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Sample Vessel for Low Vapor Pressure Measurements

IP.com Disclosure Number: IPCOM000019438D
Publication Date: 2003-Sep-12
Document File: 8 page(s) / 113K

Publishing Venue

The IP.com Prior Art Database

Abstract

It is of great importance to know the saturated vapor pressure at file operating temperature when designing vapor corrosion inhibitors (VCI), fluid dynamic bearing motor oils and Zdol based reservoir lubrication. Vapor pressure is required to calculate the minimum amount of VCI to be placed in the file to last the lifetime of the file. The concept of the lubricant reservoir system uses a volatile lubricant that is positioned in the file's enclosure. The vapor pressure is employed to control the lubricant's thickness and to model the vapor phase concentration distribution over the lifetime of the file. One key parameter of the fluid dynamic bearing motor oil is to have a vapor pressure that is low enough to avoid evaporation at the highest operation temperature. Since these materials have very low vapor pressures, the options for apparatus to measure vapor pressures are limited. This invention relates to the design of a sample holder to be used for vapor pressure measurements

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Sample Vessel for Low Vapor Pressure Measurements

The present invention describes the design of a sample holder to be used for vapor pressure measurements. Specifically, the design is based upon the Knudsen effusion principle, conducted by Martin Knudsen in 1901. The novel idea involves modification of a TA Instruments hermetic pan such that the pan's base is increased from 0.9 mm to 4.8 mm to accommodate larger sample volumes and to create a larger evaporation surface area. Hermetic lids with 75 m to 127 mm, diameter orifices are typically used to measure the boiling point and vapor pressures in the range of 0.7 kPa to 40 kPa using the differential scanning calorimeter (DSC). Vapor pressures are calculated from the experimental data using the
Clausis-Clapeyron's model.

In the present invention circular orifices with diameters of 127 mm are drilled in the top cover of TA Instruments DSC hermetic pans to allow a path from which molecules can flow.

3. Description: Describe how your invention works, and how it could be implemented, using text, diagrams and flow charts as appropriate. In the present invention circular orifices with diameters of 127 mm are drilled in the top cover of TA Instruments DSC hermetic pans to allow a path from which molecules can flow. Knudsen cells are customarily used to measure the evaporation rate of materials with very low vapor pressures by measure the mass loss of the material by effusion as a function of time. The redesigned Knudsen is fabricated from 1100 (dead soft) aluminum. The aluminum Knudsen cell required an additional anneal (300 °C) to further soften the aluminum for crimping purposes.

In the Knudsen method, the mass of gas effusion through a small orifice in a given time interval is monitored. The rate at which sample mass is loss from the container is proportional to pressure, p. The saturated vapor pressure is derived from

⋅⋅p = z 2π R T

M

 

z is the collision frequency

z =

∆m

A ∆t

⋅ (2)

1

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R = the universal gas constant

M = molecule mass

T = temperature

m = sample mass

A = area of orifice

t = effusion time

T...