Variable volume piezometers

A variable volume piezometer for fluid density measurements at high-temperature high-pressure was used by Franck s group in Karlsruhe for more than two decades. The original high-pressure cell design (Lentz, 1969) is shown in Figure 2.5 and it allows pVT measurements up to 500 °C and 300 MPa. 

For pVTx measurements in highly concentrated electrolyte solutions, Franck s group used a variable volume piezometer designed by Hilbert (1979) and shown schematically in Figure 2.6. The sample is filled in a pure nickel bellows to prevent corrosion and to permit elastic volume changes. This cell is located inside a stainless-steel autoclave immersed in an internally heated, argon-filled vessel. 

Figure 2.6 Metal bellows variable volume piezometer (Hilbert, R. (1979). Ph.D. Dissertation, Universitat Fridericiana Karlsruhe).

In the 1960s, in an Amsterdam laboratory under the supervision of A. Michels, pvT behavior of Freons-12, -13, and -22 was investigated in the interval T = 273-423 K at pressures up to 40 MPa. The study was carried out on a well-known experimental apparatus which allowed the use of the pressure-relieved variable-volume piezometer. The samples contained no less than 99.95% pure freon. The error in experimental data of pv values was usually evaluated to be 0.05%. The experimental results for Freons-12 and -13 obtained at this laboratory were published in 1966 and used to construct the tables of thermodynamic properties in Ref. [0.28]. However, the results of direct measurements for Freon-22 were not published and were known from the computational work of Martin [3.50]. 

The method of a piezometer of variable volume in glass cells has been used to measure the density of superheated light and heavy water. Experimental data have been obtained in the range of temperatures (0.7-0.95)7 and pressures from the saturation line to those close to the boundary of spontaneous boiling-up. 

A dilatometer can also be considered as a piezometer of variable volume, where the change in volume can be achieved using pistons. Older devices often used mercury as a liquid piston with the consequent environmental risk and moderate accuracy ( 0.5-1%) of density measurements (Ravich and Borovaya, 1971a-b Urusova, 1975). 

For the experimental investigation of compressibility of superheated and saturated vapors, the method of the constant-volume nonballasted piezometer was employed. The fundamental variables (pressure, temperature, and mass of the substance) were measured with instruments of the same class as those used in the work of MEI. The distinguishing feature of the piezometric rig was the use of a membrane zero indicator of pressure of the electrocontact type. The sensitivity of this instrument reached 1 mm water column, but the zero point drifted by 10 mm water column. The authors in Refs. [2.19, 2.21] evaluated the scatter of experimental points on the saturation curve as 0.4% for pressure, and the repeatability of presssure during the forward and reverse tracing along the constant-volume lines was 0.15%. 

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