work properly (blocked capillary). The fluid of choice is obviously helium since its freezing pressure (Appendix E) is higher than any other gas/fluid. For low-temperature measurements (below typically 100 K) this choice becomes unavoidable. However helium is able to penetrate into many compounds, the best known case is ice to form clathrates. The compatibility of the pressurizing fluids used in liquid and gas cells is hence an important consideration which has to be clarified well before the start of an experiment. Helium has also the well known inconvenience of being difficult to seal, in other words its viscosity is so low that even small leaks can become a serious problem, in particular at low temperatures. The use of more standard fluids would hence be desirable but is in practice rarely possible since (a) it has to be compatible with neutron measurements (no hydrogen, low neutrons absorption), and (b), it has to have a high freezing pressure, even at low temperatures. In practice, only other rare gases can be considered, such as argon, neon, and sometimes nitrogen.
