IEC/TR 61946 Mineral insulating oils - Characterization of paraffinic/naphthenic nature - Low temperature differential scanning calorimetry (DSC) test method
9 Procedure
Into a clean specimen holder, weigh 10 mg +/- 2 mg of the sample to an accuracy of 0.01 mg.
Seal the specimen holder with a lid under ambient conditions.
Load the test specimen into the instrument chamber. Purge the chamber with dry nitrogen (or other inert purge gas) at a rate recommended by the manufacturer of the instrument. Typical rates are in the range of 10 ml/min to 100 ml/min.
Select appropriate heat flow axis sensitivity on the recorder to yield a 25 % to 95 % deflection of the recorder.
NOTE 1 Pre-analysis on a similar test specimen may be run to provide this information. Most modern instruments contain computer data analysis which permits optimize the curve after the run. In these cases, a pre-analysis is not necessary.
Select the appropriate scales for the time and temperature on the recorder to yield an area of 30 cm2 to 60 cm2 under the fusion endotherm (or crystallization exotherm).
NOTE 2 Heat flow measurements are always made by integrating the peak area as a function of time. Transition temperatures, however, are measured as a function of temperature on the X axis. Most modern thermal analysis instruments contain computer data analysis which permits these measurements to be made simultaneously.
If a simultaneous measurement is not possible, two experiments should be performed to obtain both transition temperature and heat information.
Heat the sample holder from room temperature to 50 °C at 100 K/min rate. Maintain such temperature for 1 min, then cool the sample holder to the initial temperature (at least -80 °C) at 10 K/min +/- 1 K/min. Hold the sample at such temperature for 10 min to allow the equipment to re-equilibrate, then heat the test specimen at 10 K/min +/- 1 K/min until a baseline is re-established above the melting endotherm.
Other periods or heating and cooling rates may be used, but must be noted in the report.
NOTE 3 If helium is used as purge gas, do not allow the equipment to exceed 100 °C at any time.
10 Calculations - Classification of oils
Under the operating conditions described in Clause 9, naphthenic oils do not give any signal in the thermogram, which is the same than the base line (see Figure A.1 ).
NOTE If the thermogram is recorded at very low temperatures, i.e. lower than -120 °C, it should be possible to see a second order transition corresponding to the glass transition of the oil constituents. However, this transition does not give a peak like first order transitions corresponding to melting or crystallization. Glass transitions are easily recognized as sudden changes in the response of the base line.
Under the operating conditions described in Clause 9, paraffinic oils show a wide peak when heating through the melting point and a narrow peak with a wide tail when cooling (see Figure A.2).
11 Report
The report shall include the following:
- complete identification of the sample analysed;
- description of the instrument used for the test;
- material of the specimen holder;
- description of the temperature calibration procedure;
- identification of the atmosphere by pressure, gas flow rate, purity and composition, including humidity, if applicable;
- classification of the oil as "paraffinic" or "naphthenic".
