Method for calibrating a vacuum thermogravimetric analyzer for determination of vapor pressures of compounds
View Patent ↗The invention provides a method for accurately calibrating a vacuum thermogravimetric analyzer (VTGA). The invention solves the problem of calibrating a VTGA by using the actual magnetic transitions and associated transition temperatures, or Curie temperatures, T c 's, of a set of standards which can be used in-situ at the location of the sample holder obviating the difficulties associated with indirect methods of calibration. The invention encompasses a method of using a set of calibration standards comprised of a plurality of ferromagnetic slugs to provide a temperature calibration for a VTGA. The method permits accurate calibration through sufficiently numerous calibration points over a rather limited low-temperature range for determining vapor pressures of compounds. Through the use of these standards, highly accurate measurements can be made of the vapor pressures of critical hard disk drive compounds, such as lubricants, and corrosion inhibitors, that are crucial to competitive disk drive technology.
1. A method to provide a temperature calibration for a vacuum thermogravimetric analyzer (VTGA) comprising:
selecting a plurality of ferromagnetic slugs wherein each slug includes an alloy including nickel and copper and wherein the amount of cooper is within the range of 15% to 28%;
determining the Curie temperature of a each slug;
placing each slug in a sample holder of a VTGA within a magnetic field;
setting a temperature of the VTGA to a temperature corresponding to a first set-point temperature greater than the Curie temperature of each slug by an amount equal to a first offset value;
holding the temperature of the VTGA at a temperature corresponding to a said first set-point temperature for a first time interval sufficient to allow the VTGA to thermally equilibrate;
increasing the temperature of the VTGA to a second set-point temperature greater than an immediately preceding set-point by an amount equal to a second offset value;
holding the temperature of the VTGA at a temperature corresponding to a said set-point temperature for a second time interval sufficient to allow the VTGA to thermally equilibrate; and,
if each slug does not lose magnetization, then repeating the previous two operations, afterwards retuming to this test; but if not, then recording the set-point temperature at which each slug loses magnetization as the apparent Curie temperature of each slug.