Barometric corrections

TABLE 2.11 Barometric Correction for Gravity—Metric Units... 

For rapid determination of closed cup flash points between 5 and 105 °C, rapid equilibrium method, ISO 3679-1983, ASTM D 3941-1980, ASTM D 3828-1981), the Setaflash tester is recommended (Fig. 3.15). An aluminium block is heated to 3 °C below the expected flash point then 2 cm of the sample is injected into the sample hollow by a syringe. After 60 s, ignition is attempted by a 3.5-mm long test flame with a momentary opening of the sliding cover on the sample hollow. With repeated trials, the flash point is bracketed at first within 5 then within 1 C, using fresh sample portions in each test. For calculation of the closed cup flash point, the above barometric correction is again used. 

The value of the pressure calculated in one of the ways just described is the pressure at some reference point in the instrument, e.g. at the level of the mercury in the reservoir of a Fortin barometer, or at the base of the piston in a piston gauge. If, when a barometric measurement is made, the height of the reservoir differs from that of the point at which the pressure needs to be known, a correction for the hydrostatic head of air may be required at atmospheric pressure, for example, a difference in height of 1 m corresponds to about 12 Pa. Tables are available for making barometric corrections, but in apparatus for vapour-pressure measurement the situation is complicated by the fact that some parts of the system may contain a gas (probably nitrogen or helium) and some parts will contain the vapour (which is of different density) the corrections are small and may be calculated by treating both gas and vapour as ideal gases, i.e. the... 

After barometric corrections of the thermometer readings have been made, if required (see 10.3), the following data require no further calculation prior to reporting initial boiling point, dry point, end point (final boiling point), decomposition point, and all pairs of corresponding values... 

If the overall distillation range of the sample does not exceed 2 C, combined thermometer (bore irregularities and bulb shrinicage) and barometric corrections may be made on the basis of the difference between the observed 50 % boiling point and the true boiling point at 760 mm as given in Table 3. 

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