ASTM D1015 Standard Test Method for Freezing Points of High-Purity Hydrocarbons
7. Sampling
7.1 Sampling from Bulk Storage:
7.1.1 Cylinder - Refer to Practice D1265 for instructions on introducing samples into a cylinder from bulk storage.
7.1.2 Open Containers - Refer to Practice D4057 for instructions on introducing samples into open-type containers from bulk storage.
8. Calibration of Thermometric System and Conversion of Resistance Readings to Temperature
8.1 Calibration of Resistance Bridge - The Mueller-type resistance bridge should have its calibration checked at appropriate intervals by measurement of a suitable external certified resistance, with intercomparison of the resistances of the bridge.
8.2 Calibration of Resistance Thermometer - The platinum-resistance thermometer is provided with four calibration constants certified by the National Institute of Standards and Technology for use in converting the resistance of the thermometer into temperature according to the International Temperature Scale, for use in the range from −190 to 500°C, namely, R0, C, δ, and β. If the thermometer has been properly constructed and annealed, the certified constants C, δ, and β will not change significantly with time, but the value of R0 may change slightly.
NOTE 2 - International Practical Temperature Scale (IPTS) - In 1968, a new IPTS was adopted, replacing the previous scale in use since 1948. The 1948 IPTS was based on the boiling point of oxygen, the sulfur point, ice point, and steam point. The 1968 IPTS is based on the triple point of water, tin point, zinc point, and boiling point of oxygen. The differences in the two temperature scales T68–T48 vary. Above 100°C the differences are plus; below 100°C they may be either plus or minus.
If the measured freezing point is to be used for the determination of purity according to Test Method D1016, the measured freezing point tf, and the freezing point of the pure material tfo, should be on the same temperature scale. The values of tfo given in Test Method D1016 are on the 1968 IPTS. Therefore, values of tf determined using thermometers calibrated on the 1948 scale should be converted to their 1968 IPTS equivalent. This conversion can be made by applying the appropriate correction from Table 1.
8.3 Checking of the Ice Point - Frequent measurements (at least once every month) should be made of the resistance of the given platinum thermometer at the ice point, 0°C, as measured on the given resistance bridge. This value should differ only slightly from the certified value of R0. If the difference becomes appreciable (approaching 0.001 Ω), the calibration of the bridge should be checked. If the bridge has not changed, the change has occurred in the thermometer, and a recalibration of it is recommended.
8.4 Conversion of Resistance Readings to Temperature - When determinations are made on a number of substances having freezing points at different temperatures, time will be saved by making up a table giving values of the resistance, R, for each unit degree of temperature in the given range. Values of resistance for unit degrees, for the ranges from −190 to +50°C and +50 to 290°C, with differences between successive unit degrees tabulated for linear interpolation (which is permissible), may be easily placed on a single 300 by 400-mm (14 by 16-in.) sheet for each range. Calculate values for the resistance, R, from unit values of temperature, t, by means of one of the following equations:
For temperatures below 0°C:
R = R0{1 + Ct[(1 + 0.01δ) - 10(-4)δt - 10(-3)β(t - 100)t2]}
For temperatures above 0°C:
R = R0(1 + Ct[(1 + 0.01δ) - 10(-4)δt]}
where:
t= given temperature, °C, on the International Temperature Scale (see Note 2),
R = resistance of the thermometer in ohms at the temperature t,
R0 = resistance of the thermometer in ohms at 0°C, and
C, δ, and β = constants certified for the given platinum thermometer by the National Institute of Standards and Technology.
