Vapor pressure isoteniscope method

ISO 3007-99. Petroleum products and crude petroleum - Determination of vapor pressure - Reid method. ASTM D 2879-96. Standi test method for vapor pressure-temperature relationship and initial decomposition temperature of liquids by isoteniscope. 

The gas saturation method obviously suffers from errors of a different nature than the isoteniscope method. Vapor lost to surfaces within the measurement system and low vapor-trapping efficiency may lead to significant errors for low vapor pressure compounds. Spencer et al. (15) found vapor capture efficiencies as low as 90 percent, while Wasik, et al. (16) arrived at a 95-percent confidence level of +9 percent. Thomas and Sieber (17) reportedly obtained an error of +1 to 3 percent using a column-gas saturator. Spencer noted, however, that for some very low vapor pressure compounds, up to an order of magnitude variation has been reported. 

A simple and frequently used technique to determine the vaporization enthalpies of volatile liquids is the isoteniscope method [96,97]. The isoteniscope was devised to trap a small amount of the liquid and its vapour in a part of a vessel that is totally immersed in a thermostatted bath, separated from the measuring device by an open-end manometer which is also immersed in the bath. The liquid in this inner manometer is usually the liquid being studied. The pressure in the outside space is measured by conventional methods. When the inner manometer shows that the outside pressure is the same as that inside (the vapour pressure of the liquid at the bath temperature) the outside pressure is registered. This method has been widely employed to obtain vapour pressures of mixtures [98],... 

The vapor pressure is the pressure of the vapor of a substance in equilibrium with the pure liquid at a given temperature. Two procedures are used an isoteniscope procedure (standard) for measuring vapor pressures from 1x10" to 100 kPa and a gas-saturation pressure for measuring vapor pressures from IxlO"" to 1 In the isoteniscope method, a... 

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