IEC/TR 61946 Mineral insulating oils - Characterization of paraffinic/naphthenic nature - Low temperature differential scanning calorimetry (DSC) test method
6 Apparatus
The test method uses the following apparatus:
a) differential scanning calorimeter having the following performance:
- heating or cooling rate up to 20 K/min;
- automatic recording of differential heat flow between the test specimen and the reference material;
- heat flow sensitivity to provide an accuracy of +/- 1 %;
- time base precision of +/- 1 % over the time base range from 0.1 min/cm to 2.0 min/cm (10.0 cm/min to 0.5 cm/min) of chart;
- temperature sensitivity sufficient to provide a specimen temperature accurate to at least +/- 0.1 K;
- cooling device able to cool the oven at least at -80 °C. Preferred operating range -100 °C to + 100 °C;

b) planimeter, or other means of area measurement with an accuracy and precision of +/- 0.1 %;

NOTE 1 Most modern equipment has built-in area integration systems.

c) specimen holders, inert to the oil specimen over the prescribed temperature range, composed of aluminium or other material of high thermal conductivity;

d) dry nitrogen (at least 99.9 % purity) or other inert gas supply for purging purposes. The dew point of the selected gas shall be below the lowest operating temperature;

NOTE 2 Due to its high thermal conductivity, helium may be used very efficiently at very low temperatures. However, the use of helium at temperatures above 100 °C may cause damage in the equipment.

e) analytical balance, accurate to the nearest 0.01 mg.

7 Sampling
Sampling shall be made in accordance with the procedures described in IEC 60475. Samples are measured as received.

8 Calibration
Using the same heating rate, purge gas and purge gas flow rate to be used for samples, calibrate the heat flow, time base and temperature axis of the instrument using the instrument manufacturer's procedures with one or more of the substances mentioned in Tables 1 and 2, having at least 99.9 % purity.

The extrapolated onset Te (see Figure A.2) is taken as the melting temperature.

The decomposition of some calibration materials, such as benzoic acid, may occur above the melting temperature. For such materials, a fresh test specimen shall be used for each analysis.

NOTE The sublimation pressure of benzoic acid requires that it be encapsulated in a sealed specimen holder.

For more accurate temperature measurements at very low temperature, i.e. below 0 °C, the following standards with 99.9 % minimum purity can be used.