The variable-volume hydrogenator

An important advantage of the variable-volume technique over the Sieverts technique becomes apparent by re-casting Eq. (7.6) for the variable-volume scenario  

12 Minimal variable-volume hydrogenator, constructed from a basic Sieverts apparatus by including a variable component in V ef, such as a motor-driven piston, with displacement volume ax ax- 

1 for amplifying the effect of the sample volume on the calculation. As the number of isochoral steps is greatly decreased to possibly one per isotherm, the cumulative errors intrinsic to the Sieverts technique are also largely eliminated. 

The basis of the calibration of the volumes in the hydrogenator is as discussed in Section 7.5.1. The availability of the variable reference volume greatly facilitates calibration of any other volume in the system, particularly if it is under computer control. Vref is first calibrated at enough piston positions x to accurately measure dV Jdx. The unknown volume to be calibrated, Vu. which can include the constant part of Vref, is connected and an initial pressure p is established. The piston is then driven in and the pressure and all relevant temperatures are recorded against x. The amount of gas in the system is constant, so assuming for clarity that all of Vref is at the same temperature and the compression sequence starts at x = 0  

In summary, the variable-volume technique promises to confer some valuable advantages  

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